by Denise M. Finney and Nancy G. Creamer

Special Topic: Cultivation Practices for Organic Crops

by David W. Monks, Katie M. Jennings and Wayne E. Mitchem

Organic farmers struggling to develop effective and economical weed management practices are not alone. Farmers rank weeds as the number one barrier to organic production (Walz, 1999). And organic farmers cite weed management as their number one research priority.

In approaching weed management within an organic system, it is important to remember the central goal: to reduce weed competition and reproduction to a level that the farmer can accept. In many cases, this will not completely eliminate all weeds. Weed management should, however, reduce competition from current and future weeds by preventing the production of weed seeds and perennial propagules — the parts of a plant that can produce a new plant. Consistent weed management can reduce the costs of weed control and contribute to an economical crop production system.

This chapter describes weed control strategies for organic farms based on weed characteristics and an integrated cropping system approach:

• What is a weed? Weedy plants share common characteristics that must be considered.
• Preventing weeds. Crop rotations, ,cover crops, stale seedbed preparation, soil solarization, proper sanitation, and composting can prevent weeds from emerging and spreading.
• Increasing crop competitiveness. Choosing the right cultivar, using transplants, seeding correctly, ensuring crop health, and applying mulches can give crops a competitive advantage.
• Special topic: Cultivation practices for organic crops. Using the right cultivation tools at critical times can contribute to a cropping system that limits both emerged and future weeds.
• Additional tools for weed management. Animals and approved herbicides can supplement cultural practices for weed control on organic farms.
• What researchers are doing. High-tech weed control, natural weed control, crop breeding, and cropping systems are key weed research areas.
• Advantages of organic production. Organic practices can create conditions that naturally limit weeds.

Figure 1. Yellow starthistle (Centaurea solstitialis).

Photo courtesy of USDA.

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Figure 1. Yellow starthistle (Centaurea solstitialis).

Photo courtesy of USDA.

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What Is a Weed?

Preventing Weeds

Crop Rotations

Cover Crops

Stale Seedbed Preparation

Solarization

Sanitation and Composting

Increasing Crop Competitiveness

Crop Cultivar Selection

Planting Strategies

Crop Health and Vigor

Applying Mulch

Special Topic: Cultivation Practices

Overview

Cultivation Tools: Cultivation Before Planting

Cultivation After Planting – In-row

Cultivation After Planting – Between-row

Weeds that Resist Cultivation

Additional Tools for Weed Management

Animal Labor

Approved Herbicides

What Researchers Are Doing

Advantages of Organic Production

Contacts for Further Information

Recommended Reading

Despite its general acceptance, the term weed is not easily defined. What some farmers consider a weed, others will find innocuous or even charming. And what one may call a weed in a soybean crop, another may call a wildflower in a forest setting. Our perceptions of what a weed is will vary based on location, plant species, population size, and other factors. On a farm, weeds are those plants that negatively affect crop production. First and foremost, weeds compete with market crops for resources, such as light, nutrients, and water, and potentially reduce crop yields. Weeds also lead to increased production costs — the costs of controlling them and the insects and diseases they harbor. During harvest, weeds can interfere with machinery and further reduce crop quality through contamination. Despite the lack of a clear definition for every circumstance, plants that fall into the weed category have shared characteristics that earn them the “weedy” distinction.

Weed Characteristics

Weeds are highly competitive and successful organisms. Most weeds exhibit rapid seedling growth and an ability to reproduce when young, especially when they experience stress. Weeds mature quickly compared to most crop species, and many species thrive under a broad range of conditions. They can tolerate a wide range of adverse environmental conditions, such as drought stress and soil compaction. Weeds can scavenge and compete for resources, and they respond rapidly to favorable growing conditions. Furthermore, weeds have several characteristics that enhance reproductive capability:

• They reproduce via seeds, vegetative propagation, or both.
• They exploit different mechanisms for seed dispersal.
• They display self-compatibility (Zimdahl, 1999) — a single propagule is enough to start a sexually reproducing colony of plants. Self-compatible flowering plants can usually produce seed without visits from specialized pollinating insects.
• They produce a great number of seeds. Examples of species that produce a great number of seeds per plant include redroot pigweed (Amaranthus retroflexus, 117,000 seeds per plant), common purslane (Portulaca oleracea, 52,000), shepherd’s purse (Capsella bursa-pastoris, 38,000), common lambsquarters (Chenopodium album, 28,000), and yellow foxtail (Setaria glauca, 12,000) (Anderson, 1977).

Weed seeds also can be dispersed across time through extended dormancy. A classic longevity study involved 20 species of weed seeds that were buried in soil for more than 80 years (Darlington and Steinbauer, 1961). After 20 years, 11 of the buried species were still viable; after 40 years, 8 were still viable (including purslane, redroot pigweed, shepherd’s purse, annual ragweed (Ambrosia artemisiifolia), and plantain (Plantago major); and after 80 years 3 species were still viable: curly dock (Rumex crispus), common evening primrose (Oenothera biennis), and moth mullein (Verbascum blattaria).

Together, these qualities make weeds tough adversaries, both for neighboring crops and the farmers who manage them.

Weed management within an organic farm relies on an integrated cropping-system approach. An organic farming system should be designed to create a balance between crop plants and weeds. Within such a system, farmers can take action to tip the balance in favor of crop plants whenever possible:

• Cultural practices, including crop rotation, cover cropping, mulching, and cultivating, are important tools in a farming system that puts weeds at a disadvantage. These practices can also have secondary benefits for soil fertility, disease, and pest management.
• Solarization and stale seedbed preparation can keep weeds from emerging when the planting season begins.
• Proper sanitation and composting practices can stop the spread of weeds between fields and from outside the farm.

Crop Rotation

Organic farmers often use mixed cropping systems and long rotations to enhance soil fertility and economic diversity. Crop rotation also can be a cornerstone in a weed management plan. Through long-term variations of crop species and planting times, rotations create a changing environment and prevent the dominance of a particular weed species. Researchers have compared emerged weed densities in test crops grown in rotation versus continually grown test crops. For most of the crops studied, weed densities were lower when a crop was grown in rotation (Liebman and Dyck, 1993).

Knowledge of potential weed problems allows a farmer to select the rotation best suited to a particular field. When making a crop production plan, a farmer should design rotations for each field with weed management and potential weed problems in mind. For example,

• When a crop with a dense, closed canopy, such as potatoes, is grown prior to growing a crop that is less competitive with weeds, the dense crop reduces the development of weeds.
• Where late-germinating weeds are a concern, an early crop can be followed with tillage and a vigorous, competitive summer annual crop to suppress these weeds.
• If perennial weeds are a problem, a market crop can be rotated with a perennial crop that can be mowed repeatedly or grazed as a management strategy. (Bond and Grundy, 2001)

Further, rotations should be evaluated regularly to determine if problem weeds are surviving in the crop rotation scheme and to determine what adjustments need to be made for more effective management.

Cover Crops

Cover crops offer many benefits to an organic farming system, including protection against soil erosion, improvement of soil structure, soil fertility enhancement, and weed suppression. Cover crops can be used in a variety of ways to suppress weeds. Cover crops can suppress weeds, reduce weed populations in the subsequent crop, and reduce weed seed contributions to the soil seedbank:

• Annual or short-term perennial cover crops can be used in place of a fallow period to reduce soil erosion and maintain soil fertility while competing with weeds for resources, such as light, water, and nutrients.
• Cover crops that develop rapidly and form a dense canopy can keep sunlight from newly emerged weeds and outcompete them.
• Cover crops can also provide organic mulch or act as a living mulch to further suppress weed populations during the cropping season.

Cover Crop Residues as Mulch

Annual cover crops may be killed or left to die naturally and used as mulch. By altering light, soil moisture, and soil temperature, mulches limit the germination and growth of weed seedlings. Dead cover crop residues serve as excellent mulch for no-till and reduced-tillage systems when left in a field. Cover crop residues may also be moved from one field to another. There is, however, a risk of transporting weeds into a field with mulch, including cover crop residues that are moved from one field to another. (Mulches applied after planting will be discussed later in this chapter.)

Consider the Market Crop

If cover crop residues will be used as mulch for no-till production, a farmer must consider the market crop that will follow the cover crop. For instance, if the market crop will be planted in early spring, it is best to choose a winter annual cover crop that will die back early, such as a mixture of oats and crimson clover. If the market crop will be planted in late spring or early summer, a mixture of longer-lived species, such as rye and hairy vetch, is preferred.

Effective Cover Crop Kill

Another key to the successful use of cover crop residues is effective cover crop kill. Many no-till systems now used in the midwestern United States rely on chemical herbicides to kill cover crops. Organic farmers, however, must kill crops mechanically, which can be a considerable challenge.

Mechanical methods of killing cover crops that will be left on the soil surface include mowing, rolling, roll-chopping, and undercutting. The success of these methods depends, in part, on the species and growth stage of the cover crop. Optimal mechanical management promotes rapid desiccation and limits the regrowth of the cover crop species while leaving residues intact for mulch. Because mowing generates small pieces of residue that decompose quickly, this may not be the best method of mechanical kill. Rolling and undercutting cover crops can create a longer-lasting surface mulch that can provided extended weed suppression. Rolling also can be accomplished at higher speeds, with lower machinery maintenance costs and reduced fossil fuel consumption compared to mowing. Various methods have been tried for rolling and roll-chopping cover crops. Depending on conditions, an effective kill can result from breaking, cutting, crushing, or crimping stems. (See the recommended reading list: Creamer and Dabney, 2002; Creamer et al., 1995.)

Creamer et al. (1995) designed a modified undercutter to sever the cover crop roots and flatten the intact above-ground biomass on the surface of raised beds. This implement was designed to kill a cover crop with minimal soil disturbance, while leaving the maximum amount of cover crop residue on the soil surface and avoiding shredding the residue. The standards holding the undercutting blades are placed on the outside of the bed to prevent soil and residue disturbance. A rolling basket follows the blades to flatten and distribute the undercut cover crop and aid residue flow through the implement. The undercutter leaves a thicker, longer-lasting mulch on the soil surface than mowing and a noncompacted soil, which can facilitate transplanting of vegetable crops.

Cover Crops as Living Mulch

Certain cover crops also may be used as living mulches (this is often referred to as intercropping). Living mulches can be established before planting, or they can be seeded with or after the main crop has been planted. Seeding with or after the main crop is referred to as interseeding or underseeding. Living mulches may be annual or perennial cover crops, and they can be used with both annual and perennial cash crops.

Researchers have demonstrated that living mulches can effectively suppress weeds when grown with a cash crop. In 51 research trials in which main crops grown with a living mulch were compared to the main crop grown alone, weed biomass was lower in the living mulch system in 47 cases (Liebman and Dyck, 1993). In most instances, the researchers attributed weed suppression to competition from the intercrops, although it is possible that allelopathy — the suppressive effect of chemicals emitted by one species on another — also played a role in some systems (Vandermeer, 1989).

Considerations

The most significant challenge a farmer faces in using living mulch systems for crop production is competition between the living mulch and the market crop. Many examples of successful living mulch systems exist for vineyards and fruit orchards, but many attempts to use living mulches in annual cropping systems (Miura and Watanabe, 2002; Ateh and Doll, 1996; Mohler, 1995) or early in the establishment of perennial crops (Paine et al., 1995) have resulted in reduced growth and yields for the market crops.

Cover crops that are suitable for use as living mulches in intercropping systems should do the following (Enache and Ilnicki, 1990):

• Compete minimally with the market crop for resources, including light, water, and nutrients.
• Have characteristics that control weeds.
• Provide a regular and sufficient source of nitrogen.
• Have low maintenance costs.

The spatial arrangement, seeding rate, and planting time of living mulches should also create favorable conditions for the market crop. For most crops, it is best to confine the living mulch to between-row spaces.

During the growing season, a farmer may need to suppress a living mulch to reduce competition with the market crop. This can be done in an organic system by mowing, partial rototilling, and complete tillage, if necessary. In addition to above-ground competition for light, root competition between the living mulch and crop may reduce yield. (See Liedgens et al. in the “Recommended Reading” list.)

Effective species

Clovers, particularly white clover (Trifolium repens), kura clover (Trifolium ambiguum), and subterranean clover (Trifolium subterraneum), are species with great potential for use as living mulches (Table 1). These low-growing legumes are planted in late summer or fall and grow until winter dormancy. The clover crop flowers in late spring and then sets seed for the following fall. After flowering, vegetative growth dies, leaving a thick mulch. Annual cash crops can be planted into the clover while it is still growing in the spring. As the clover dies in late spring and early summer, it creates a weed suppressive mulch and is no longer a potential source of competition for the market crop.

For further information on living mulch systems, see the “Recommended Reading List” at the end of this chapter. The National Sustainable Agriculture Information Service (ATTRA) recommends the publication by Leary and De Frank (2000).

Allelopathic Cover Crops

In addition to physically suppressing weeds, cover crops can also suppress weeds through chemical means, a process known as allelopathy. Allelopathy is defined as "any direct or indirect harm induced in one plant through toxic chemicals released into the environment by another" (Rice, 1974). Research is underway to determine how plants that produce allelochemicals can be exploited to help manage weeds in cropping systems. Approaches being explored include the use of allelopathic cover crops in rotation with market crops, breeding for allelopathic crop cultivars, and biosynthesis of useful natural herbicides from plants and microorganisms.

Effective species

Many cover crop species produce allelochemicals as they grow and during decomposition, meaning that both living cover crops and decaying residue (incorporated or on the surface) can help to suppress weeds. Commonly used cover crops known to produce allelochemicals and effectively suppress weeds are listed in Table 2. Because these crops also physically suppress weeds, it is difficult to determine if allelopathy is a significant factor in weed control by these species. Despite this unknown, it is generally advantageous to include allelopathic cover crops in crop rotations to promote weed suppression.

Many researchers have documented effective suppression, particularly of small-seeded, broadleaved weeds, by these species. Putman et al. (1983) demonstrated that rye residues reduced the emergence of annual ragweed by 43 percent, green foxtail (Setaria viridis) by 80 percent, redroot pigweed by 95 percent, and common purslane by 100 percent. Through a similar study in North Carolina, Worsham and Blum (1992) found that three pigweed species (A. retroflexus, A. spinosus, and A. hybridus) were controlled by 80 to 100 percent in crops planted into residues of rye or subterranean clover based on weed control ratings.

Considerations

Farmers should be aware of several warnings when using allelopathic cover crops.

• Residues of killed tall fescue (Festuca arundinacea), creeping red fescue (F. rubra, and perennial ryegrass (Lolium perenne), all of which exhibit allelopathy, can significantly reduce crop seedling establishment (Weston, 1990).
• Small-seeded and slow germinating crops are more likely to be adversely effected by allelopathic cover crops than are large-seeded, rapidly germinating crops (Weston, 1996).
• Inhibition of transplant growth by allelopathic cover crops, though not extensively documented, has been observed in woody seedlings grown in a living sorghum-sudangrass cover crop (Geneve and Weston, 1988).

Farmers should select cover crops that can be easily managed and that do not negatively affect seedling establishment to reduce the risk of poor crop germination.

Stale Seedbed Preparation

This weed management strategy consists of preparing a fine seedbed, allowing weeds to germinate (relying on rainfall or irrigation for necessary soil moisture), and directly removing weed seedlings via light cultivation or flame weeding. Seeds or transplants can then be planted into the moist weed-free soil. This technique helps to provide an opportunity for crop emergence and growth before the next flush of weeds. If time allows, this can be done twice before planting. (This strategy for suppressing weeds is discussed in more detail in the “Special Topic” section of this chapter.)

Soil Solarization

Solarization consists of heating the soil to kill pest organisms, including fungi, bacteria, and weed seeds. It also reduces populations of various pathogens and nematodes. Soil is covered in summer with clear or black polyethylene plastic and moistened under the plastic, which is left in place for six to seven weeks or longer. Weed seeds and young seedlings are killed by the heat and moisture and through direct contact with the plastic, which causes scorching.

Research has demonstrated that solarization from July to October with clear or black plastic provides weed control comparable to methyl bromide fumigation in strawberries without reducing fruit yield (Rieger et al., 2001). Solarization can also be used to produce weed–free soil or potting mix for container production in warm climates (Stapleton et al., 2002), and it has been used in Mediterranean climates to reduce weed competition and increase yields of field-grown cauliflower and fennel (Campiglia et al., 2000).

Considerations

In general, solarization is more effective against annual weed species and less effective against perennial weeds. The degree of weed suppression achieved with solarization varies with weed species, depth of seed in the soil, and length of solarization. The drawbacks of solarization include the use of plastics in agriculture and their associated disposal problems (though sheets may be re-used if they are not used as in-season mulch), and the fact that land is taken out of production during the summer.

Guidelines

Solarization can be accomplished on raised beds using a traditional bed layer to lay the plastic, or it can be done on a flat field. Special glues are available to hold the plastic together on a flat field. When solarizing on raised beds, plastic can be left in place and cashcrops planted through it when solarization is complete. To use solarization successfully, farmers should rely on these practices:

• Soil must be finely tilled, and the plastic tarp must fit tightly over the soil.
• Plastic should be from 0.03 to 0.08 inches (0.75 to 2 millimeters) thick, and it should have an ultraviolet inhibitor added to prevent degradation.
• Solarization must be performed during the summer months, due to the temperatures required for effective soil treatment.
• The recommended soil temperatures for solarization are 140°F at a depth of 2 inches and 102°F at a depth of 18 inches (Peet, 1996).

For more details on solarization, see Elmore et al. in the “Recommended Reading” list at the end of this chapter.

Sanitation and Composting

Where do weeds come from? Many on-farm weed populations exist because of the natural movement of weed seeds and propagules from both neighboring and distant populations by wind, animals, people, and other carriers. Human activity is a major culprit in the introduction of weeds to a farm or to new areas on a farm. Paying close attention to sanitation and seed sources on the farm can help prevent the introduction and movement of weeds:

• Clean farm equipment regularly. If machinery and tools are used in more than one location, they should be thoroughly cleaned before use in a different field. Cleaning is especially important when equipment is transferred between farms.
• Limit the amount of off-farm traffic visiting production areas, either by vehicle or foot.
• Apply mulch and compost that is free of weed seeds. Straw mulch, for instance, may contain seeds that will later be a nuisance. To avoid carrying weeds into a field with straw mulch, wet the straw and allow weeds to germinate. Once weed seeds have germinated, dry out the straw bale to kill seedlings by breaking it apart.
• Compost animal manures properly. Animal manures often contain weed seeds, with the source of the manure affecting the number and species of viable weed seeds introduced. To kill weeds and other harmful organisms, compost manures properly before field application. To kill the majority of weed seeds in cattle manure, compost materials at a temperature of at least 180°F (82°C) for no less than three days (Wiese et al., 1998). This temperature is relatively easy to reach in most composting systems.
• Inspect seeds and transplants before planting. Crop seeds, especially grains, may be contaminated with weed seeds. Transplants may have weed seeds in the potting medium if it was not sterilized before use. Buy seeds and transplants from reputable suppliers, and always examine them before planting.

Organic farmers can give their market crops a competitive advantage over weeds by choosing the right cultivar and planting it to ensure vigorous growth that outcompetes weeds for light, moisture, and soil nutrients. Mulching the crop can help to ensure vigorous growth and keep weeds from emerging.

Crop Cultivar Selection

Crop cultivars vary in their abilities to compete with and adapt to weeds. Several characteristics can enhance a cultivar’s ability to compete with weeds, including its physical structure. Tall grain crops, for example, are generally more competitive with weeds because they intercept light. A large leaf area and high biomass production can also contribute to a cultivar’s competitive abilities.

Planting Strategies: Date, Density, and Arrangement

For many row and horticultural crops, rapid growth and early canopy closure can result in the suppression of weeds. For this reason, using transplants when possible for horticultural crop production is advantageous. Use of transplants will increase production costs, so the economic benefit of using transplants must be weighed against cost. When it is economically viable, as is the case with many vegetable crops, use of transplants should be considered.

Research indicates that the planting date, density, and spatial arrangement of a crop can maximize the space it occupies early in the season and put competitive pressure on weeds (Mohler, 2001).

Planting Date

The optimal planting date for a crop may vary from year to year depending on weather and soil conditions. Although these factors must be considered when a farmer determines a planting date, planting can be timed to limit competition from potentially troublesome weed populations. In some instances, it is wise to seed or transplant a cash crop early to get canopy closure as soon as possible.

Alternatively, some farmers believe that planting on the later side of the window of recommended planting dates makes sense from a weed management perspective. Later planting allows one or two precultivations of weeds, and also can give the cash crop a jump start because of warmer soils.

Crop Density

Many researchers have demonstrated that increasing crop density decreases weed competition, though this strategy poses several risks. First, lodging and disease may increase in certain crops as crop density increases (Mohler, 1996).

Second, increasing the crop density may affect the marketability of some crops. Farmers should examine the trade-off between yield gains due to reduced weed competition and any potentially negative effects on yield. This strategy is best suited to seed crops (such as corn and wheat) and not well-suited to most fruiting crops for which increased plant density reduces fruit size. Higher plant density for row crops also helps to buffer against losses caused by mechanical injury from cultivation.

Arrangement

Theoretically, narrow row spacing decreases weed emergence and growth. Research, however, does not overwhelmingly support this conclusion because of inconsistent results in studies of row spacing and weed populations (Mohler, 2001). If narrow row spacing is possible with available planting and cultivation equipment and if it can be done without negatively affecting yield, it can be tried as a weed management tactic.

Crop Health and Vigor

Healthy, vigorous crops are better competitors with weeds for resources, such as light, water, and nutrients. Some crops are inherently better competitors than others, but farmers who make sure that seedlings and transplants have adequate access to nutrients and water will help their crops overcome weed competition. Careful management of soil fertility is essential for successful weed management. Farmers can unknowingly promote weed populations by careless placement or over-application of nutrients, especially nitrogen.

Applying Mulch

Applying a mulch after planting can offer some benefits in many cropping systems. Mulches reduce weed competition by limiting light penetration and altering soil moisture and temperature cycles. Although black plastic is commonly used as a mulching material, its environmental impacts conflict with the goals of regenerative and sustainable production. Synthesized from petroleum, plastic represents a significant use of nonrenewable fossil fuels. In addition, the disposal of plastic mulch has contributed to current landfill problems throughout the United States. The discussion in this chapter will be limited to organic and reusable or biodegradable inorganic mulching materials.

Organic Mulches

Organic mulches include many materials that can be produced on-farm such as hay, straw, and livestock or poultry bedding. Other materials, such as leaves, composted municipal wastes, bark, and wood chips, may be available from off-farm sources. Farmers must consider both the quantity and type of mulch to be applied, and the cost of the mulch and the equipment needed to manage it.

Cost

In situations that require hauling and applying organic mulches, the use of organic mulches can be cost-prohibitive for farmers. Farmers can reduce the costs of purchasing, hauling, and applying mulch by using these strategies:

• Investigate locally available organic mulches. Municipalities will often deliver organic materials for free because it saves them landfill costs.
• Investigate ways in which mulches can be produced on farm.
• Have organic mulch materials analyzed for both nutrient concentration and potential contaminants such as heavy metals, especially those procured off-farm.
• Consider growing vigorous summer cover crops, such as sorghum-sudangrass and pearl millet, that can be cut during the season to produce hay mulch for on-farm use.
• Leave cover crop residues in place for no-tillage planting.

Benefits and Drawbacks

Using organic materials as mulch can help to increase soil organic matter, promote soil biological activity, and enhance soil structure, water infiltration, and aggregate stability. Organic matter is biodegradable and does not contribute to landfill problems. Despite these advantages, however, several drawbacks must be considered:

• Organic mulches high in carbon may temporarily reduce the availability of soil mineral nitrogen as they decompose.
• Allelopathic interactions between mulch materials and the crop are possible.
• Mulches of any type may delay soil warming early in the season. Delayed warming can slow or reduce germination of annually seeded crops or lead to delayed fruit set and harvest in perennial systems. Delaying mulching until two to four weeks after planting or delaying planting can reduce this effect, as can proper cultivar selection.

Quantity

The amount of mulch required for effective weed suppression varies with the type of mulch used. In general, weed suppression improves with increasing mulch thickness and uniformity of distribution. Researchers have examined optimal quantities of various mulching materials. Ligneau and Watt (1995) demonstrated sufficient suppression of annual weed emergence with 3 cm (1.18 in.) of composted materials. Researchers have also evaluated the use of shredded and chopped newspaper as mulch in North Carolina and West Virginia (Monks et al., 1997). Newspaper mulch is generally less expensive than other types of organic mulches for small-scale producers. In West Virginia, a thickness of at least 7.6 cm (3 in.) of chopped newspaper mulch was required to provide 90 percent weed control. Another interesting research result was that chopped newspaper performed better than shredded newspaper for conserving moisture, controlling weeds, and maintaining yield.

Researchers are also investigating how much biomass is needed for effective weed management when cover crops are grown for mulch in no-till systems or to apply to other field. Mohler and Teasdale (1993) suggest that residue levels two to four times the recommended seeding depth for a crop provide sufficient weed control. Other factors, however, also influence the effectiveness of cover crop residues. Weed control is enhanced if residues are intact (rather than chopped or cut) when applied to the field, as this delays breakdown of the mulch material and extends the period of weed suppression. As described earlier, cover crops exhibiting allelopathy may also exhibit greater weed suppression and require lower residue biomass.

Finding the optimal level of cover crop residue may involve on-farm trials of various crops to find the mulch system that is most reliable and effective. Researchers from Virginia Tech suggest that at least 3 tons of cover crop biomass are needed for successful no-till vegetable production (Schonbeck and Morse, 2004). For more information, see the Sustainable Agriculture Network entry in the “Recommended Reading List” at the end of this chapter.

Inorganic Mulches

Several inorganic mulches that are more environmentally friendly than disposable plastics are available. Reusable materials such as black polypropylene mulch can be used for long-term weed management in nurseries and some perennial cropping systems (such as blueberries). Reusable cloth mulch has also been used in lettuce production to promote seed germination and prevent weeds. Research is underway to investigate alternatives to plastic mulch such as degradable plastic mulches and paper mulches.

Degradable plastic mulches

are either photodegradable, breaking down after 30 to 60 days of exposure to sunlight, or biodegradable, broken down by soil microorganisms. Degradable materials do not need to be removed from the field following the growing season, and some may be incorporated into the soil to speed degradation.

Paper mulches

will also degrade naturally and can be applied using traditional bed shapers. Paper mulches are typically coated with biodegradable materials to slow degradation and provide color. Because biodegradable plastics and coated papers are incorporated into the soil, farmers must discuss guidelines for use with their organic certifier to ensure that the product they wish to use meets National Organic Plan (NOP) standards.

How they compare

Researchers at Cornell University compared the performance of three coated paper mulches, a biodegradable plastic mulch, and traditional black plastic. They reported that all of the mulch products remained intact throughout the watermelon production season. Paper coated with plant-derived oils (soybean and linseed) was difficult to lay and, in this study, did not provide soil warming or moisture retention like other products tested. Watermelon yields were higher on black plastic and the biodegradable plastic than on coated paper mulch (Rangarajan and Ingall, 2002).

Further study of biodegradable plastics has demonstrated that yields are variable on these products compared to traditional black plastic, though many provide sufficient weed control (Rangarajan et al., 2003). Several companies are working to develop biodegradable plastics that are easy to apply and make holes in for planting and that will provide adequate soil coverage throughout the growing season. Information on alternative mulches derived from field trials conducted by Cornell University is available online.

Contributed by: David W. Monks, Katie M. Jennings, Wayne E. Mitchem

A cropping system that works to prevent weed emergence provides a strong foundation for optimal weed management. Cultivation practices that limit competition from weeds are key parts of such a cropping system. This “Special Topic” insert discusses cultivation practices and other strategies that organic farmers can use to eliminate emerged weeds and prevent the spread of weed seeds and propagules.

Cultivation: An Overview

Cultivation of the soil with a variety of different tools can control emerged weeds and disrupt weed reproduction cycles. Farmers use cultivation to supplement the use of herbicides and as a stand-alone treatment for controlling weeds. It is the basis for weed management programs for vegetables produced conventionally (without no-till practices or plasticulture). For example, in North Carolina, sweetpotatoes and pickling cucumbers are cultivated two to three times per growing season to control weeds.

Research indicates that repeated cultivation can reduce the number of weed seeds in the seed bank (Cardina and Hook, 1989; Chancellor, 1985; Gunsolus, 1990; Johnson and Mullinix, 1995). Under optimum rainfall conditions, 50 percent of the weed seeds in the plow layer germinate within six weeks of cultivation (Bond and Baker, 1990). The time needed to reduce the seed number in the seed bank, however, varies by weed species and tillage history.

Considerations

Cultivation is influenced by soil type, rainfall, and crop canopy characteristics. Friable soils with few or no rocks can be easily cultivated. Rainfall, especially during seasons with above average rainfall, can delay or prevent timely cultivation, making emerged weeds more difficult to control. This can lead to reductions in crop yield and quality. Rainfall can also prevent cultivators from working properly, promote survival of weeds that are uprooted by cultivators, and stimulate rooting at the nodes of weeds such as large crabgrass. Finally, crop canopy characteristics should be considered prior to cultivation because cultivators can damage crops that are in a stage that is not suited for cultivation (for example, sweet potatoes at the vining stage or when sweet corn is more than 18 inches tall).

Timing

The timing of cultivation should be based on the critical weed-free period of the cash crop and the weed species present in the field where the crop is growing. The critical weed-free period is the time during the season that weeds must be controlled to ensure optimum crop yield and quality. Critical weed-free periods vary by crop and weeds (Table 3). That is, some crops are more competitive with weeds than other crops, and some weeds are more competitive than other weeds (Table 4).

Source: Data gathered from a survey completed by A. Straw, J. Norsworthy, A. S. Culpepper, W.E. Mitchem, D.W. Monks, and K.M. Jennings, University of Tennessee, Clemson University, University of Georgia, and North Carolina State University, cooperating.

Sources: Bowman, 1997; Hotte et al., 2000

Cultivation Tools for Weed Control

Organic farmers have several options for controlling emerged weeds during the critical weed-free period. These options include hand removal (pulling, hoeing, or cutting), mowing, mechanical cultivation, and flame cultivation. Table 5 lists the cultivation tools that are commonly used for weed control and indicates when they are most effective based on crop and weed size.

Cultivation Before Planting

As previously discussed in this chapter, cultivating a stale seedbed before planting is one way to reduce weed populations. Cultivation may be used to encourage weed emergence and subsequently remove emerged weeds. The success of a stale seedbed approach depends on establishing an advantage in crop growth over weed growth. The time between field preparation and crop planting may be as short as three to four weeks and up to several months.

USDA-ARS researchers reported that in a stale seedbed system, shallow cultivation prior to planting crops gave better control of certain weeds than a nonselective herbicide (Johnson and Mullinix, 1998). In their study, effective results were observed when the plot was prepared for planting approximately four weeks ahead of planting and then shallow-tilled at two weeks prior to planting and again just prior to planting.

Mechanical Cultivation

While any form of shallow tillage may stimulate weed emergence before planting, soil cultivation by an implement that will destroy clods, provide better soil-weed seed contact, and create soil conditions favorable to weed seed germination is desirable. Flex tine weeders, rotary hoes, or power tillers are effective on small weeds.

Flex tine weeders

Flex tine weeders are effective on small weeds at the white-thread to cotyledon stage, whereas rotary hoes are effective at the white-thread to 2-inch stage for rotary hoe. The white-thread stage occurs just after weed seed germination when the radical or first root resembles a white thread. Weeds 4 inches or taller and grasses can tolerate cultivation with a flex tine weeder or a rotary hoe. (These tools may also be used after crop seeding if the crop is seeded deep enough to avoid seed disturbance or injury by the implement.) Tillage depth of flex tine weeders is easily adjusted by pressure on the tines or by using gauge wheels. The speed of the rotary hoe affects weed control results because increased operation speed reduces soil penetration.

A self-propelled or tractor-mounted rotary power tiller can also be used to kill weeds. Shallow tillage gave excellent weed control in a stale seedbed system at the University of Georgia (Johnson and Mullinix 1995, 1998, 2000). The plant bed was prepared approximately four weeks ahead of planting, tilled two weeks prior to crop planting, and tilled again just prior to cucumber planting. Rotary tillers are very effective in controlling weeds, and depth of tillage can be adjusted easily.

Flame Cultivation

Broadcast flame cultivation prior to seeding the crop can be used effectively on most organically produced crops. It is more effective on a smooth soil surface than a rough or cloddy surface (Smilie et al., 1965). And it is more effective on broadleaf weeds than grasses, but its effectiveness decreases as weeds mature. Grasses and perennial weeds are most tolerant to flaming. Flaming burns grasses and perennial weeds to the soil surface, but sometimes these weeds are capable of regrowth. Seeding or transplanting crops after flame cultivation must be done carefully to prevent soil disturbance that can lead to weed seed germination and establishment.

Cultivation After Planting – In-row

Finger Weeders

Finger weeders consist of steel cones fitted with rubber-coated fingers capable of controlling small weeds growing in the crop row. These fingers are ground-driven by hard spike tines located on the bottom of the steel cones. They give good control of small weeds less than 1 inch tall and can be used on established crops until they are approximately 8 to 10 inches tall. Finger weeders generally are very safe for crops and do not reduce crop stand if used properly. They must be used with a sweep cultivator to control weeds across the entire bed.

Flexible spider weeder

Spider weeders consist of a circular disk fitted with flexible wire fingers. They are ground-driven and are very effective in controlling weeds. Because crop stands are often reduced by this weeder, the crop must be established at above-optimum seeding rates when farmers use this tool. If crop stands are not optimum, using this weeder may result in low productivity. Flexible spider weeders must be used with a sweep cultivator to control weeds across the entire bed.

Cultivation After Planting – Between-row

Torsion weeders

Torsion or spring hoe weeders are made of flat metal strips that control weeds in the white-thread stage and emerged weeds. These weeders not only uproot or clip weeds (or both), but they also move soil over weeds in the crop row, resulting in weed death.

Rod torsion weeders are made of steel rods that are positioned on each side of the row. These rod weeders run shallow under the soil surface and vibrate so that small weeds are uprooted. Torsion weeders are used with rotating spiders that either push the soil away from the crop row or push soil onto the row around the crop. Cultivators fitted with spider weeders and with torsion weeders are aggressive and are suited for many types of soils.

Sweep cultivation

Sweep cultivation is aggressive and very effective in controlling weeds less than 4 inches tall. The height difference between weeds and the crop allows soil to be moved around the crop to cover small weeds. This cultivator is not a precision cultivator, but a sliding fender or a rolling star-shaped fender will protect crops in the small seedling stage and prevent crop coverage while the soil is being moved by the cultivator. For perennial weeds and large weeds, sequential cultivations spaced approximately 7 to 10 days apart will often increase control. On many cultivators, sweeps can be adjusted to a sharper angle so they till deeply for large weeds, or they can be adjusted to a flat position so that they can till shallowly for small weeds. Sweeps come in different sizes and can be sharpened on the edges to increase control of large weeds.

Rolling cultivators

Rolling cultivators are aggressive and effective in controlling most annual and perennial broadleaf weeds. Rolling cultivators are not very effective, however, in controlling weedy vines or perennial weeds because their long stems interfere with proper turning of the rolling cultivator units. These weed species must be controlled when they are in the seedling stage. Rolling cultivators effectively control weeds near the crop row, and sweep cultivators are often used with rolling cultivators to control weeds between rows.

Basket weeders

Basket weeders are precision cultivators that can be used to cultivate multiple rows on beds. These weeders consist of two axles, each with rolling wire baskets. The front axle is fitted with a large sprocket, and the back axle is fitted with a small sprocket that causes the back axle to turn at a faster rate than the front axle. This weeder is very effective at controlling small weeds, but it does not control perennial weeds, such as nutsedge, effectively. Basket weeders work best on soils that do not contain rocks or a high percentage of clay, and that are composed primarily of sand, muck, or high organic matter.

Flame cultivation

Some vegetable crops are suited to flame cultivation after they are planted (Table 6). The objective of flame cultivation is to create a temperature high enough to dehydrate or rupture the plant cells so that weed death occurs. Flame cultivation effectively controls most broadleaf weeds, especially those that are less than 2 inches tall. There are three types of flame cultivation — parallel flaming, cross flaming, and middle flaming:

• Parallel flaming involves directing burners to the rear so that the flame patterns run parallel with the crop row. Parallel flaming is used when crops lack tolerance to flaming, either because the crop species is susceptible to flaming or because a crop commonly tolerant to flaming is in a susceptible stage.

• Cross flaming can be done by directing the burners so that the flame patterns are across the crop row from each other, but not directly across. Burners set directly across from each other can create turbulence and cause flames to damage crop leaves (Diver, 2002). Cross flaming can be accomplished when the crop is in a tolerant stage of growth — when the crop is taller than the weeds, has a woody stem, or both. To increase crop tolerance, a sprayer can be fitted on the flamer to spray water on the crop just above the burners.

• Middle flaming uses burners located beneath a hood over the row middle. The hood directs the flame to the row middles but protects the crop. Infrared weeders are similar in principle to flame weeders. With infrared weeders, however, the flame is directed to a ceramic element or steel plate that radiates heat at 1,800 to 2,000°F (Diver, 2002).

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Tests have shown that the length of time the plant is exposed to the flame is the main controlling factor in plant damage, rather than the fuel pressure. Therefore, tractor speed is an important consideration in flaming. A good indicator of efficacy is to squeeze a flame-treated leaf between your fingers. If a thumbprint remains, the foliage has been adequately flamed (Diver, 2002). With heavy weed infestations, sequential flaming approximately three to five days apart is more effective and safer for the crop than heavy flaming at slow speed (Smilie et al., 1965).

Hand removal

Hand removal (hoeing, pulling, cutting) effectively controls most small annual weeds. Removal of weeds by the time they are 3 to 4 inches tall will usually result in effective control. At low densities, escaped mature weeds that have produced seeds can be removed carefully from the field when the soil is damp due to rain or dew and then burned if local laws allow it. Care must be taken to prevent seed loss as weeds are being removed from the field.

Mowing

Mowing may also be an option for weed control in low-growing crops such as sweetpotatoes. Approximately 50 percent of North Carolina sweetpotato growers mow weeds such as Palmer amaranth, common cocklebur, sicklepod, common ragweed, and others that extend above the crop canopy. Mowing gives effective control, but care must be taken to mow above the crop without damaging it. In general, it appears that the critical period for weed control is similar for cultivation and mowing. Multiple mowings are required. The first mowing should occur when weeds extend 6 to 10 inches above the crop canopy. After the initial mowing, most lateral branches of weed plants will grow to similar length, resulting in some weeds becoming more competitive with the crop if no further mowing occurs. Weeds should be mowed again as their new branches grow to 6 to 10 inches.

Weeds that Resist Cultivation

Some weeds survive cultivation. Those weeds include nightshades, pigweed species, common and pink purslane and most perennial weeds (such as johnsongrass, purple and yellow nutsedge, bigroot morningglory, and passionflower). Cultivation can spread perennial weeds. For example, nutsedge populations sometimes increase after hand weeding breaks up the root system as the plant is pulled from the soil (Mitchem, personal communication). Table 7 lists some of these troublesome weeds and the characteristics that allow them to resist cultivation. Table 8 is an efficacy table that can be used to determine the effectiveness of each cultivation tool on weeds.

Perennial Weeds

Perennial weeds pose a considerable challenge to organic farmers. Some perennial weeds, such as curly dock, perennial sowthistle (Sonchus arvensis), and dandelion, rely primarily on seed production. But many others, including wild onion (Allium vineale), field bindweed (Convolvulus arvensis), hedge bindweed (Calystegia sepium]), alligatorweed (Alternanthera philoxeroides), purple nutsedge (Cyperus rotundus), and yellow nutsedge (Cyperus esculentus), rely on vegetative reproduction. Their ability to reproduce vegetatively allows such weeds to persist despite many cultural and mechanical controls and some chemical controls. Their problematic vegetative propagules vary by species and can include stolons, rhizomes, bulbs, creeping roots, and tap roots.

Key:

Poor (P) = less than 70% control. ↲

Fair (F) = 70 to 79% control. ↲

Good (G) = over 80% control. ↲

Figure 2. Field cultivator

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Figure 2. Field cultivator

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Figure 3. High residue cultivator

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Figure 3. High residue cultivator

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Figure 4. High-residue cultivator

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Figure 4. High-residue cultivator

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Figure 5. Flex-tine weeder with torsion rods

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Figure 5. Flex-tine weeder with torsion rods

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Figure 6. Broadcast flame weeder

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Figure 6. Broadcast flame weeder

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Figure 7. Flame weeder on tractor

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Figure 7. Flame weeder on tractor

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Figure 8. Flexible spider weeder

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Figure 8. Flexible spider weeder

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Figure 9. Torsion weeder⎯two pairs of torsion rods adjusted for twin row cabbage

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Figure 9. Torsion weeder⎯two pairs of torsion rods adjusted for twin row cabbage

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Figure 10. Rolling unit⎯two-row unit

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Figure 10. Rolling unit⎯two-row unit

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Figure 11. Rolling unit plus sweep

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Figure 11. Rolling unit plus sweep

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Animal Labor

The integration of animals into an organic farming system offers benefits, such as enhanced nutrient cycling and conservation, effective use of crop residues, and an alternative source of income for the farm (Clark and Gage, 1996). Animals also can be used as effective tools for weed management. In particular, the use of “weeder” geese has seen a revitalized interest. Prior to the advent of chemical herbicides, geese were popular for weed control in cotton. Geese have also been used for weed control in strawberries, melons, beans, asparagus, potatoes, onion, garlic, tomatoes, turnips, and in vineyards, nurseries, and orchards. In North Carolina, geese have been used in bramble fruits, though year-old plants need to be protected from grazing.

Weeder geese are selective grazers and prefer to eat grasses over broadleaved weeds. They will eat immature seedlings of johnsongrass, bermudagrass (Cynodon dactylon), crabgrass (Digitaria spp.), and other common grass weeds, but tend to avoid broadleaves such as pigweeds and common lambsquarters. Fortunately, they also will avoid most (but not all) broadleaf crops.

To be effective, geese need to be in place when weed grasses emerge because they are most effective at grazing small grasses. Once weeds are removed, geese will forage on crops in the absence of other food sources. They should, therefore, be removed from fields or provided with food. Because geese are selective feeders, populations of weeds that they do not eat may increase. Therefore, geese are best used in combination with other weed management strategies (Wurtz, 1995). Stocking rates vary by crop, field conditions, weed populations, and other factors. Historically, recommendations have ranged from 2 to 10 geese per acre. Higher stocking rates are necessary in crops with sod intercropping. For example, in orchards where the entire floor is sod, stocking rates can be 50 to 80 geese per acre.

White Chinese and African geese are the most effective weeders. In addition, young geese (goslings) make more effective weeders than mature geese because they are more active and consume seedlings at a higher rate. Goslings can be purchased by mail-order in the spring and then sold for meat in December, in time for winter holidays. Geese are generally fit to be in the field at about six weeks old. Because they require shade, water, and protection at night from predators, movable pens should be constructed to manage the flock. Geese also respond to electrical fencing. ATTRA has a free packet of information on using geese for weed management.

Approved Herbicides

A limited number of natural substances can serve as herbicides on organic farms.

Corn Gluten Meal

The most widely used of these products is corn gluten meal, a by-product of cornstarch production. Corn gluten meal may be applied as a pre-emergence herbicide. Time of application is extremely important, as the gluten must be present when weed seeds germinate to inhibit root formation.

Weeds affected by corn gluten meal include redroot pigweed, black nightshade (Solanum nigrum), common lambsquarters, curly dock, creeping bentgrass (Agrostis palustris), purslane, common dandelion (Taraxacum officinale), and smooth crabgrass (Digitaria ischaemum ). Of weeds that have been tested, barnyardgrass (Echinochloa crus-galli ) and velvetleaf (Abutilon theophrasti) are the least susceptible to corn gluten meal (Bingaman and Christians, 1995). Broadleaf species are generally more susceptible than grasses to corn gluten meal. In field studies, weed cover has been reduced up to 84 percent when corn gluten meal was incorporated prior to planting (McDade and Christians, 2000).

Researchers do not recommend incorporating corn gluten meal prior to direct seeding crops, as crop seedling survival is reduced in the presence of this broad-spectrum herbicide. Transplants, however, are not adversely affected by this product (McDade, 1999). An additional benefit of corn gluten meal is its high nitrogen content. Currently, the Organic Materials Review Institute (OMRI) lists commercially available corn gluten meal under the category corn gluten.

The suggested application rate is 20 pounds per 1,000 square feet, though farmers should consult product specifications to determine the application rate suited to their production program. Research on corn gluten in a broad range of production systems and in various regions has not yet been conducted. Farmers should try this product and other organic herbicides on a small scale before applying them in large-scale cropping systems.

Contact Herbicides

Several OMRI-certified contact herbicides are also available. The active ingredients of these herbicides include citric acid, garlic, thyme and clove oils, and acetic acid (vinegar). The OMRI maintains the most up-to-date list of commercially available products accepted for use in certified organic production. These materials are listed in the category nonsynthetic herbicides.

The use of vinegar for weed control is growing in popularity, but the making of homemade vinegar solutions is not recommended. Effective weed control requires a highly concentrated acetic acid solution, which may be dangerous to handle. Acetic acid formulations are commercially available, and most are in compliance with the USDA National Organic Standards.

Exercise care when using acetic acid and other natural weed control products, as most are not selective and may damage crops as well as weeds. A recent study of vinegar and clove oil demonstrated that both products provide good control of small-seeded broadleaved weeds, less control of velvetleaf and common ragweed, and were not effective in controlling giant foxtail. When applied at a rate of 60 gallons per acre, a vinegar application of 20 percent vinegar and 80 percent water was needed to achieve 80 percent control of broadleaved species susceptible to this product (Curran et al., 2005).

Another product often identified for use as a contact herbicide is soap. According to the National Organic Standards Board list of approved substances, soap-based herbicides may be used only for farmstead maintenance and on ornamental crops.

Researchers around the globe are working to refine and expand the weed management tools that serve as alternatives to synthetic herbicides. These strategies include new mechanical technologies, the use of biocontrols (such as natural plant products and soil bacteria), plant breeding to enhance crop competitiveness with weeds, improved models to predict weed populations, and further development of production systems designed to limit weed competition.

Natural Weed Controls

Researchers are investigating the various agents of weed control available in nature: phytochemicals produced by plants that suppress the growth of other plants (allelopathy), soil bacteria that inhibit seedling growth, and insects that prey upon weed seeds.

Phytochemicals

As previously discussed, the phytochemicals in cover crops can be used to suppress weeds (allelopathy). Researchers continue to investigate ways to optimize this interaction. Research is also underway to identify, extract, and synthesize the plant chemicals responsible for suppressing weed growth to create natural herbicides that could be used in organic farming (Duke et al., 2002).

Weed inhibition by bacteria

Deleterious rhizobacteria (DRB) and other soil microbes can suppress weed species. DRB are specific rhizobacteria (bacteria that naturally occur in association with crop and weed root systems) that reduce or prevent plant growth. Investigators are currently working to identify DRB that inhibit specific weed species, to study the effects of cropping systems on DRB populations (Li and Kremer, 2000), and to develop procedures by which DRB can be isolated and applied as a biological weed control agent (Hardin, 1998).

Seed predation by insects

Carabid (ground) beetles (Coleoptera: Carabidae) and field crickets (Orthoptera: Gryllidae), for example, have been identified as important seed consumers in temperate climates. These species are found naturally in agricultural production systems. Research is underway to determine the effects of various agricultural practices on the presence and effectiveness of such species in lowering weed populations (Menalled et al., 2000). Existing research suggests that ground beetles may be more abundant on organic farms than on farms that apply synthetic pesticides (Dritschilo and Wanner, 1980). Efforts have been made to identify additional insect species that may be effective in reducing weed populations and to develop appropriate methods of introducing seed predators to agricultural fields. For example, the North Carolina Department of Agriculture Biological Control Program has identified and released a weevil that consumes musk thistle seed.

Breeding for Crop Competitiveness and Weed Suppression

Plant breeding is one way to improve weed management in organic systems. By using crops with increased competitive ability and enhanced weed suppressive qualities, farmers will have yet another advantage over weeds. Crop qualities that promote crop competitiveness include early, rapid establishment in less favorable conditions, crop structures that limit weed access to light and nutrients (such as increased ground cover by vegetative portions) and increased plant hardiness (Lammerts van Bueren et al., 1998).

Breeding can also lead to crop varieties with improved seedling resilience and transplant vigor, particularly in systems that rely on cover crop residue. Efforts to improve cover crops for weed management are also a research priority. Desirable cover crop qualities include increased allelochemical production, early establishment, improved structure, complete natural dieback, self-seeding, and high biomass, all of which may be introduced through plant breeding (Foley, 1999).

System-level Approaches to Weed Management

Perhaps the most promising and practical area of research in weed management is the study of agricultural systems. Farming systems can be designed to integrate the various cultural practices that farmers use to manage weeds, such as cover crops, crop rotations, increased crop competitiveness, and direct controls. This line of research is based on the recognition that organic agriculture is holistic (Barberi, 2002). It relies on many natural processes working in concert to supply nutrients, build soil organic matter, deter pests, and decrease disease incidence. The same is true of weed management: No single solution or isolated practice will reduce weed competition within an organic farming system.

Research groups at many land-grant universities are designing and conducting studies to assist growers in developing their own farm-specific production programs. For example, the Center for Environmental Farming Systems (CEFS) at North Carolina State University is currently conducting a long-term study and numerous short-term studies of cropping systems for horticultural and commodity crops.

This new focus on integrated cropping systems will require both field trials of these systems and accurate models for assessing weed management within a system. Such models must predict both the negative impact of weeds on crop production (as traditional models have done) and the impact of crops and cropping systems on weed populations (Bastiaans et al., 2000). Modeling can be used to help farmers make decisions about their cropping systems, avoid potential weed problems, and design the most beneficial systems for their farms.

Weeds as Indicators of Soil Condition

Recognizing that an agricultural production system will never be entirely weed-free, many farmers have sought to find some value and utility in the weeds on their farms. Can weeds serve as indicators of various soil conditions, such as nutrient deficiencies or compaction? Anecdotal evidence suggests that weeds can serve as indicators of some soil conditions, such as low pH (acid soils), high pH (alkaline soils), high nitrogen, low nitrogen, calcium deficiency, severe compaction, and poor drainage (Hill and Ramsay, 1977). Scientific research on this topic, however, is inconclusive. The information gleaned from weeds cannot substitute for analytical tools, such as soil nutrient testing. But it can be useful for preliminary soil assessments. For a complete list of potential indicator species and soil conditions, see Hill and Ramsey (1977) in the “Recommended Reading” list at the end of this chapter.

Despite the fact that many organic farmers and would-be organic farmers cite weeds as a major impediment to farming organically, going organic may naturally reduce weed competition. In a review of recent research on weed population dynamics in organic systems, Ngouajio and McGiffen (2002) conclude that though the number of weed species found in an organic system may be higher, the total weed density and biomass are often smaller in organic systems than conventional systems.

Organic production systems have three key features that can positively affect weed management:

• Greater soil microbe, insect, and plant species diversity. Increased plant species diversity discourages the outbreak of large populations of a single weed species through resource competition and limited niche availability.
• Soil conditions favorable to beneficial microbes. Increased soil microbe and insect populations deplete the weed seed bank through weed seed predation.
• Suitable habitat for beneficial insect populations. Phytophagous insects limit seedling growth by consuming newly emerged weeds.

These conditions take time to develop. The transition from conventional to organic may lead to short-term increases in weed competition when the equilibrium established by conventional production is disrupted. As the transition progresses, a new equilibrium is established in which weed competition is limited. Over the long term, a well-managed organic system may contain natural limits to weed populations, decreasing both the inputs required for weed management and crop losses due to competition.

ATTRA — Appropriate Technology Transfer to Rural Areas P.O. Box 3657 Fayetteville, AR 72702 Telephone: 1-800-346-9140

Ecological Agriculture Projects McGill University (Macdonald Campus) Ste-Anne-de-Bellevue, Quebec, Canada H9X 3V9 Telephone: 514-398-7771

OMRI – Organic Materials Review Institute

CEFS –Center for Environmental Farming Systems

Sources Cited

Anderson, W.P. 1977. Weed Science: Principles. 2nd Ed. St. Paul, MN: West Publishing Company.

Ateh, C.M. and J.D. Doll. 1996. Spring-planted winter rye (Secale cereale) as a living mulch to control weeds in soybeans. Weed Technology. 10(2):347-353.

Barberi, P. 2002. Weed management in organic agriculture: Are we addressing the right issues? Weed Research. 42(3):177-193.

Bastiaans, L., M.J. Kropff, J. Goudriaan, and H.H. van Laar. 2000. Design of weed management systems with a reduced reliance on herbicides poses new challenges and prerequisites for modeling crop-weed interactions. Field Crops Research. 67(2):161-179.

Bellinder, R.R., J.J. Kirkwyland, R.W. Wallace, and J.B. Colquhoun. 1999. Weed control and potato (Solanum tuberosum) yield with banded herbicides and cultivation. Weed Technology. 14:30-35.

Bingaman, B.R. and N.E. Christians. 1995. Greenhouse screening of corn meal gluten as a natural control product for broadleaf and grass weeds. HortScience. 30 (6): 1256-1259

Bond, W. and A.C. Grundy. 2001. Non-chemical weed management in organic farming systems. Weed Research. 41:383-405.

Bond, W. and P.J. Baker. 1990. Patterns of weed emergence following soil cultivation and its implications for weed control in vegetable crops. Pages 63-68 in British Crop Protection Council. Monogr. 45.

Bowman, G. 1997. Steel in the Field. Sustainable Agriculture Network. Handbook Series Book 2: 14-16.

Campiglia, E., O. Temperini, R. Mancinelli, and F. Saccardo. 2000. Effects of solarization on the weed control of vegetable crops and on the cauliflower and fennel production in the open field. Acta Horticulturae. 533:249-255.

Cardina, J. and J.E. Hook. 1989. Factors influencing germination and emergence of Florida beggarweed (Desmodium tortuosum). Weed Tech. 3: 402-407.

Chamen, W.C.T. 2000. A new methodology for weed control and cereal crop production based on wide span vehicles and precision guidance: Biotrac. In: Proceedings 4th Workshop of the EWRS Working Group on Physical and Cultural Weed Control, Elspeet, The Netherlands, 51-54.

Chancellor, R.J. 1985. Tillage effects of annual weed germination. World Soy. Res. Conf. III Proc. 3:1105-1111.

Clark, M.S. and S.H. Gage. 1996. Effects of free-range chickens and geese on insect pests and weeds in an agroecosystem. American Journal of Alternative Agriculture. 11(1): 39-47.

Coleman, E. 1995. The New Organic Grower. White River Junction, VT: Chelsea Green Publishing Company.

Creamer, N.G., B. Plassman, M.A. Bennett, R.K. Wood, B.R. Stinner, and J. Cardina. 1995. A method for mechanically killing cover crops to optimize weed suppression. American Journal of Alternative Agriculture. 10(4):157-162.

Creamer, N.G. and S. Dabney. 2002. Killing cover crops mechanically: review of recent literature and assessment of new research. American Journal of Alternative Agriculture. 17(1): 2-40.

Curran, W.S, D.D. Lingenfelter, and C.B. Muse. 2005. Effectiveness of vinegar and clove oil for control of annual weeds. Poster Presentation at the Weed Science Society of America Annual Meeting. Honolulu, Hawaii. February 7-10, 2005.

Darlington, H. and Steinbauer, G.P. 1961. 80 year period for Dr. Beal’s seed viability experiment. American Journal of Botany. 48(4):321-&.

Diver, S. 2002. Flame weeding for vegetable crops.

Dritschilo, W. and D. Wanner. 1980. Ground beetle abundance in organic and conventional corn fields. Environmental Entomology. 9(3):629-631.

Duke, S.O., F.E. Dayan, A.M. Rimando, K.K. Schrader, G. Aliotta, A. Oliva, and J.G. Romangi. 2002. Chemicals from nature for weed management. Weed Science. 50:138-151.

Ellis, D.R., K. Guillard, and R.G. Adams. 2000. Purslane as a living mulch in broccoli production. American Journal of Alternative Agriculture. 15(2):50-59.

Elmore, C.L., J.J. Stapleton, C.E. Bell, and J.E. Devay. 1997. Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds. Publication 21377. University of California Cooperative Extension, Davis, CA.

Enache, A.J. and R.D. Ilnicki. 1990. Weed control by subterranean clover (Trifolium subterraneum) used as a living mulch. Weed Technology. 4:534-538.

Fogelberg F. 2000. Electroporation: Can we control weed seeds by the use of electric pulses applied in soil? In: Proceedings 4th Workshop of the EWRS Working Group on Physical and Cultural Weed Control, Elspeet, The Netherlands, 50.

Foley, M.E. 1999. Genetic approach to the development of cover crops for weed management. Journal of Crop Production, 2(1):77-93.

Foulds, C.M., K.A. Stewart, and R.A. Samson. 1991. On-farm evaluation of legume interseedings in broccoli. In W.L. Hargrove (ed.) Cover Crops for Clean Water. Soil and Water Conservation Society, Ankeny, IA. pp. 179-180.

Friesen, G.H. 1978. Weed interference in pickling cucumbers (Cucumis sativus). Weed Sci. 26:626-628.

Friesen, G.H. 1979. Weed interference in transplanted tomatoes (Lycopersicon esculentum). Weed Sci. 27:11-13.

Geneve, R.L. and L.A. Weston. 1988. Growth reduction of eastern redbud (Cercis canadensis L.) seedlings caused by interaction with a sorghum-sudangrass hybrid (sudex). Journal of Environmental Horticulture. 6:24-26.

Grubinger, V.P. and P.L. Minotti. 1990. Managing white clover living mulch for sweet corn production with partial rototilling. American Journal of Alternative Agriculture. 5(1):4-12.

Gunsolus, J.L. 1990. Mechanical and cultural weed control in corn and soybeans. Am. J. Alter. Agric. 5:114-119.

Hardin, B. 1998. Underground Biocontrol Allies? Agricultural Research Magazine 46(10):14-16.

Heisel, T, J. Schou, C. Andreasen, and S. Christensen. 2002. Using laser to measure stem thickness and cut weed stems. Weed Research 42:242-248.

Hill, S.B. and J. Ramsay. 1977. Weeds as indicators of soil conditions. McGill University Ecological Agriculture Projects Bulletin 67.

Hotte, Marie-Josee, D.L. Benoit, and D. Cloutier. 2000. Use of mechanical cultivators for market vegetable crops. Ontario, Canada: Agriculture and Agrifood Canada.

Ilnicki, R.D. and A.J. Enache. 1992. Subterranean clover living mulch: An alternative method of weed control. Agriculture, Ecosystems, and the Environment. 40: 249-264.

Johnson, W.C. and B.G. Mullinix, Jr. 2000. Evaluation of tillage implements for stale seedbed tillage in peanut (Arachis hypogaea). Weed Tech. 14:519-523.

Johnson, W.C. and B.G. Mullinix, Jr. 1995. Weed management in peanut using stale seedbed techniques. Weed 43:293-297.

Johnson, W.C. and B.G. Mullinix, Jr. 1998. Stale seedbed weed control in cucumber. Weed Sci. 46:698-702.

Lammerts van Bueren, E.T., M. Hulscher, J. Jongerden, M. Haring, J. Hoogendoorn, J.D. van Mansvelt, and G.T.P. Ruivenkamp. 1998. Sustainable organic plant breeding. Louis Bolk Institute.

Leary, J. and J. DeFrank. 2000. Living mulches for organic farming systems. HortTechnology 10(4): 692-698.

Levett, M.P. 1992. Effects of various hand-weeding programmes on yield and components of yield of sweet potato (Ipomoea batatas) grown in the tropical lowlands of Papua New Guinea. J. Agric. Sci (Camb.) 118:63-70.

Li, J. and R.J. Kremer. 2000. Rhizobacteria associated with weed seedlings in different cropping systems. Weed Science. 48:734-741.

Liebman, M and E. Dyck. 1993. Crop rotation and intercropping strategies for weed management. Ecological Applications. 3(1): 92-122.

Liedgens, M., A. Soldati, and P. Stamp. 2004. Interactions of maize and Italian ryegrass in a living mulch system: 1. Shoot growth and rooting patterns. Plant and Soil. 262(1-2): 191-203.

Ligneau L.A.M. and T.A. Watt. 1995. The effects of domestic compost upon the germination and emergence of barley and 6 arable weeds. Annals of Applied Biology. 126(1):153-162.

Mallet, J.Y. and R.A. Ashley. 1988. Determination of summer squash’s tolerance to weed interference: A critical period study. Proc. Northeast. Weed Sci. Soc. 42:204-208.

McDade, M.C. and N.E. Christians. 2000. Corn gluten meal – A natural preemergence herbicide: effect on vegetable seedling survival and weed cover. American Journal of Alternative Agriculture. 15(4):189-191.

McDade, M.C. 1999. Corn gluten meal and cluten hydrolysate for weed control. M.S. thesis. Iowa State University, Department of Horticulture, Ames.

Menalled, F.D., P.C. Marino, K.A. Renner, and D.A. Landis. 2000. Post-dispersal weed seed predation in Michigan crop fields as a function of agricultural landscape structure. Agriculture, Ecosystems, and Environment. 77:193-202.

Mohler, C.L. 2001. Enhancing the competitive ability of crops. In: Ecological Approaches to Weed Management, M. Liebman, C.L. Mohler, and C.P. Staver, Eds. Cambridge: Cambridge University Press.

Mohler, C.L. 1996. Ecological bases for the cultural control of annual weeds. Journal of Production Agriculture. 9:468-474.

Mohler, C.L. 1995. A living mulch (white clover)/dead mulch (compost) weed control system for winter squash. Proceedings of the Annual Meeting of the Northeastern Weed Science Society of America. 49:5-10.

Mohler, C.L. and J.R. Teasdale. 1993 Response of weed emergence to rate of Vicia villosa and Secale cereale residue. Weed Res. 33:487-499.

Monks, D.W. and J.R. Schultheis. 1998. Critical weed-free period for large crabgrass (Digitaria sanguinalis) in transplanted watermelon (Citrullus lanatus). Weed Sci. 46:530-532.

Monks C.D., D.W. Monks, T. Basden, A. Selders, S. Poland, and E. Rayburn. 1997. Soil temperature, soil moisture, weed control, and tomato (Lycopersicon esculentum) response to mulching. Weed Technology 11:561-566.

Neary, P.E. and B.A. Majek. 1990. Common cocklebur (Xanthium strumarium) interference in snap beans (Phaseolus vulgares). Weed Tech. 4:743

Nerson, H. 1989. Weed competition in muskmelon and its effects on yield and quality. Crop Prot. 8:439-443.

Ngouajio, M. and M.E. McGiffen. 2002. Going organic changes weed population dynamics. HortTechnology. 12(4):590-596.

Nordell, A. and E. Nordell. 1998. A whole-farm approach to weed control: A strategy for weed-free onions. Sharing the Lessons of Organic Farming Conference, January 30-31, 1998, Ontaria, Canada: University of Guelph. Paine, L., H. Harrison, and A. Newenhouse. 1995. Establishment of asparagus with living mulch. Journal of Production Agriculture. 8(1):35-40.

Peet, M. 1996. Sustainable vegetable production practices for the South. Newburyport, MA: Focus Publishing.

Rangarajan, A., B. Ingall, and M. Davis. 2003. Alternative mulch products 2003.

Rangarajan, A., and B. Ingall. 2002. Alternative mulch products 2002.

Rice, E.L. 1974. Allelopathy. New York: Academic Press.

Rieger, M., G. Krewer, and P. Lewis. 2001. Solarization and chemical alternatives to methyl bromide for preplant soil treatment of strawberries. HortTechnology. 11(2): 258-264.

Schonbeck M. and R. Morse. 2004. Choosing the best cover crops for your organic no-till vegetable system. The New Farm.

Seem, J.E., N.G. Creamer and D.W. Monks. 2003. Critical weed-free period for ‘Beauregard’ sweetpotato (Ipomoea batatas). Weed Tech. 17:686-695.

Smilie, J.L., C.H. Thomas, and L.C. Standifer. 1965. Farm with Flame. Agr. Ext. Pub. 1364:1-16

Stapleton, J.J., T.S. Prather, S.B. Mallek, T.S. Ruiz, and C.L. Elmore. 2002. High temperature solarization for production of weed-free container soils and potting mixes. HortTechnology. 12(4): 697-700.

Sustainable Agriculture Network. 1998. Managing Cover Crops Profitably, 2nd Ed. Washington, DC: Sustainable Agriculture Research and Education Program. 212 pp.

Talatala, R.L., A.M. Mariscal, and A.C. Secreto. 1978. Critical periods for weed control in sweetpotato. Philipp. J. Weed Sci. 5:1-6.

Terry, E.R. and W.M. Stall. 1997. Smooth amaranth interference with watermelon and muskmelon production. Hort. Sci. 32(4):630-637.

Walz, Erica. 1999. Third Biennial National Organic Farmers’ Survey. Santa Cruz, CA: Organic Farming Research Foundation.

Weston, L.A. 1996. Utilization of allelopathy for weed management in agroecosystems. Agronomy Journal. 88:860-866.

Weston, L.A. 1990. Cover crop and herbicide influence on row crop seedling establishment in no-tillage culture. Weed Science. 38:166-171.

Wiese, A.F., J.M. Sweeten, B.W. Bean, C.D. Salisbury, and E.W. Chenault. 1998. High temperature composting of cattle feedlot manure kills weed seed. Applied Engineering in Agriculture. 14(4):377-380.

William, R.D. and G.F. Warren. 1975. Competition between purple nutsedge (Cyperus rotundus) and vegetables. Weed Sci. 23:317-323.

Worsham, A.D. and U. Blum. 1992. Allelopathic cover crops to reduce herbicide inputs in cropping systems. In: Proceedings of the First International Weed Control Congress, pp. 577-579. Richardson, R.G., Ed., Weed Science Society of Victoria, Melbourne, Australia.

Wurtz, T.L. 1995. Domestic geese: biological weed control in an agricultural setting. Ecological Applications. 5(3):570-578.

Vandermeer. J. 1989. The Ecology of Intercropping. Cambridge, England: Cambridge University Press.

Zemenchik, R.A., K.A Albrecht, C.M. Boerboom, and J.G. Lauer. 2000. Corn production with kura clover as a living mulch. Agronomy Journal. 92(4): 698-705.

Zimdahl, R. 1999. Fundamentals of Weed Science, 2nd Ed. New York: Academic Press.

Additional Reading

Holmes, G.J., D.W. Monks, J.R. Schultheis, and K.A. Sorensen. 1999. NC Crop Profile: Cucumber. AG-598-8:1-4.

Lal, R., E. Regnier, D.J. Eckert, W.M. Edwards, and R. Hammond. 1991. Expectations of cover crops for sustainable agriculture. In W.L. Hargrove (ed.) Cover Crops for Clean Water. Soil and Water Conservation Society, Ankeny, IA. pp 1-11.

Liebman, M. and A.S. Davis. 2000. Integration of soil, crop and weed management in low-external-input farming systems. Weed Research. 40: 27-47.

Miura, S. and Y Watanabe. 2002. Growth and yield of sweet corn with living mulches. Japanese Journal of Crop Science. 71(1):36-42.

Putnam, A.R. 1988. Allelochemicals from plants as herbicides. Weed Technology. 2:510-519.

Putnam, A.R., J. DeFrank, and J.P. Barnes. 1983. Exploitation of allelopathy for weed control in annual and perennial cropping systems. Journal of Chemical Ecology. 9(8):1001-1010.

Schultheis, J.R., K.A. Sorensen, D.W. Monks and G.J. Holmes. 1999. NC Crop Profile: sweetpotato. Ag-598-24:1-4.

Teasdale, J.R. and C.L. Mohler. 1993. Light transmittance, soil temperature, and soil moisture under residue of hairy vetch and rye. Agronomy Journal 85:673-680.

Denise M. Finney

SANREM Coordinator, Department of Crop Science

Nancy G. Creamer

Director, Center for Environmental Farming Systems

Department of Horticultural Science

Special Topic Insert Prepared by

David W. Monks

Extension Specialist, Horticultural Science

K. M. Jennings

Research Assistant Professor, Horticultural Science

Wayne E. Mitchem

Extension Associate, Horticultural Science

Photographs Provided by

Ken L. Fager

Research Specialist, Horticultural Science

College of Agriculture and Life Sciences

North Carolina State University

The Organic Production Publication Series was developed by the Center for Environmental Farming Systems

a cooperative effort between North Carolina State University,

North Carolina A & T State University, and the

North Carolina Department of Agriculture and Consumer Services

The USDA Southern Region Sustainable Agriculture Research and Education Program

and the USDA Initiative for Future Agriculture and Food Systems Program

provided funding in support of the Organic Production publication series.

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North Carolina Agricultural and Technical State University

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North Carolina Agricultural and Technical State University

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NC State University

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NC State University

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North Carolina Department of Agriculture and Consumer Services

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North Carolina Department of Agriculture and Consumer Services

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