Supplemental hand weeding accounts for the large portion of landscape bed maintenance costs. But, most of the costly and unsightly weed problems can be minimized with a little planning. Developing a landscape weed management plan involves five basic steps.

(1) Site assessment. Survey the site for cultural aspects as well as weed species.

(2) Define the type of planting. The type of planting, woody shrubs vs. bedding plants etc., will define the post-plant weed management options available.

(3) Selection of ornamentals species and compatible weed management options; based upon design, cultural, and weed management criteria.

(4) Site preparation. Control weeds which cannot be controlled after planting.

(5) Installation and implementation; of the plants and the plan.

These steps will be discussed separately, but keep in mind that each step relates to, and is dependent upon the decisions made in the other steps. The goal is to provide a process by which an effective and economical weed management plan may be developed.

Adequate site assessment will allow proper species selection based upon design criteria, cultural suitability, and management regime, including weed management. Take soil samples for pH and nutrient analysis. Note soil type and physical condition, drainage patterns, compaction, disturbance, exposure and edaphic aspects, and potential maintenance problems such as gutter down-spouts, chlorine from the pool, or traffic patterns. Identify the weeds in the area, with particular emphasis on perennial weeds. Ask yourself: "Can these weeds be controlled after planting?" Some species which are difficult or impossible to control after planting include: bindweed, nutsedge, mugwort, Canada thistle, goldenrod, bamboo, Japanese knotweed, wild violet, and field horsetail. Also, inspect the surrounding areas for weeds which may encroach, such as: ground ivy, wild strawberry, yarrow, bermudagrass, creeping speedwell, quackgrass, or other creeping perennials.

The best time to scout for weeds is in mid- to late summer, when annual and perennial weeds can be identified. Scouting in late fall or early spring, is likely to miss many of the important species. Also, scouting in the summer will allow adequate time for decision-making and site preparation before planting.

The species to be planted will define the intensity of management required and, to a large degree, govern your future weed management options. A planting of woody trees and shrubs will allow the most post-plant weed management options. In contrast, a mixed planting of woody and herbaceous plants will have fewer post-plant options. Table 1 provides an overview of the weed management limitations and options for the different types of plantings. Tables 2 and 3 list the herbicides registered for use in landscape ornamentals, and the suggested types of plantings where they can be used. Understanding these limitations and options will help guide you in the following steps toward developing an effective weed management plan.

Table 1. Weed management options and limitations for the five types of landscape plantings.

Tree and Shrub Beds: Densely shaded plantings exclude weeds.

Two options set woody tree and shrub beds apart from the rest; one is the possibility of post-plant perennial weed control, the other is the use of geotextile fabrics for annual weed control. Perennial weeds may be controlled by manual removal, spot applications of systemic herbicides (such as glyphosate or triclopyr), or, in some instances, dormant-season applications of diclobenil (Casoron). Care should be taken not to contact desirable foliage with systemic herbicides; also, use diclobenil only on labeled species as injury is likely to result if applied to other landscape plants. Annual weeds may be controlled using geotextile fabrics, organic or inorganic mulches, and/or herbicides. It is often necessary to combine these treatments for complete weed control. Geotextile fabrics are somewhat expensive to install but become cost-effective if the landscape bed is to remain in place for more than four years. Preemergence herbicides are less expensive and equally effective, but must be reapplied annually (usually two applications per year, late winter and late summer). Escaped weeds may be controlled manually or with spot applications of postemergence herbicides.

Woody Ground Covers

Ultimately, woody ground covers should exclude most weeds; however, weed encroachment during establishment is likely. After planting, it is difficult to make spot applications of Roundup or other nonselective herbicides without injuring desirable plants; therefore, perennial weeds should be eliminated before planting. An exception are perennial grasses, which can be selectively controlled after planting with clethodim, fluazifop-p, or sethoxydim. Annual weeds may be controlled with mulch plus a preemergence herbicide, supplemented with some hand weeding.

Annual Flower Beds

Weed control in annual flower beds can be simple if perennial weeds are eliminated before planting. Perennial grasses can be selectively controlled with clethodim, fluazifop-p, or sethoxydim. but perennial sedges and broadleaf weeds cannot be selectively controlled after planting. Nonselective herbicides will kill or severely injure annual bedding plants and should be avoided after planting. Geotextiles are generally not used in annual flower beds. Annual weeds may be controlled with mulches, preemergence herbicides, and/or hand weeding. However, relatively few preemergence herbicides are labeled for use in bedding plant, and careful product selection is required to avoid injury to your plants.

Herbaceous Perennial Beds

Weed management options in herbaceous perennial beds are similar to those for annual flowers, except (1) it is more important to eradicate perennial weeds, as there will be no opportunity to cultivate or renovate the bed for several years; and (2) fewer species are included on herbicide labels.

Ornamental Grasses

The use of ornamental grasses in commercial and residential landscapes continues to grow. Native grass are often promoted as low-input, sustainable plant selections. However, many native grasses are slow to establish, providing an opportunity for weeds to establish. Furthermore, many species generically included in “ornamental grass” plantings are, in fact, not grasses. Sedges, rushes, irises and others are often mistaken for ornamental grasses. This complicates weed management. Few herbicides are labeled for use in ornamental grasses and fewer still are labeled for use on ornamental sedges or rushes. Site preparation to eliminate perennial weeds is critical. In particular, perennial grass weeds like bermudagrass or quackgrass will be very difficult to control after planting. For specific guidelines on weed control in ornamental grass plantings see: .

Mixed Plantings

In mixed plantings of woody and herbaceous ornamentals, site preparation is as critical as for herbaceous perennials because post-plant choices are few. One option for such areas is to plant the woody species first; control the perennial weeds in the first two growing seasons, and then introduce the herbaceous species. Another option may be to define use-areas within the bed which will receive different weed management programs - such as a section devoted to annual flowers in an otherwise woody tree & shrub bed

Based upon the type of planting desired and the site assessment, we can now select the species for planting. The criteria for species selection should include design and site suitability, and maintenance aspects including disease and insect resistant species/varieties. But including considerations of post-planting weed control in the design selections can improve overall landscape maintenance , as well as weed management option compatibility. Selecting the proper weed management option(s) will depend upon weed species present, your flexibility in planting design and species selection, economics, as well as some personal choice. Generally, it is best to control perennial weeds before planting (See "Site Preparation"). However, in the “real world” of landscape installation, the schedule for site preparation and installation is usually dictated by customer expectations and not by the life cycles of weeds on the site. If perennial weeds cannot be controlled before planting, it is possible to modify ornamental plant species selections to enable a more effective weed management plan.

A landscape weed management plan will utilize several tools and strategies including mulches, preemergence herbicides, postemergence herbicides, sanitation to prevent introduction and spread of weeds, and (of course) some hand weeding.

Mulches

Mulches are our first line of defense against annual weeds. Many types of mulches are available including barks, wood products, pine straw, various hulls (pecan, cocoa, buckwheat, etc.) municipal composts, crushed rocks, and others. All mulches suppress annual weeds by excluding light, which is required for seed germination and seedling growth. When mulches are too fine, too thick, or begin to decompose, they stay wet between rains, allowing weeds to germinate and grow directly in the mulch. Therefore, a mulch that is fairly coarse-textured, with a low water-holding capacity would be preferred. To effectively suppress weeds in perennial landscape beds, organic mulches should be applied to a depth of 3 to 4 inches. DO NOT EXCEED 4 inches of mulch. More than that will encourage weed growth in the mulch and may damage landscape plants. Inorganic mulches should be 2 to 3 inches thick, as they do not decompose or settle as quickly. Plan for periodic replenishment.

In annual color beds, mulches are used to provide temporary weed suppression while the bedding plants are establishing. A thinner layer, about 1 to 2 inches thick, of a finer-textured organic mulch is recommended. This thin layer of mulch will be incorporated into the soil when beds are seasonally transitioned. Therefore, inorganic mulches are not recommended for use in seasonal color beds.

Mulch material selection is largely a matter of personal choice, aesthetics, and product availability, but the physical properties can affect mulch performance. For example: pine bark nuggets are an effective and attractive mulch, but will float. If used on a slope or near a drainage area, pine bark nuggets will wash away. Not only does this expose the soil beneath and allow weeds to grow, but also creates a mess your landscape maintenance crew will have to clean up after every heavy rain. Fibrous mulches that will knit-together are more stable on slopes.

Geotextiles are synthetic fabrics that allow water and air to pass, but prevent weed seedling emergence. Never use sheets of plastic as a mulch. Although geotextiles are relatively expensive and time-consuming to install, they can be cost-effective if the planting is to remain in place for 4 or more years. Geotextiles are not suggested where the area is to be replanted periodically, as in annual flower beds, where the fabric would inhibit rooting and spread of ground covers, or in heavy clay soils where the fabric may contribute to saturated soils and root rot. Geotextiles must be covered by a mulch to prevent photodegradation. Use a shallow layer of mulch, as roots of weeds germinating on top can penetrate the fabric. Many perennial weeds can grow through geotextile mulches. If weeds do grow into or through the geotextile, remove them when they are small to prevent holes in the fabric.

It is important to note that mulches control weeds from seed. Mulches do not control perennial weeds emerging from vegetative reproductive organs such as tubers, rhizomes, stolons, bulbs or perennial crowns. Furthermore, some weed seeds do not require light to germinate and will emerge in mulches. Thus, when used alone, mulches rarely provide 100% weed control. Supplemental hand weeding or spot spraying escaped weeds are generally necessary. In many situations, the amount of supplemental weeding required can be burdensome and expensive; therefore, many landscapers will choose to use preemergence herbicides in addition to the mulch..

Herbicides are relatively inexpensive and effective, and, when properly chosen and applied, can be used in nearly any type of planting. If you decide to use a herbicide, consider the following selection criteria:

1. herbicide safety on the ornamental species;
2. efficacy on the weeds present (no herbicide will control all weeds);
3. proximity of susceptible species, and the likelihood of exposure;
4. potential residual effects on subsequent plantings (such as an annual flower bed);
5. type of application equipment (granular or spray); and
6. cost.

There are two broad categories of herbicides to choose from: preemergence and postemergence.

Preemergence herbicides

Preemergence herbicides are applied after planting but before weeds germinate, and provide a residual barrier in the surface of the soil that prevents weed seedlings from establishing. See Table 2 for a list of herbicides commonly used for preemergence weed control in landscape plantings. Most labeled herbicides control annual grasses and small-seeded broadleaf weeds but few control nutsedges (Table 3). S-metolachlor and dimethenamid-p are effective on annual grasses and suppress yellow nutsedge. Herbicides also differ in their efficacy on larger-seeded broadleaf weeds. A good “rule of thumb” to remember is that if a herbicide is effective on a wide range of broadleaf weeds, then that product has a higher potential to injury tender ornamentals compared to a product that controls grasses but few broadleaf weeds.

A typical schedule for preemergence herbicide applications in landscape plantings is late winter, early summer and again in late summer. Preemergence herbicides applied in late winter will control spring-germinating weeds like crabgrass and spurge. Most preemergence herbicides do not provide season-long weed control, thus a second application in early summer is usually needed to maintain control of summer-germinating weeds. An application in late summer or early autumn will control fall-germinating winter annual weeds like annual bluegrass, henbit or chickweed.

A few tips for using preemergence herbicides:

• Apply before top-dressing mulch
• In new plantings, the soil should be settled before herbicides are applied
• Apply granular herbicides when the foliage is dry (to prevent foliar injury).
• Apply uniformly and accurately (per area basis, not ounces per gallon)
• Wear your PPE
• Irrigate after herbicide application.

Postemergence herbicides

No preemergence weed management plan is perfect and some weeds will emerge. Emerged weeds may often be controlled with selective or non-selective postemergence herbicides. Glyphosate, diquat, glufosinate, and pelargonic acid are non-selective, injuring any vegetation contacted. Nonselective herbicides are widely used for general weed control in landscape plantings but contact with desirable plants must be avoided. In contrast, selective herbicides control some weeds but not others. For example: clethodim, fluazifop-p and sethoxydim control grasses but not broadleaf plants. In contrast, clopyralid controls many broadleaf weeds but not grasses. Some selective herbicides may be applied over the foliage of landscape plants whereas others cannot. A thorough understanding of herbicide safety to ornamental plants is necessary before using selective postemergence herbicides over landscape plants.

Postemergence herbicdes are also categorized as ‘contact’ or ‘systemic’, based on whether or not they move (translocate) within plants. Glyphsate and clopyralid are systemic, translocating from treated foliage to the roots, thereby killing the entire plant. Thus, systemic herbicides are generally effective on annual and perennial weeds. In contrast, diquat and pelargonic acid are contact-type herbicides, controlling small annual weeds but only "burning-back" perennial or larger annual weeds. Before using these products, carefully check the lists of weeds controlled and the ornamentals species over which the herbicide may be safely used. For example: Casoron will control many perennial weeds but will injure all herbaceous ornamentals and many woody ornamentals. Flumioxazin can be used over many conifers but will injure many other ornamentals.

For more detailed and extensive information on herbicides registered for landscape uses, see your local Cooperative Extension office, the NC State Weed Management portal, and the suggested references.

When difficult-to-control weeds are present in landscape beds, it is sometimes possible to modify landscape plant selection to facilitate a more effective weed management plan. The following two examples demonstrate how this can result in improved weed management.

Example 1. In a potential planting bed you identified yellow nutsedge as a major weed. Since yellow nutsedge is not controlled by mulches, geotextiles, or most preemergence herbicides, and postemergence herbicides applied in late summer or fall would have little or no effect on tuber emergence in the spring, your options are limited. Preemergence applications of Pennant Magnum, can suppress nutsedge, but this herbicide will injure many annual flowers and ornamental grasses. Pennant Magnum should not be applied to begonia or impatiens, but it is safe on many woody species and herbaceous perennials. Therefore, an important species selection criteria would be to choose from those listed on the Pennant Magnum label. If you must plant begonias, then you will have to hand weed nutsedge plants weekly throughout spring and summer.

Example 2. Your planting site has Florida betony or bamboo. These weeds are very difficult to control and require a multi-year effort. Diclobenil provides nearly complete control of Florida betony but damages many landscape plants. Diclobenil can be used around deciduous trees, junipers, arborvitae, and several woody shrub species. Planting trees and shrubs that tolerate diclobenil treatment will facilitate control of Florida betony.

Once the species selection and weed management options have been chosen, you are ready to prepare the site for planting.

The best time to control perennial weeds is before planting. There are basically two options: repeated cultivation or glyphosate. Table 5 provides guidelines for controlling several perennial weeds which are difficult to control after planting. Note that spring applications of glyphosate are less effective on several species compared to fall applications. Therefore, where possible, plan ahead, and do your site preparation in the fall. Note also that even glyphosate does not control all weeds. For species like nutsedge, field horsetail (Equisetum), and wild violet, other measures may be necessary.

If the site is to be amended with top soil or organic matter, inspect the sources of these materials for weeds. Top soil from farm land or stream banks are notorious sources of nutsedge tubers and weed seed. Inspect piles of compost or mulch for signs of weeds. In particular, improperly composted municipal yard waste is often full of weeds. Although, properly composted organic materials are typically weed free. Some species frequently found in mulch piles include mugwort, thistle, and bindweed. If these weeds are present, find an alternate source!

Once you have gone to so much trouble to prepare a weed-free planting area, don't introduce weeds. Many perennial weeds are introduced in soil balls of field-grown nursery stock. Look for signs of mugwort, bindweed, field horsetail, of nutsedge. Again, inspect your source of mulch for weeds. If herbicides are to be used, apply them carefully. In short, implement the plan which you have developed.

No single weed management strategy will control all weeds. An integrated approach, utilizing all options at your disposal is the most economical and effective means of controlling weeds. To achieve this, remember to PLAN BEFORE YOU PLANT.

NC State Extension Weed Management portal

Weeds of the Northeast by Uva, Neal and DiTomaso. Cornell University Press. https://www.cornellpress.cornell.edu/book/9780801483349/weeds-of-the-northeast/#bookTabs=1

Weeds of Southern Turfgrass by Murphy et al. https://extension.uga.edu/publications/detail.html?number=SB31&title=Weeds%20of%20Southern%20Turfgrasses

2017 Southeastern US Pest Control Guide for Nursery Crops and Landscape Plantings (205 pages). by Chong, J.C., J.C Neal, J. Williams-Woodward. (eds.). https://content.ces.ncsu.edu/southeastern-us-pest-control-guide-for-nursery-crops-and-landscape-plantings

Weed management in shrub production, by Neal, J. C. and J. F. Derr. In: White, S. ed. IPM for Shrubs in Southeastern US Nursery Production: Vol I. Chapter 6. http://wiki.bugwood.org/IPM_Shrub_Book

Weed management in shrub production. by Neal, J.C., J.F. Derr and S.C. Marble. In: Chappel, M. ed. IPM for Shrubs in Southeastern US Nursery Production. Vol II. (https://wiki.bugwood.org/IPM_Shrub_Book_II)