Manual removal of weeds is time consuming, expensive, and often results in damage to landscape plants when intertwined roots of both the weed and the ornamental plant are pulled up. Consequently, postemergence herbicides are often used to remove weeds. Few selective postemergence herbicides are available for use in landscape plantings or nursery stock production. Consequently, nonselective herbicides (which must be selectively applied to avoid injury to desirable plants) are typically used for postemergence annual and perennial weed control. Choosing the right herbicide for the situation is an important decision. Nonselective herbicides (as the term indicates) are not selective about which plants they kill. Any green plant that they contact will be injured or killed. The level of weed control (or ornamental plant injury) resulting from these herbicides depends upon the chemical characteristics, mode of action of the herbicide, and the season of application.

Postemergence, nonselective herbicides are classified as either contact or systemic. Contact herbicides kill only the green tissues that are contacted by the spray. Systemic (sometimes also called translocated) herbicides move within the plant from the point of application to other plant parts. Some systemic herbicides are more mobile (move easily and farther) in plants than others. Also, in perennial weeds the amount of systemic movement of the herbicide often depends upon the season. Therefore, to obtain optimum control, herbicides must be applied at the time of year when weeds are most susceptible.

Several postemergence, nonselective herbicides are labeled and commonly used in landscapes and nurseries. Diquat, pelargonic acid, vinegar-based products and natural oils or acids are contact herbicides. Glufosinate and glyphosate are systemic. Glufosinate transport is limited compared to glyphosate that will move from the foliage to growing points in the roots and shoots. Some characteristics of each are compared in Table 1. None of these products have residual activity (i.e.: no root uptake and no preemergence weed control) in soils containing greater than 1% clay. However, in sandy soils, muck soils or soilless media, root uptake of systemic herbicides is possible. Each product has a place in landscape management, but choosing the right herbicide for the right situation requires information and an understanding of each herbicide's strengths and weaknesses. The following descriptions should provide some background information to facilitate your herbicide selection.

Diquat (several trade names) is a postemergence contact weed killer. It kills small annual weeds. Large annual weeds and perennials will be injured but not killed. In our research, spray volumes of 2 gal per 1000 square feet were more effective than lower volume treatments. Advantages of Reward include rapid kill of small seedling weeds and relatively low cost. Also, small amounts of spray drift will cause only cosmetic damage to landscape plants and will not translocate to kill desirable plants. Additionally, Reward is not temperature sensitive, working well in cool and warm weather. Disadvantages of this herbicide are lack of control of perennial or well-established weeds and the relatively high mammalian toxicity (compared with Scythe, Finale and Roundup-Pro).

Pelargonic acid (Scythe) is a postemergence, contact herbicide that controls small seedling weeds but only injures larger annual weeds and perennials. In cold weather, Scythe is not as effective as diquat. Scythe is perceived by many people to be an alternative to traditional herbicides. Customers who do not wish to have pesticides applied to their properties will sometimes accept the use of soaps (such as insecticidal soaps) and may accept the use of Scythe, often considered to be a "herbicidal soap." Ammonium nonanoate (Axxe) is essentially the same as pelargonic acid, but from natural sources and is OMRI certified for use in organic systems. As with Reward, spray drift on desirable plants will cause cosmetic damage but will not translocate to kill the entire plant. In all applications, avoid contact with desirable vegetation. The main disadvantages of pelargonic acid products are higher costs, somewhat less effective than diquat, and the odor is persistent and offensive to some people. Spray drift can be a severe eye irritant.

Glyphosate (many trade names and manufacturers) is a systemic, postemergence herbicide which kills annual and perennial weeds. Advantages of glyphosate are the systemic kill of annual and perennial weeds and low mammalian toxicity. The main disadvantage is that small amounts of spray drift can severely injure certain ornamentals. Table 2 provides guidelines for optimum doses and application timing for controlling several perennial weeds.

Glufosinate-ammonium (Finale XL or Cheetah Pro) is also a non-selective, postemergence herbicide. Many practitioners consider glufosinate to have 'contact action'; however this is some translocation. Absorption and transport within treated leaves occurs but longer distance transport to new growing points does not occur. Glufosinate produces symptoms more rapidly than glyphosate (often within 48 hours, see Table 1) but only provides temporary suppression of most perennial weeds (such as: bindweed, goldenrod, and mugwort). Reduced translocation of glufosinate may offer advantages over glyphosate in some landscape trim and edge applications. Generally, glufosinate is less effective than glyphosate on perennial weeds and grasses, but there are exceptions. Glufosinate was more effective than glyphosate on horsenettle, white clover, cutleaf evening primrose, and glufosinate controls many glyphosate-resistant weeds.

*LD₅₀: the dosage required to kill 50% of the test animals, expressed as the milligrams (thousandths of a gram) of herbicide active ingredient per kilogram (1000 grams) of body weight. The larger the LD₅₀, the safer it is.

**REI: Worker protection standard prescribed Arestricted-entry intervals@ for agricultural uses. These are applicable to nurseries but not for landscape uses. For this time interval following a herbicide application, agricultural workers are not to enter treated areas without wearing personal protective equipment (PPE) for activities that would bring them in contact with treated surfaces. Depending upon the herbicide, the PPE required may be as simple as shoes, socks, coveralls, and rubber gloves. Check the AGRICULTURAL USE REQUIREMENTS section of the label for required PPE.

Glyphosate is labeled for control of annual and perennial weeds in ornamental plantings. For maximum weed control, use no more than 25 gal of final spray solution per acre, apply when the foliage is not wet, allow at least 1¹⁄₂ hours without rain or irrigation after application and apply glyphosate at the time of year that weeds are most susceptible. A 1% solution (1.25 oz of product per gallon of water) will give excellent control of most weeds. For some species, increasing the percentage to 1.5% or 2.0% is advisable (see Table 2). For wick (or wiper) applications use one part glyphosate product and two parts water (33%, by volume). Glyphosate is currently available under many brand names which differ in many ways. The liquid products range from a 0.5% ready-to-use formulation to concentrates of up to 53.8% active glyphosate. There are also several formulations which contain additional herbicides such as diquat and pelargonic acid which provide some rapid symptoms, or residual herbicides such as imazapic or indaziflam. Check each label for specific instructions and precautions. Be especially wary of.... no, correct that, DO NOT USE, products containing imazapyr in landscape plantings. Severe injury to landscape plants is likely.

By taking advantage of periods of high and low susceptibility of ornamental plants and problem weeds, you can manage many difficult problems with glyphosate. In general, conifers and broadleaf evergreen shrubbery are most tolerant to glyphosate in late fall and early winter, whereas small quantities will cause severe damage in the spring. In contrast, deciduous plants are more tolerant in winter and spring, while much more severe damaged results from glyphosate contact in the late summer and early fall. As little as a half dozen leaves on a 2-inch branch sprayed with a 1 percent solution can cause death of the entire branch. Table 2 has been developed based upon research and label guidelines.

Amount of a 41% Active Formulation of Glyphosate to Obtain 90% or Better Weedy Species Control One Season Later Application Timing for Best Control

In general, the application times given above have been more effective than applications made at other times of year.