Prepared by:
W.A. Skroch, S.L. Warren, and G.E. Mahnken
Published by: North Carolina Cooperative Extension Service
Publication Number: AG-508-7
Last Electronic Revision: March 1996 (JWM)
Weed management in landscape areas is necessary to preserve aesthetics. Weeds can mar the color patterns of an annual or perennial flower bed. Weeds also compete with desirable vegetationfor water, nutrients, and space, which may prevent landscape plantsfrom achieving maximum growth and health. Effective weed management requires a combination of cultural practices and chemical control. This publication addresses both methods and discusses the best management practices to minimize the effects of herbicides on water quality.
The particular plants used in an area may influence the weed population and the methods of weed control used at the site. When designing a site, determine which plants are best suited to its conditions, such as the amount of sunlight the area receives, amount of irrigation or rainfall available, site drainage, and amount of traffic through the site. Plants that do not grow well in a site are more likely to succumb to damage from disease, insects, or environmental stress such as drought. If a plant dies, weeds will exploit the unoccupied space. If other plants in the area are not growing well, weeds could spread throughout the planting, because weeds may be better competitors for nutrients and moisture.
Use of chemical weed control should also be considered during plant selection. This is especially true when designing areas that include perennial species that require several years to reach full size or when designing areas that will have open ground after plants have reached maturity. If herbicide use is anticipated, consider the compatibility between plant species and herbicides likely to be used for weed con- trol. Species that do not tolerate certain herbicides can be grouped so that some of the area could be treated without damaging sensitive plants.
Avoid using plants that may be contaminated with weeds. Even though the aboveground portion of the weed may have been removed from a container-grown plant, reproductive structures such as roots, tubers, rhizomes, or seeds could still be present in the container substrate. Yellow nutsedge and dogfennel are commonly transferred to a new site in this manner. Some annual weeds such as prostrate spurge, common groundsel, and oxalis produce large numbers of seed that contaminate not only the container in which the plant is growing but also nearby containers. Plants should be obtained from nurseries that practice good weed management.
In order to minimize the amount of post-planting weed control in landscaped areas, it is important to kill all unwanted vegetation before planting. A site that is weed free initially will require less weed control during the first year and thereafter.
Initial site preparation. Existing vegetation can be killed with a broad-spec- trum postemergence herbicide such as glufosinate (Finale). Glufosinate kills only the vegetation it contacts, so uniform coverage is necessary to kill weeds. However, glufosinate may not be effective on perennial weeds that reproduce from underground rhizomes, stolons, or tubers such as bermudagrass and yellow nutsedge.
Glyphosate (such as Roundup and others) 1S another broad-spectrum herbicide used for site preparation. Glyphosate is a systemic herbicide which kills weeds by interfering with a biochemical pathway in plants. Glyphosate requires more time for plant kill but usually provides more effective control of perennial species. If an area is being renovated and desirable plants are already present in the landscape area, herbicides should be used with care to prevent contact with desirable vegetation. This is especially true with glyphosate, which can be quite damaging to many woody species if applied improperly. See North Carolina Cooperative Extension Service publication AG-427, Weed Control Suggestions for Christmas Trees, Woody Ornamentals, and Flowers for more details on the use of glyphosate near woody species. Contact your Extension Service Agent for publication cost and ordering information.
Depending upon the weed species present, it may be possible to prepare a site by mowing and then tilling to eliminate unwanted vegetation. The site should then be left undisturbed for 7 to 14 days to allow weeds to germinate. Weed seedlings should then be removed through shallow cultivation, disturbing the soil as little as possible. Too much soil disturbance brings new weed seeds to the surface where they germinate and continue the cycle of weed production. This type of preparation works best in an area with no perennial weeds. Perennial weeds are difficult to kill through cultivation alone. Cultivation often makes a perennial weed problem worse by breaking up the reproductive structures of the plant and spreading them throughout the cultivated area.
Mulches. Once existing vegetation has been removed, steps should be taken to prevent weed seed germination and growth of new weeds. Mulches will prevent weed germination and suppress weed growth. Materials such as pine or hardwood bark, leaves or needles, grass clippings, sawdust, woven polypropylene fabrics, or gravel can be used as mulch. Tree leaves, sawdust, and grass clippings, however, should be composted before use. Woven synthetic fabrics will need to be covered with a thin layer of mulch to improve the appearance of the fabric-covered area. Woven fabrics are not always effective against perennials such as yellow nutsedge or bermudagrass that can penetrate landscape fabrics.
If mulch is used without woven synthetic fabrics, it should be spread to a depth of 2 to 4 inches which will prevent most weed seed from germinating. Mulch should not be deeper than 4 inches as this can interfere with plant growth. In addition, deep, fine-textured mulches can support the germination and growth of weeds that may be transported into the area from outside sources.
As with plant material, mulches made from organic materials should be free of weeds and weed seeds. Organic mulches vary in their durability, with pine bark more durable than cedar chips and longleaf pine needles. These three materials are more durable than hardwood bark, which in turn is more durable than shortleaf pine needles.
In addition to weed control, mulches will reduce soil erosion, conserve moisture, and moderate soil temperature. Reducing the amount of sediment eroded from a site decreases the amount of sediment that reaches lakes and streams, thereby protecting water quality.
Preemergence weed control using herbicides. Preemergence herbicides control only those weeds that germinate after herbicide application. Thus, for best results, preemergence herbicides must be applied before weed seeds germinate. To maximize effectiveness and minimize losses to volatization and photodegradation, preemergence herbicides must be watered, either through irrigation or rainfall, to wet the upper 1 to 2 inches of soil. Table 1 lists the maximum number of days before water needs to be applied for each pre-emergence herbicide. Preemergence herbicides can be used with mulches. The sediment control of mulch also reduces loss from herbicides that are bound to soil particles.
Chemical name Trade name Activation Benefin Balan 1 Bensulide Betasan 1 DCPA Dacthal 4 Dichlobenil Casoron 1 EPTC Eptam 1 Isoxaben Gallery 21 Metolachlor Pennant 14 Napropamide Devrinol 3 Oryzalin Surflan 21 Oxadiazon Ronstar 1 Oxyfluorfen Goal 25 Pendimethalin Pendulum, others 30 Prodiamine Barricade 14 Pronamide Kerb 1 Simazine Princep 10 Trifluralin Treflan 2
Once a site has been established, weed control must be maintained. Weeds can move into a landscape area from surrounding areas. Preemergence herbicide applications lose their effectiveness several weeks to a few months after application, and no preemergence herbicide will control all weeds. Mulches also can control many weeds, but weeds may be able to establish within the landscape if the mulch is allowed to deteriorate and is not replaced.
Cultural practices. Weeds that escape preemergence methods of weed control may be controlled by hand pulling or hoeing. When cultivating, the soil surface should be disturbed as little as possible so that seeds in the soil profile will not be brought to the surface where conditions are favorable for germination. Cutting weeds at the soil surface may be better than pulling weeds, because removing roots may bring weed seeds to the soil surface. To interrupt the reproductive cycle, weeds should be removed before seeds are produced.
Most weeds are easiest to control when they are small, generally 2 to 4 inches tall. Tools used for cultivation should be cleaned before moving to another area to prevent spread of weed seeds or other reproductive parts. To control weeds through cultivation, an area may need to be scouted every 10 to 14 days in spring and fall when summer or winter annual weeds are germinating. Scouting may not be needed as frequently during hot, dry periods of the summer.
Herbicides. You may prefer to use chemical weed control in landscape areas where frequent scouting and mechanical control would be expensive and time consuming. Undesirable effects, such as water quality degradation or overapplication, from herbicide use can be minimized if the correct herbicide for a situation is selected and properly applied. Before an appropriate herbicide can be selected, the weeds in the landscape must be correctly identified so that the applicator can determine which herbicide will be most effective in controlling weeds without damaging desirable plants. Weed Control Suggestionsfor Christmas Trees, Woody Ornamentals, and Flowers (AG-427), referenced earller, provides information on the selection of herbicides for specific situations.
Postemergence herbicides can be used to eliminate weeds that escape cultural or chemical control. Some postemergence herbicides, such as Finale and Roundup, are nonselective and kill any vegetation they contact, whereas others are selective and kill only certain types of vegetation. The use of postemergence herbicides may decrease off-target effects because these herbicides can be spot-sprayed only where needed. Because only infested areas are sprayed, the amount of herbicide applied will be considerably less than would be applied with a broadcast application. Postemergence herbicides are usually directed to the foliage of weeds, decreasing the amount of herbicide applied to the soil, which, in turn, could lessen the amount of herbicide lost in leaching or runoff. To ensure adequate weed control using postemergence herbicides, weeds should be sprayed at the proper growth stage, usually when the weeds are about 2 to 4 inches tall. Some established perennials are best controlled when herbicides are applied at flowering. Before using any herbicide, carefully read the product label.
When more than one herbicide could be used to control problem weeds in a particular landscape, the potential for the herbicide to be lost in surface runoff or to leach through the soil profile should be factored into your decision. Herbicides that leach through the soil can contaminate groundwater, and chemicals that move with surface runoff water or eroded sediment can contaminate lakes or streams. The tendency for herbicides to either leach through the soil profile or to be lost in surface runoff is influenced by properties of both the herbicides and the soil.
Herbicide properties that influence leaching and run-off include the following:
A herbicide that binds tightly to soil organic matter is not likely to leach through the soil profile unless the soil contains little organic matter. A herbicide that binds tightly to organic matter would normally be lost only by sediment erosion. A highly water-soluble herbicide that does not bind tightly to soil organic matter or clay minerals has a greater tendency to leach through the soil profile. A persistent herbicide also has greater potential to be lost through leaching or surface runoff, because it is present at the application site for an extended period of time. Higher herbicide application rates can also increase losses through leaching and runoff. Herbicides may also be lost from the application site through photodegradation (breakdown by sunlight) or volatilization.
Soil properties that affect leaching and runoff include the followiing:
Soil organic matter and soil clay minerals can bind various herbicides and decrease the amount of herbicide leached. Soil pH influences the amount of binding of some herbicides with soil organic matter and soil clay minerals. A porous soil, such as one with a sandy texture, has a high rate of infiltration and drainage, increasing the probability that a herbicide will be lost through leachiing rather than runoff. A fine-textured clay soil limits water infiltration and herbicide leaching, but herbicide losses in runoff can be high if sediment erodes from the site.
As discussed, applying mulch to the soil surface helps to minimize erosion and runoff. Another management practice to protect water quality is to plant vegetative buffer strips around water bodies or drainage areas. These buffer strips interrupt water flow, allowing sediment to settle out of the water stream and more water to infiltrate the soil profile.
Table 2 lists the herbicide leaching and runoff potential for several commonly used landscape herbicides. The loss potential for each herbicide is based on the herbicide's soil binding capacity, persistence of the herbicide, rate applied, and amount of herbicide reaching the soil surface. When more than one herbicide option exists for weed control, the loss potential should be considered. If the application site has a very permeable soil with low organic matter content and shallow groundwater, the best choice might be a herbicide that has a low leaching potential. In a situation where surface runoff and erosion are problems but the groundwater is deep, the best choice might be a chemical with a moderate leaching potential.
Chemical Trade Application Leachate Runoff name name method potential* potennal* Benefin Balan Soil VL H Bensulide Betasan Soil M H Bentazon Basagran T/O Foliage M L DCPA Dacthal Soil L H Dichlobenil Casoron, others Soil M M EPTC Eptam Soil L L Fenoxaprop Acclaim Foliage VL M Fluazifop Fusilade Foliage VL M Glyphosate Roundup, others Foliage VL H Isoxaben Gallery Soil L L Metolachlor Pennant Soil M M Napropamide Devrinol Soil L M Oryzalin Surflan Soil L L Oxadiazon Ronstar Soil L M Oxyfluorfen Goal Soil VL M Pendimethalin Pendulum, others Soil VL H Prodiamine Factor Soil VL H Pronamide Kerb Soil VL M Simazine Princep Soil H M Tnfluralin Treflan Soil L L
*Leachate potential was taken from the 1995 North Carolina Agricultural Chemicals Manual. Runoff potential is based on information available in the research literature. VL = very low, L = low, M = Moderate, H = high.
The following guidelines are not meant to provide complete information on the various aspects of herbicide application. For more information, see the current copy of the North Carolina Agricultural Chemicals Manual or contact your county Cooperative Extension Center. Also, carefully read herbicide labels before use, and carefullyfollow label directions and guidelines.
After you have selected a herbicide, it is important to apply the correct amount properly. Liquid sprayers and granular spreaders must be calibrated in order to deliver the correct amount of herbicide to a given area. Improper calibration can result in overapplication of the herbicide, which may injure susceptible plant species and promote excessive losses of herbicide in leachate or runoff water. For information on the proper calibration of sprayers and spreaders, contact your county Cooperative Extension Center.
Careful mixing of spray solutions helps protect groundwater and surface water. Before mixing herbicides, the size of the area to be sprayed should be determined so that the applicator can prepare the correct amount of spray solution. The goal is to minimize the residual spray solution. Spray solutions should be prepared as far from a wellhead as possible to prevent any spilled spray solution from entering the well. To prevent material from back-siphoning into the welL the hose used to fill the spray tank should never be submerged into the spray solution. In addition, empty herbicide containers must be disposed of properly. Liquid herbicide containers should be triple rinsed and the rinsates added to the spray solution. Herbicide labels provide instructions on proper disposal of empty containers.
After spraying is completed, the sprayer must be rinsed to prevent contamination of subsequent spray solutions, and the rinsate should be sprayed on an area, such as the application site, where the herbicide can be safely applied.
Weather conditions should be considered before deciding to spray. Postemergence herbicides should not be applied just before rain or irrigation, because the herbicides could be more susceptible to loss in surface runoff or through leaching. Also, some postemergence herbicides require a certain drying time before irrigation or rainfall for maximum effectiveness (Table 3). Conversely, most preemergence herbicides require 1/2 inch or greater of precipitation to bring them into contact with the soil and increase their effectiveness. If preemergence herbicides are watered in with irrigation, water should not be allowed to run off the site.
To prevent sprays or granules from drifting into nontarget areas, herbicides should be applied when winds are relatively calm. Spray nozzles that produce very fine droplets should not be used, because small droplets are much more likely to drift than coarse droplets. Similarly, herbicide granules that are fine and lightweight are more likely to drift than larger, heavier granules. Somewhat volatile spray solutions, such as 2,SD esters, should not be sprayed when the temperature added to the percent humidity, a measurement known as the comfort index, is 150 or more. These conditions cause the herbicide to evaporate, and the herbicide vapor can be moved by wind into off-target areas.
Chemical Trade Drying name name Time Bentazon Basagran 8 hours Clethodim Prism 1 hour Clopyralid Stinger 8 hours Diquat Reward 0.5 hour Fenoxaprop Acclaim 1 hour Fluazifop Fusilade 1 hour Glufosinate Finale 4 hours Glyphosate Roundup, others 6 hours Imazaquin Image 6 hours Oxyfluorfen Goal 0 hour Paraquat Gramoxone 0.5 hour Sethoxydin Vantage 1 hour DeMoss 2 hours Sharpshooter 2 hours
Several practices can be used to control weeds in landscape areas while protecting water quality. When designing the site, plants should be selected that will grow well at the site. All unwanted vegetation should be removed before the planting is installed to decrease subsequent weed pressure. After installation, the soil surface should be disturbed as little as possible to prevent buried weed seeds from being brought to the surface where they can germinate.
Potential for herbicide movement in surface runoff water or in leachate should be considered when selecting a herbicide. When applying the herbicide, application equipment should be calibrated to ensure that the correct amount of herbicide is being applied. Before using any herbicide, carefully read the label and follow all directions.
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
AG-508-7