Weed Control In Landscape Beds

Features - Lawn Care

Controlling weeds in sensitive landscape beds is easier when the bed is well prepared and the planting is well planned.

December 1, 1999

Weed management in landscape plantings is a complex task. The diverse nature of the ornamental plant material and weeds present and differences in the size and slope of the beds create quite a challenge. Herbicide options are limited by the mixture of woody and herbaceous ornamentals, as well as health and environmental concerns. Thus, the integration of multiple strategies is necessary to keep weeds from detracting from the beauty and quality of a landscape.

DESIGN FOR CONTROL. The ideal landscape according to a weed scientist might not win any awards for diversity, but grouping similar plant material together allows for more weed control options in the beds. Herbaceous plants, especially annual flowers, are more sensitive to herbicides than ornamental shrubs or trees. In such flower beds, mulching and hand pulling of weeds are often the only weed control options. More herbicides are registered for use around woody ornamentals, which also help shade out weeds that might otherwise germinate.

Certainly, a rich variety of plant types and colors in a landscape is desirable and attractive, but designers should at least consider the require-ments for maintaining the landscape over time.

ON THE SITE. Before establishing a planting, evaluate the site for soil type and slope and identify existing weeds so suitable plant material, mulching and herbicide use can be selected. Control existing weeds with a nonselective herbicide before the beds are established.

Avoid introducing weeds or their propagules (seeds, rhizomes) into the landscape, and eliminate weeds that emerge before they begin to form seed. Root balls of field-grown nursery stock may contain tubers or rhizome fragments of perennial weeds, which are then transplanted into the landscape with the shrubs.

COMPARING MULCHES. Mulches can be classified as organic (bark, wood chips, composted leaves, pine needles), inorganic (crushed rock, gravel) or synthetic (black plastic, landscape fabric). Mulches limit light and physically block seedling growth.

Coarse-textured organic mulches can be applied up to 4 inches deep and provide long term weed control. Fine-textured mulches pack more tightly and should be limited to a depth of 2 inches. They degrade more quickly and consequently provide weed suppression for a shorter period of time.

The optimum mulch is relatively coarse-textured with a low water-holding capacity. Because mulches rarely provide complete weed control, preemergence herbicides can be applied to improve the level of control.

Perennial weeds such as bindweed and mugwort often have sufficient root reserves to penetrate even thick mulch layers. Even annual weeds can grow through mulches or germinate on top of a mulch as it decomposes. Weeds with wind-borne seeds such as horseweed, common groundsel and dandelion are most likely to establish in the mulch.

Natural inorganic mulches like gravel or stone are generally more expensive than organic mulches. However, they are stable over time, allow good water drainage and air flow and can make very attractive mulches.

WEED BARRIERS. Black plastic mulch has been used for years and provides excellent control of annual weeds and suppression of perennials. However, nonporous black plastic restricts water penetration and air exchange; thus, it is not recommended for long-term use in landscape plantings.

Porous, black landscape fabrics (geotex-tiles) have been developed to replace black plastic in landscapes. Landscape fabrics form a barrier and block sunlight from reaching weed seeds, but allow water and gas exchange necessary for plant health. Although relatively expensive and labor intensive, landscape fabrics are cost-effective if the planting is to remain in place for several years.

Landscape fabrics are most useful for long-term weed control around trees and shrubs, but not for annual flower beds that are replanted periodically or where a fabric could inhibit rooting and spread of ground covers. Landscape fabrics can eventually be damaged by tree and shrub roots, and pulling up a fabric may be difficult due to root growth within the material.

When installing a fabric, first remove existing weeds and stones. Cut the fabric to fit snugly around tree trunks and shrubs. For unplanted beds, cut an “X” in the fabric for each planting hole. Avoid leaving soil from the planting hole on top of the fabric since this will serve as a source of weed seeds.

After planting, fold the fabric back down to keep the sheet as continuous as possible and secure to the soil with U-shaped pegs. Apply a thin layer of organic or rock mulch on top of the fabric to prevent its photo-degradation. Remove any weeds that grow into or through the fabric when they are small to prevent holes from forming in the fabric.

Alternative Methods for Applying Herbicides

    Landscape fabrics impregnated with trifluralin are now available (i.e. Biobarrier II). Once in place, the fabric should be covered with 1 to 2 inches of organic mulch. This approach integrates the use of nonchemical and chemical weed control methods. In addition, the trifluralin is applied without the need for calibrating, mixing, spraying, or cleaning up.

    Customized hand pruners (i.e. KlipKleen) that apply a postemergence herbicide to the stem as it is cut are useful for controlling woody brush and vines such as poison ivy. Herbicides that are effective with these pruners are one-fourth to full strength solutions of glyphosate, 2,4-D, or triclopyr. Even though concentrated herbicide is needed, the overall amount of herbicide used will be reduced and its placement will be more precise because it is applied directly to the cut stem instead of being sprayed over the entire plant.

THE HERBICIDE OPTION. Landscape managers often use herbicides to improve the efficiency of weed control. Some factors to consider before selecting an herbicide are:

  1. What weeds are present and what weeds are expected to emerge? Choose an herbicide or combination of herbicides that will be effective on these weeds.

  2. What ornamental species are present in the planting? Be sure the herbicide is registered for use on these species.

  3. How close are susceptible ornamentals and turf, and what is the risk that they will be injured by the herbicide?

  4. What is the potential for residual effects of the herbicide on subsequent plantings, especially those containing annual flowers?

  5. What precautions need to be taken to protect the applicator and property owners?

  6. What method will be used to apply the herbicide (granular or spray formulation)?

  7. How much will the treatment cost?

In the Northeast, herbicides are typically applied in the early spring to prevent summer annual weeds, and in the late summer to prevent winter annual weeds.

The dinitroanilines, a group of preemergence herbicides including Treflan™ (trifluralin), Surflan™ (oryzalin), Pendulum™ (pendimethalin) Barricade™ (prodiamine) and Team™ Pro (trifluralin + benefin) inhibit root development in germinating seedlings. They are most active on annual grasses, but also prevent the emergence of some broadleaf weeds. Other preemergence herbicides for landscapes are Devrinol™ (napro-pamide), Pennant™ (metolachlor), Dacthal™ (DCPA), Ronstar™ (oxadiazon), Gallery™ (isoxaben) and Goal™ (oxyfluorfen).

Devrinol, Pennant and Dacthal prevent the emergence of annual grasses and some broadleaf weeds. Pennant, in addition, has preemergence activity on yellow nutsedge.

Gallery controls a wide spectrum of broad-leaf weeds, while Ronstar and Goal have greater preemergence activity on broadleaf weeds than grasses.

Herbicide combinations increase the weed control spectrum. Applicators may mix two of the “grass” and “broadleaf” herbicides listed above or use one of the following granular herbicide combinations: Rout™ (oxyfluorfen + oryzalin), Ornamental Herbicide 2™ (oxyfluorfen + pendimethalin), Snapshot™ TG (isoxaben + trifluralin), Team or XL™ (oryzalin + benefin). Product labels must be checked carefully for lists of registered ornamental species.

MULCH + HERBICIDES. The characteristics of the organic mulch can dramatically affect herbicide performance. A mulch composed primarily of fine particles can absorb herbicides, making the chemical less active, whereas a mulch composed of coarser particles is less likely to affect herbicide efficacy.

The depth of the mulch layer is also a factor. For an herbicide to work when applied on top of the mulch, it has to leach through to the zone in which weed seeds are germinating. Many of the herbicides used in landscapes have low solubility in water. Thus, if the mulch layer is too thick, the herbicide may not move through the mulch.

Most herbicides work best when applied underneath the mulch layer. Such placement is possible only if the herbicide is applied before the mulch is deposited or if additional mulch is spread after herbicide application. Another reason to apply herbicides under mulch is to reduce volatilization losses.

If tough-to-control weeds such as mugwort, thistles or field horsetail are problems in a planting, the granular herbicide Casoron™ (dichlobenil) may be an appropriate choice. Casoron can be used around established woody ornamentals such as yews, arborvitae and juniper, but not around firs, spruces, or hemlocks. Because of its volatility Casoron should be applied during cool weather and either covered with mulch or watered in soon after application. Of the herbicides registered for landscapes, Casoron provides the best preemergence control of biennial and perennial weeds.

POSTEMERGENCE APPLICATIONS. Postemer-gence herbicides generally provide minimal residual weed control — they are either inactivated by binding to soil particles or rapidly degraded by soil microorganisms. Roundup Pro™ (glyphosate), Finale™ (glufosinate), Reward™ (diquat) and Scythe™ (pelargonic acid) are nonselective herbicides which must only be applied as directed or spot treatments in which ornamentals and turf are not contacted. Reward and Scythe rapidly kill vegetation contacted by their spray, but perennial weeds can regrow. Finale is primarily a contact herbicide, but limited translocation occurs in the plant. Roundup Pro, a slower acting herbicide, is readily translocated to roots and growing points of plants, whereby it kills both annual and perennial weeds.

Several postemergence grass herbicides are registered for use in landscapes. These products include Vantage™ (sethoxydim), Fusilade™/Ornamec™ (fluazifop), Envoy™ (clethodim) and Acclaim™ (fenoxaprop). The postemergence grass herbicides can be safely sprayed over the top of most woody and herbaceous plants (except ornamental grasses), but check the label carefully for any precautions about susceptible species.

The author is assistant agricultural scientist at the Connecticut Agricultural Experiment Station, Windsor.

Gallery, Surflan, Treflan, Snapshot, Team and XL are registered trademarks of DowElanco, Indianapolis. Deverinol, Fusilade and Reward are registered trademarks of Zeneca Professional Products, Wilmington, Del. Barricade and Pennant are registered trademarks of Novartis, Greensboro, N.C. Finale and Acclaim are registered trademarks of AgrEvo, Wilmington, Del. Vantage is a registered trademark of BASF, Research Triangle Park, N.C. Envoy is a registered trademark of Valent, Walnut Creek, Calif. Pendulum and Image are registered trademarks of American Cyanamid, Wayne, N.J. Scythe is a registered trademark of Mycogen, San Diego, Calif. Goal is a registered trademark of Rohm & Haas, Philadelphia, Pa. Ronstar is a registered trademark of FMC Corp., Hoopeston, Ill. Roundup Pro is a registered trademark of Monsanto, St. Louis, Mo. Dacthal is a registered trademark of ISK Biosciences, Marietta, Ga. Rout and Ornamental Herbicide 2 are registered trademarks of The Scotts Co., Marysville, Ohio. Casoron is a registered trademark of Uniroyal Chemical, Middlebury, Conn.

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