15 Weed Management

 

Contents

Weed Control in Lawns. 1

Control of Broadleaf Weeds in Established Lawns. 1

Chemical Herbicides. 1

Special Situations. 1

Nonselective Vegetation Control 2

Weed Control in Fruit Plantings. 2

Eliminate All Perennial Weeds Before Planting  2

Prevent Weed Seeds from Germinating  2

Remove established weeds when they appear 3

 

 

Check the herbicide label for the proper way and most effective time to apply herbicides. When used according to directions on the package label, herbicides should not be a hazard to people, pets, turfgrass, or desirable plants in the garden or landscape. To avoid injury to sensitive ornamentals it is best to use phenoxy herbicides such as 2,4-D or MCPP in the fall or early spring when temperatures are lower and herbicide volatility will be reduced, and on days with little wind action. 

Weed Control in Lawns

For a general discussion of lawn weed management without pesticides, see Part I, Cultural Methods or see “Weeds and Your Lawn” at www.nysipm.cornell.edu.

For established lawns the use of herbicides to control weeds is more practical for large areas, weeds that are difficult to pull such as perennials, and small patches of persistent weeds. Large areas can be treated with sprays or granular formulations, which provide reasonable weed control with minimal injury to turfgrass. Small infestations of weeds can be spot treated, and formulations are available for homeowners for these sites.

When weeds infest an area of any size and the populations are high enough throughout the area to be objectionable, an overall treatment with a selective herbicide that will kill the weeds and leave the grass unharmed may be necessary. Broadleaf weeds such as dandelion and chickweed require different management practices than do annual grasses such as crabgrass or goosegrass or perennial grasses such as quackgrass. See Table 20 for herbicide recommendations for grass weeds in lawns. Products for difficult to control broadleaf perennials are currently available as well. Where weed infestation is serious (greater than 40 percent) and turf condition is poor, consider a total renovation and replanting with a turfgrass mixture well suited for your particular site.

Control of Broadleaf Weeds in Established Lawns

The basic ingredient in many broadleaf herbicides is 2,4-D. Used alone, it controls a wide range of broadleaf weeds. Formulators often combine one or more chemicals with 2,4-D, such as MCPP or dicamba, to increase the number of weeds controlled. Such mixtures control many broadleaf weeds commonly found in lawns. Chemical control of broadleaf weeds is best accomplished in the fall or spring.

A specific exception is the weed Veronica, a small-leaved, low-growing plant with a purplish flower in the spring. If repeated applications of one of the mixtures of broadleaf weed killers fail to eliminate a weed that fits this description, it may be Veronica. Currently there are very limited options for effective control of Veronica. Ground ivy and wild violet are also perennial weeds that are exceptionally difficult to control in shaded lawns. Mixtures containing triclopyr aid in control of ground ivy, violet and veronica. Repeated herbicide applications may eventually provide full control of these weeds. Otherwise, selection of a more competitive turfgrass mixture for shadier sites may help to outcompete these shade –loving weeds. Fine-leaf fescues are competitive in both shady and droughty sites and, depending on the cultivars, can be exceptionnally weed suppressive once established.

Weed Control in Vegetable and Flower Gardens and Landscape Plantings

See Part I, Cultural Methods, for a discussion of the use of mulch.

Chemical Herbicides

Most herbicides labeled for use in flower and vegetable gardens and landscape plantings are effective against annual weeds; a few control certain perennial weeds. Always read the label and follow the instructions carefully. Failure to do so may result in injury or death of desirable plants.

Table 21 contains nonpesticidal as well as pesticide guidelines for weeds in vegetable gardens. In many instances use of herbicides will not be needed if alternative cultural methods are used.

Special Situations

Perennial weeds such as quackgrass and mugwort can virtually destroy ornamental ground covers (such as ivy or pachysandra) and cause great problems in flower and vegetable gardens as well as landscape and fruit plantings. The best course of action is to eliminate them before making a new planting. This usually requires extensive hand removal of weeds and soil-borne propagules or the use of an herbicide. Apply glyphosate (Roundup) in early fall. Spot treat escaped weeds the following spring. Quackgrass and other grasses may be controlled selectively around many ornamentals with fluazifop-p (Grass-B-Gon) or Vantage. Several other herbicides are as good or better but require a special permit from the New York State Department of Environmental Conservation. If you are contemplating new plantings it is best to find out beforehand whether you have a serious perennial weed problem.

Perennial weeds may also be controlled with glyphosate (Roundup or Kleenup) at higher rates. The season of application will affect the level of weed control achieved. Round Up is most effective at moderate temperature when weeds are not water stressed. Table 23 lists optimal application times for controlling several perennial weeds with glyphosate.

For controlling quackgrass and other weedy grasses without injuring most broadleaf trees, shrubs, groundcovers, or flowers, Fluazifop-p (Grass-B-Gon or Fusilade 2000), Ornamec, or sethoxydim (Poast, Vantage) may be used. Some exceptions exist, so read the label carefully.

Nonselective Vegetation Control

(In driveways, patios, hedgerows, and other noncrop areas where complete kill of all vegetation is desired)

Sometimes people wish to control weeds or nuisance vegetation for long periods of time. Frequently this can be accomplished with only one or two applications of an herbicide during the entire growing season. Areas lending themselves to such treatments are gravel driveways, stone patios, the bases of fences and fence posts, and outdoor storage areas where recurring vegetation is annoying. Because chemicals used in such situations usually kill all vegetation that is treated (thus the term non­selective) they should be used with discretion.

Special caution should be exercised when using nonselective, persistent herbicides on areas bordering desirable plantings. This is especially true if the areas to be treated are on higher ground (such as a slope) or drain into a water source (such as a lake, pond, or stream). A sudden heavy rainfall can wash the herbicide into other areas, resulting in serious damage. Round-up (glyphosate) is not persistent in soil but is generally effective in controlling most vegetation. Newer product mixtures which are effective and moderately persistent with limited potential to leach are now available for sidewalk weed control. The use of flame burners for vegetation management is also on the rise. Be careful to avoid flaming around dry brush that may cause runaway fires.

Note: Several commonly used lawn herbicides (such as 2,4-D) are volatile and can damage many fruits, vegetables, and landscape shrubs within several hundred yards from the site of application because of volatilization. Grapes, asparagus, tomatoes, roses, and other crops are especially sensitive to 2,4-D.

Weed Control in Fruit Plantings

One of the most important aspects of fruit culture is weed control. The growth of many fruit crops is negatively related to the amount of weed growth in the area surrounding the roots. In addition, weeds can harbor insect and disease pests or create an environment favorable for their development. Controlling weeds is the most important component of pest control. One should strive to maintain a weed-free area around the plant; the size of this area depends on the growth habit and age of the plant. For example, at least 18 inches should separate a strawberry plant from any other weeds or grassy areas; this distance should be 3–6 feet or more for large fruit trees. The successful fruit grower must employ several strategies for controlling weeds.

Eliminate All Perennial Weeds Before Planting

A key to successful fruit growing is eliminating perennial weeds the year before the plants are set. One could repeatedly cultivate an area for an extended period of time until weeds cannot regrow or plant a cover crop such as rye, buckwheat, sorghum, or sudan­grass to suppress the weeds. A heavy sheet of black plastic placed over the area to be planted will eliminate most weeds if left in place for one season. The most effective method is to apply a nonselective herbicide such as glyphosate to the planting area one year before planting. Nearly all plants will die within three weeks of application, then the area can be worked and a cover crop planted. Retreatment of difficult to control perennials may be necessary. The cover crop will temporarily prevent weed seeds from germinating and add organic matter to the soil when it is plowed under in the spring.

Prevent Weed Seeds from Germinating

Organic mulches not only prevent seed germination but gradually improve soil structure and moisture. Straw is excellent mulch for strawberries and raspberries, sawdust or shredded bark work well on blueberries, and wood chips or pine needles are best for tree fruits. Soil should be well drained if organic mulches are to be used. Wet soils can lead to insect and disease problems. Landscape fabrics can be used for blueberries and tree fruits but are expensive and difficult to install. A limited number of herbicides can also be used to prevent weeds from becoming established. Always follow label directions when using herbicides.

If mulches are not used, one should regularly cultivate or mow the weed-free area around the plants, but cultivation should be no deeper than 2 inches. The root systems of most fruit crops are very shallow, and deeper cultivation can cause considerable damage. Keep the surrounding lawn or field mowed to prevent weeds from flowering and releasing seeds into the weed-free area. The selection of a low-growing, less competitive turf or ground cover will suppress weeds and interfere less with established trees. Consider establishment of fine-leaf fescues to minimize weeds in the orchard.

Remove established weeds when they appear

Hand weeding is unavoidable for the fruit grower. Regularly pull weeds that become established in the area surrounding the fruit plants so they will not flower or develop an extensive root system. Regular cultivation also helps eliminate established weeds. Glyphosate (Roundup) may be wiped on established weeds surrounding tree fruits with a wick applicator if the trees are well established. It can also be used near grapes in spring until the end of bloom. Do not allow glyphosate to come into contact with any part of the tree or vine, especially when actively growing. The timing and rates of application are critical when using herbicides, so read the label carefully before using. Research on fruit crops has shown that weed competition during May and June is most detrimental to growth and yield, whereas weeds that appear later in the season have less impact.

See Part I, Cultural Methods, for nonpesticidal alternatives.

 

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Table 20. Control of grass weeds in lawns
Weed	Herbicide	Comments
Crabgrass and other annual grasses	Balan, Betasan, Tupersan, Pre-M, Halts, Team, and others	Apply in the spring before crabgrass germinates. These products can be formulated with fertilizer.
Crabgrass, nutsedge, and certain other grassy weeds, postemergent	Methanearsonate (MSMA or DSMA)	Apply to growing weeds in early to midsummer. These products are available only in liquid formulation. To avoid injuring turf, do not overapply.
Orchardgrass, quackgrass, and other perennial weed grasses	glyphosate (Roundup)	Dig out offending plants or spot treat with glyphosage then reseed or sod the area.

 

Table 21. Control of weeds in vegetable gardens
Plant	Weed	Management practices
Seeded crops	Annual weeds (pigweed, lambsquarters, purslane, crabgrass, foxtail) Perennial and biennial weeds (quackgrass, thistles, bindweeds, yellow rocket, curly dock)	Good cultivation before planting, shallow cultivation, hoeing, hand pulling. Organic mulches such as straw are also effective. Black plastic sheeting is of limited usefulness. Treflan and glyphosate have very specific uses in vegetable gardens. Read and follow label directions when using any of these materials. They cannot be used on all crops.
Transplanted crops	Annual weeds (pigweed, lambs-quarters, purslane, crabgrass, foxtail)	Same as for seeded crops. Black plastic mulch (1.5–4 mils thick) or geotextile fabrics can be satisfactory if properly installed. Organic mulches including straw are also effective in suppressing weeds in transplanted beds. Treflan and glyphosate have very specific uses in vegetable gardens. Treflan will provide preemergent control of annual grasses and a few broadleaf weeds.  Round Up can be spot applied to control weedy escapes.  Read and follow label directions when using any of these materials. They cannot be used on all crops.

 

Table 22. Herbicides for use in flower gardens and ornamental plantings
Herbicide	Uses
Trifluralin (Treflan or Preen)	Controls many annual grasses and a few broadleaf weeds in established ornamentals and bedding plants. Apply to soil preemergence, before weeds germinate. The granular formulation is easy to apply. Irrigate or till in immediately after application.

 

 

 

 

 

 

 

Table 23. Optimal times for controlling perennial weeds with glyphosate
Spring	Summer	Fall
Quackgrass	Honeysuckle	Japanese knotweed or bamboo
Other perennial grasses	Kudzu	Goldenrod
	Poison ivy (Many persons havebeen afflicted with poison ivy from touching dead poison ivy plants.  It usually takes a year or so before the toxic oils dissipate.)	Brambles
		Trumpet creeper
		Mugwort (chrysanthemum weed)
		Most other deciduous and
		herbaceous perennial weeds

 

 

Table 24. Some nonselective chemical herbicides for use on flagstones, patios, gravel driveways, and other noncrop areas (The persistence of some nonselective herbicides depends on the rates applied. Read labels carefully.)
Chemical Name	Some Trade Names	Comments
Dichlobenil	Casoron, Dyclomec, Norosac	Long residual effect. Do not use on light, sandy soils.
Glyphosate	Roundup, Kleenup, Spot-Kleen	No residual effect.
Oxyfluorfen	Goal	Selective herbicide in vegetables and ornamentals.
Prometon	Pramitol, OrthoGround Clear Triox Total Vegetation Killer	Long residual effect.
Herbicidal fatty acids	Sharpshooter	Contact desiccation of treated vegetation. No residualeffect. Controls small annual weeds but burns only the foliage of larger annual or perennial weeds
Glpyhosate plus oxyflourfen	Ortho Season-long Grass and Weed Killer Ready-To-Use	Some control and residual effects, controls perennial weeds and annuals as well.
Triclopyr	Ortho Brush-B-Gon Poison Ivy, Poison Oak, Brush Killer 1, Ortho Weed B Gon Tough Weed Killer for Lawns	Some residual effect.  Provides postemergent control of difficult to manage broadleaf weeds by translocation.

 

See Part I, Cultural Methods, for nonpesticidal alternatives.