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Lawn and Turf Weed Control
编辑:J. H. Dunn and D. D. Minner   出处:Department of Horticulture, College of Agriculture   时间:2009-3-4 15:23:24

Weed-free turf beautifies and adds greater usefulness to home landscapes, recreational areas and public and industrial grounds. Proper management is the most effective factor in weed control, but unfavorable climate, insects, diseases or abuse by man permits weeds to invade the turf. Under such conditions careful use of appropriate herbicides permits the turf to recover its original value.

Weed control principles

A dense stand of healthy grass provides the best weed control. Most weeds are opportunists which invade weakened lawns and turf; thus, the fight against weeds starts with good management. All cultural practices such as mowing, fertilizing, watering, etc. should be done in a manner and time that will favor the grass rather than the weeds. Height of mowing influences competition against weeds such as crabgrass-the higher the cut, the lower the infestation. Frequent light sprinkling encourages shallow rooted weeds and seed germination. Less frequent deepsoak watering that maintains a dry surface layer provides the grass with a competitive advantage.

Temperature, light, soil moisture and other factors determine the time and extent of weed germination and development. Some germinate in early spring while others sprout in summer or fall. If conditions are favorable a weed may be particularly abundant in a given year, but under different conditions, the next year it may be little in evidence.

Weeds such as crabgrass and knotweed, which germinate in the spring from seed and mature in summer or fall each year, are designated as summer annuals. Chickweed, henbit and others germinate in fall or late winter and mature in late spring. These are called winter annuals.

Pre-emergence herbicides (chemicals applied to soil to prevent growth from seeds) must be applied in spring to control summer annuals but in fall to control winter annuals. After weeds appear, postemergence herbicides must be used.

Perennial weeds live more than two years but may produce seeds for new plants each year. Like biennials, which require two years to complete a life cycle, they store food in fleshy roots for next year's growth. Generally, postemergence herbicides are required for control and are more effective in the first year⺳ growth. Late fall often may be a good time to apply such herbicides. Old established plants, which have large taproots, may be controlled more effectively by hand pulling or digging. Henbit--upright, squarish stems, blue & lavender flowers; winter annual. Plantain, (narrow or buckhorn)--long, narrow leaf, seedhead atop wiry stem; perennial. Plantain, (broadleaf or Rugels)--thick, oval leaves, long seedhead; perennial. Goosegrass--coarser than crabgrass, leaf base flat, whitish; summer annual. Nimblewill--fine wiry stems, frail inconspicuous seeds, winter dormant; perennial. Yellow Nutgrass (Nutsedge)--triangular stems, yellowish green; treat as perennial. Mallow--upright stems, five point leaf; summer annual or biennial. Chickweeds--common--light green delicate vine; winter annual; mouse-ear--dark green hairy leaves; perennial. Shepherds Purse--young leaf-like dandelion, triangular seed pod; winter or summer annual. Knotweed--tough viny stems, leaves small & single; spring germinating annual. Spurge (prostate or mat)--plant sap milky, stems often matted; summer annual. Bindweed--creeping vine, leaf-like arrowhead, flower white or pink; perennial.

Identification aids

Notes above give key identification characteristics of weeds more frequently found in lawns and turf. Crabgrass and dandelion are omitted because most readers are likely to be familiar with them already.

Herbicide application

Although most herbicides are formulated with reliable safety factors, application rates higher than those recommended may cause injury to turf. Many people "over-apply" herbicides, especially when using fertilizer-herbicide combinations. The user needs to follow directions on containers carefully to avoid overdoses. Lower safe rates are effective if applied when weeds are most susceptible.

Pre-emergence treatments are applied before weeds sprout from seeds. Apply 2-4 weeks ahead of germination. Less effective control may be expected if applied more than a month before. Removing clippings and sprinkling immediately after application will help move materials down to the soil.

Postemergence herbicides such as 2,4-D, Trimec, DSMA, etc. are applied after weeds appear. Liquid sprays are more effective than dry materials, especially on hard-to-kill weeds. Apply postemergence materials when weeds are growing vigorously. Tough old weeds are hard to kill, and if mature seeds are already formed, the lawn is likely to be infested again next year.

Amine forms are safest because they give off less vapors which might damage other plants. Volatile ester formulations should not be used around ornamental plants. Select a time when winds are calm to prevent spray drift. Using wax bars or granules impregnated with herbicides near ornamentals will minimize such hazards.

Fertilizer-herbicide combinations are extremely popular because they combine two operations. Combinations with pre- emergence chemicals are generally effective since both the fertilizer and herbicide action are dependent upon contact with the soil. Postemergence herbicide action depends more upon absorption by leaves, and granules in such combinations do not adhere well to smooth-surfaced leaves. They will stick better if applied when weed leaves are damp. "Weed and feed" materials present a conflict in desirable actions. Proper time for weed control often does not coincide with the most desirable time and rates for fertilizing. If used for follow-up fertilizations, there is danger of herbicide overdose.

Spot Treatments. Hard-to-kill weeds may be eradicated by spot applications of non-selective herbicides such as dalapon, amino triazole (amitrole), cacodylic acid or glyphosate. Thoroughly wet the foliage with the solutions, but avoid run-off and excess accumulation in the soil. Dalapon is effective mostly against weedy grasses. Use one of the others for broadleaf weeds. Glyphosate and cacodylic acid usually decompose in a few days, but amitrol and dalapon will require three to six weeks before the spot can be reseeded.

Small patches can be eliminated by excluding all light with pieces of tarred paper, black plastic, etc. Keep the cover in place at least three months.

Equipment. Fertilizer spreaders can be used for applying granular herbicides. Be sure to adjust the spreader to apply recommended rates. If possible, apply one-half the desired rate in one direction and the remaining half at right angles to the first.

Gravity flow applicators, compressed air sprayers or types attached to a garden hose are effective for liquid applications. High pressures cause mists subject to drift and should be avoided. Sprinkler cans or sprinkler nozzles attached to a gallon container can be used on small areas.

A sprayer used for 2,4-D applications should not be used to spray garden or flower plants. Cleaning procedures are not always reliable. To be safe, have a separate sprayer for weed killing purposes.

Eliminating weeds is of little value unless enough desirable grass is present to fill in bare spots. A reseeding program deserves first consideration if the turf is so weak that it will not recover once weeds are eliminated. Study soil and other conditions to determine reasons for low vigor of the original turf.


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