Commercial Horticulture: Turf Weed Control
Brad S. Fresenburg
Department of Horticulture
Consider any undesirable plant in turf to be a weed. Weeds are opportunistic and are virtually impossible to eradicate from turf. A proper weed control program limits weed infestations rather than attempting to eliminate them entirely. Weak stands of turf that lack density will soon be infested by weeds. In some situations, extremely competitive weeds can infest dense turfgrass. For either situation, combine chemical weed control with a management program directed at improving turfgrass density and vigor. Proper management includes mowing, watering, fertilizing and cultivation. Once weeds have been suppressed by an effective chemical and cultural weed control program, eliminate your subsequent use of herbicides or reduce them to spot treatments where problem weeds reappear.
To combat weeds, you must be able to identify them and understand their life cycles. Take weeds that are difficult to identify to your local MU Extension
center or send them to: Weed Identification, Integrated Pest Management, 45 Agriculture Building, University of Missouri, Columbia, MO 65211.
Weed type
Turfgrass weeds are generally classified as either grasses or broadleaves.
- Grasses. These weeds are distinctive in that grasses are monocots. They have leaves with parallel veins and only one seed leaf in a developing seedling.
- Broadleaf weeds
Broadleaf weeds are dicots. They have a netted leaf vein pattern and two seed leaves on a seedling plant.
- Sedges
A sedge is neither a grass nor a broadleaf. Yellow nutsedge is the most prominent weed in this group. It has a grasslike appearance with a distinct triangular stem, a deeply keeled midrib and no collar or ligule.
Turfgrass weeds are further classified according to their life cycle.
Weed life cycles
All weeds go through a cycle of seed germination, foliar and root growth, maturity and flowering, and seed development. You must understand weed life cycles because the effectiveness of herbicides depends on the stage of growth of a particular weed. For example, preemergence crabgrass herbicides are effective only when applied before weed seeds germinate in the spring. Postemergence broadleaf herbicides may be applied to perennial broadleaf weeds. But they are much more effective when applied to weeds in a juvenile vegetative stage rather than a mature flowering stage of growth. If postemergence broadleaf herbicides are applied to mature perennial broadleaf weeds, applications should be made in early fall.
- Winter annuals
Plants live for one year. Seed germinates in the fall; weed matures through the winter, sets seed and dies in the spring.
- Summer annuals
Plants live for one year. Seed germinates in the late spring and early summer; weed matures very rapidly in the summer and sets seed and dies in the fall.
- Winter perennials
Plants live for several years. Weed reaches maximum flowering and subsequent seed set in the spring.
- Summer perennials
Plants live for several years. Weed reaches maximum flowering and subsequent seed set in late summer and fall.
- Biennials
Plant lives two years. Its first year is vegetative growth; it flowers and sets seed in the second year.
Checklist for proper weed control
- Identify the weed and understand its life cycle. Then answer these questions:
- Is this weed causing an economic or aesthetic loss that cannot be tolerated?
- Can this weed be reduced to an acceptable population through management practices alone?
- If this weed is not controlled now, will it be more difficult to control in the future?
- Are there herbicides that can effectively limit the growth of this weed, and can the herbicide be safely used without harming other ornamental plants?
- Do I have the knowledge and equipment to safely handle, apply and store herbicides?
- Determine the cause of the weed problem and design a management program to increase turfgrass competition and reduce weed competition.
- Correct major problems, such as traffic patterns and poor drainage, that may be causing the weed problem.
- When needed, safely apply herbicides so that weeds may be controlled and turf may resume a competitive density.
Cultural weed control
The basic principle involved in cultural weed control is to grow a stand of grass that is dense and competitive enough to prevent weed encroachment. Weeds are not the cause of poor turf, but rather the result
Weeds require light, water and nutrients to grow. As turf loses density, light will penetrate the canopy and cause weed seeds at the thatch and soil surface to germinate. Once germinated, weeds can develop rapidly, especially if turf continues to decline.
Weeds probably will receive sufficient light to develop if you can see bare soil or thatch when you look straight down on a stand of grass. Management practices that discourage turf weeds include proper mowing, watering, fertilizing, thatch control and cultivation. The chance for weed encroachment can be reduced by using locally adapted turfgrasses and establishing them during the correct season of the year.
- Mowing
To prevent weed germination, mow frequently at the tallest recommended mowing height. Weeds germinate rapidly when turf is scalped by mowing too short or when it is not mowed frequently enough. Both mistakes decrease turf density and cause an open canopy that favors weeds. Experts recommend a range of mowing heights to meet specific turf activities. Lower mowing heights require more frequent mowing. Each mowing should not remove more than one-third of the total leaf height. Annual grassy weeds -- such as crabgrass -- are especially a problem on turfs that lack density as the result of poor mowing.
- Fertility
Apply nitrogen at least once a year. Areas that receive chemical weed control but no nitrogen are probably being mismanaged. Even in low- maintenance situations, the first defense against weeds is dense turf. For cool-season grasses, apply nitrogen in the fall. For warm-season grasses, apply it in the summer. Additional nitrogen should be supplied during the growing season to provide adequate growth and density with less attention given to turfgrass color. On cool-season grasses avoid heavy applications of nitrogen in the spring. That will encourage excessive vertical leaf growth, bringing with it a greater chance for scalped turf and an eventual decrease in turf density. Don't apply nitrogen to dormant warm-season grasses, since it will encourage winter annual weeds, such as chickweed, henbit and speedwell.
- Establishment
Weeds can prevent the establishment of grasses at certain times of the year. Spring seeding of cool-season grasses is seldom successful -- because the fertility requirements and frequent watering necessary for turf establishment will also create intense weed pressure from crabgrass and summer annual broadleaf weeds. Consequently, late summer and early fall are preferred for establishment of cool-season grasses. Cool-season turfgrasses that are germinating by early September will not be affected by summer weeds and will usually establish a competitive stand of grass before winter annuals, such as chickweed and henbit, become a problem.
- Thatch and cultivation
These have a dual effect in terms of weed control. Weed seeds below a thatch layer lack the necessary light for germination, so weeds that germinate in the thatch layer usually die because of the poor environment for seedling development in thatch. In this respect, thatch provides some degree of weed suppression. On the other hand, thatch can cause turf decline by harboring disease-causing pathogens, reducing water infiltration and tying up pesticides and fertilizers. Power-raking and verticutting are typically used to remove thatch.
- Cultivation
Soil compaction from traffic and tight soils will often decrease turf competition and favor some weeds, especially goosegrass and knotweed. Two ways to fix the problem are coring and slicing. These procedures improve the soil surface and the growing environment for turfgrass.
Chemical weed control
The proper application of herbicides can dramatically reduce turfgrass weed populations in a short period of time. A combination of their cost, effectiveness and simplicity of use often makes herbicides the primary means of weed control -- with little attention given to other weed reduction measures. Remember, herbicides are only one facet of the total weed control puzzle. Effective long-term weed control and a reduction of pesticides in the environment will be achieved only when an effective cultural weed control program has been implemented.
Types of herbicides
There are two general types of herbicides -- selective and nonselective.
- Selective herbicides kill only the target weed species, leaving the turfgrass unharmed.
- Nonselective herbicides kill all plants they contact. Selective chemicals are further divided into their type of application. Preemergence herbicides are applied before the emergence of a weed (Table 1). Postemergence herbicides are applied after a weed has emerged; they are generally applied to weed foliage (Tables 2, 3 and 5). See Table 4 for seeding and reseeding intervals in relation to herbicide application. Table 6 gives herbicide application rates. Table 7 lists various herbicides labeled for controlling weeds in various turfgrass species.
Herbicide action
The degree of mobility of a herbicide within a plant often determines the degree of weed control. Systemic herbicides provide a more complete control of mature plants, since they translocate in the xylem and phloem to kill the parts of the plant that are above and below ground.
The terms used for herbicide type and action can be combined to provide a very descriptive phrase of how herbicides work. For example, pendimethalin is a selective, contact, preemergence herbicide for controlling crabgrass. Glyphosate is a nonselective, systemic, postemergence herbicide that is often used before turf is renovated.
G6752, reviewed August 2003
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