When planting turfgrasses, it is important to use seed or vegetative propagules (sod, plugs, or sprigs) that are as weed free as possible. Inspect seed labels or vegetative propagules to determine the quantity of weed seeds or weeds present. Plant as few weeds as possible to avoid future problems. Usually, high-quality seed or vegetative propagules are more weed free than those that are less expensive. Reducing weed spread from areas adjacent to established turf can also help prevent weed infestations. Mowing weeds before flowering can reduce seed movement into turf areas. When modifying the soil with amendments (peat, sand, topsoil) prior to planting, make sure amendment is clean and weed free as possible.
Hoeing, pulling, burning, and digging weeds are mechanical methods of removing weeds in turf. Tilling areas before establishment is also a method of mechanical weed control. In small landscape settings, these methods may be acceptable and hand pulling small quantities of young weeds may prove to be an acceptable method of control. In larger areas, however, these methods are probably impracticable.
The first defense against weed infestations in turfgrass is to produce a dense stand of healthy turf. Quality turf can compete well with weeds and is best achieved by properly preparing planting soils, planting turfgrasses suitable for the given situation, and by using standard cultural practices (mowing, watering, fertilizing, and cultivating).
Primary to any turf management program is properly selecting the turf to be used. For the golf course manager, this is generally not a problem area; there is a limited choice of turfgrasses appropriate for use on greens, tees, and fairways. Golf course turf selection is usually dictated by the desired quality, budget, and amount of play, not primarily by a desire to keep weed populations to a minimum.
In any turf setting, following proper turfgrass selection, the second line of defense against weed and pest invasion is the use of correct mowing, irrigating, fertilizing, and cultivating practices. Proper mowing is critical to turfgrass health and appearance. Turfgrasses mowed too short become open, inviting weed invasion. Also, when mowed too short, turfgrass may develop inadequate root systems that allow plants to succumb to insect and disease pests. When allowed to grow too tall, turfgrasses can appear unkempt and coarse-textured. Golf course playability is also altered when turfs are allowed to grow tall. Basic mowing recommendations include frequent mowing at the tallest height recommended for the specific turf and use.
Irrigating turfgrasses is also important. Over-watered turf may grow too rapidly, develop shallow root systems, and be prone to invasions of weeds, insects, and diseases. Under-irrigated turf may become open and sparse, develop a poor appearance, and also allow weed invasions. Under most circumstances, when watering, water infrequently and deeply to the depth of the root system. When irrigated lightly and frequently, weed seeds (especially those of annual weeds) can germinate and readily develop into mature weeds because the soil surface is never allowed to dry out.
Fertilizers maintain turfgrass density, vigor, and color. Inadequate nitrogen fertilization leads to open turf that is readily invaded by weeds. Inadequate amounts of other minerals in the soil, especially potassium, phosphorus, iron, and sulfur, can also reduce turf color, disease resistance, and stress tolerance. Conduct soil tests and supply these elements as recommended by test results. It is important, however, not to supply excessive amounts of phosphorus to established turf. Excessive amounts of phosphorus at the soil surface can encourage development of germinating weed seeds.
Cultivation activities include core aerification, slicing, and vertical mowing. These activities can reduce thatch and prepare turf for overseeding. Core aerification is also useful for reducing soil compaction. Conduct cultivation activities during periods of active growth when turfgrasses are best able to recover from these practices. In areas where annual weeds are a particular problem, conduct cultivation activities during autumn.
Insect pests and disease pests can thin and weaken turves, allowing weed invasion. Control these invaders by using good culture and the appropriate pesticides when necessary.
Excessive traffic physically damages turf and also compacts soil. Thin turf, prone to weed invasion, can result. Perennial ryegrass and tall fescues can tolerate wear better than most other cool season grasses. Where appropriate, use these grasses in high traffic areas. In addition, where possible, use lightweight mowing equipment and alter the equipment's path to spread wear evenly over the use area.
Compaction reduces the quantity of large, gas-containing pore spaces in soils. Some weeds (e. g., goosegrass and knotweed) are more tolerant of low soil oxygen conditions than are most turfgrasses. Cultivate these areas so that the turf can better compete with weeds.
Along with compacted soils, other unfavorable growth environments can also create growing conditions to which weeds are more adapted than is turf. Shade that reduces light quantity and quality favors weeds (e. g., ground ivy and common chickweed) rather than turf. Prune or remove trees to allow more light to reach the growing area. This also can increase air movement and decrease disease problems. Excessively wet or dry soils also create environments in which weeds often grow better than does turf. If possible, provide external and internal drainage to move excessive water from the turf root zone. Also, supplemental irrigation can be used in dry periods. In general, try to create growing environments that favor turf and not weeds.
In the future, any number of biological agents may be employed to control weeds in turf. For example, research attempting to identify dandelion diseases has been undertaken. Other biological agents include bacteria, nematodes, insects, and viruses. At present, however, practical and effective biological weed control in turf is not a reality.
Herbicides are pesticides used to control weeds. They can be selective (controls a particular pest without harming related organisms) or nonselective (control nearly all related organisms). Preemergence herbicides are applied before the weed seedlings emerge from the soil. Postemergence herbicides are applied to weeds after they have emerged from the soil. Contact herbicides affect only the parts of the plant they come into contact with, while systemic herbicides are taken into the plant and moved throughout it. Whenever any pesticide is used, be sure to read, understand, and follow all label directions for the safest and most effective handling and application.
Broadleaf weed invasions can be minimized through proper turfgrass management. Consider use, site, and budget when selecting an appropriate turfgrass for your conditions. Follow correct selection by appropriate mowing, watering, fertilizing, and cultivating which can lead to a dense, healthy turf. Reduced weed populations result because weeds have difficulty becoming established in healthy, competitive turf.
In areas where broadleaf weeds are already a problem, initiate controls. Mechanical removal of weeds by hand pulling or hoeing can eliminate small numbers of weeds easily. Be sure to remove as much of the root system as possible to reduce regrowth of perennials.
Postemergence herbicides can also provide effective control during active weed growth. Individual herbicides, or combinations of these herbicides, are available. Be sure to read, understand, and follow the label directions for proper use of these chemicals. If mishandled or misapplied, these herbicides may damage or kill many desirable ornamental or edible plants in the landscape. Follow the general recommendations listed below when using postemergence broadleaf products.
Many recently introduced turfgrasses are vastly superior to older types, especially in their tolerance to wide ranges of cultural and environmental conditions. Unfortunately, even superior turfgrass cultivars have limits to what they can tolerate. In most cases, when cultural or environmental conditions surpass minimum turfgrass tolerance levels, problems arise. For example, the presence of annual grassy weeds such as crabgrass, goosegrass, barnyardgrass, fall panicum, and yellow foxtail indicates problem growing conditions that allow these weeds to be more competitive than the desirable turf.
Management practices that discourage annual grassy weed growth in turf include:
Proper turf management is primary to a weed control program. In some situations, turf is grown in environments that favor weed growth and development. Many annual grassy weeds are more tolerant of wet or compacted soils or shade than are turfgrasses. Altering the growing environment to favor the turfgrass can shift the competitive edge away from weeds.
The bottom line is that turfgrass breeding, selection, and evaluation have greatly improved turfgrasses. However, even new and improved turf selections are incapable of competing with weeds when mismanaged or planted into unfavorable environments. If producing high-quality turf is important, cultural practices and environmental alterations that enhance turf growth relative to weed growth are the basis of a sound weed management program.
Maintaining a dense, vigorous turf is the best weed control. Occasionally, herbicide applications are mandated to reduce weed populations to tolerable levels. When annual grassy weeds such as crabgrass, goosegrass, barnyardgrass, fall panicum, or yellow foxtail become a problem, preventive preemergence herbicides are often used for control. A number of preemergence herbicides are recommended by the University of Illinois Cooperative Extension Service. When using any pesticide, read, understand, and follow the label directions for the safest, most efficient pest control.
Several general recommendations can be made when using these products.