A successful weed control program includes prevention, early detection and diagnosis, and responsible weed control action. Prevention begins with cultural practices that promote plant health, because healthy turfgrass is less vulnerable to weed infestations. A routine program of plant health care also encourages early detection and diagnosis of weed infestations.
Responsible weed control begins with evaluating the level of weed infestation and determining whether control measures are needed. For example, on a high-maintenance site such as a golf course green, few if any weeds can be tolerated. Consequently, the decision to treat is obvious, and necessary control measures will be selected. Alternatively, a low-maintenance site, such as a highway right-of-way that does not have great visibility can withstand some level of weed infestation.
Plant health care is the use of a variety of pest control measures, including:
resistant varieties or cultivars
proper cultural practices
Integrated Pest Management (IPM) is the use of a combination of these methods. The idea is simple; all available prevention and control methods are used to keep weeds from reaching unacceptable levels. The following practices influence weed growth and their management in the turfgrass environment.
Sod can be laid anytime after new growth starts in the spring and before November. For cool season grasses the best time to seed a lawn is in September. That is when cool-season grasses normally germinate in nature, and most summer annual weeds will not be competitive, because they will not germinate until spring. A lawn seeded during this period can become established before winter and will be growing actively in the spring before summer annual weeds germinate. Planting high-quality seed from a reputable source will minimize the introduction of weed seed onto the site.
Properly fertilized turfgrasses are more vigorous and competitive. Thinned areas can result where fertilizer was applied at excessive rates or at an improper time. Excessive rates in late spring, particularly nitrogen, encourage severe incidences of spring and summer diseases of cool-season turfgrasses. Cool-season turfgrasses that are highly maintained should be fertilized only once in the early spring and twice in the fall. Nitrogen, in quick-release forms, can burn the leaf blades of turfgrasses especially when they are damp. If quickly available forms of nitrogen are used, no more than one pound of nitrogen per 1,000 square feet should be applied in a single application, then watered in.
Frequent, light waterings stimulate the germination of some weed seed, particularly crabgrass. A new sod lawn should be soaked immediately, and then water can be reduced when roots are established (2 to 3 weeks). Newly seeded areas should be watered two or three times a day to keep the seedbed moist while avoiding puddling and runoff. Once the seedbed has been watered, it should not be allowed to dry out; however, avoid letting the soil go into the night in a wet condition. As seedlings develop, apply more water but apply it less frequently to encourage root development. If the turfgrass is established and healthy, it should be watered heavily while allowing the soil to dry out between irrigations. This will help develop deep root systems and plant tolerance to stress conditions.
Lawns should be mowed frequently enough so that no more than onethird of the vertical height is removed with each mowing. Mowing cool-season turfgrasses at 2" — 3" on a regular basis maintains a dense, smooth, uniform site and reduces the competitiveness of weeds. The growing point of broadleaf weeds is near the terminal portion of the plant. Regular mowing will place broadleaf weeds in a continuous state of stress and they may often be eliminated. "Scalping" the lawn with the blade set extremely close to the soil surface will open areas that allow sunlight penetration and subsequent germination of weed seed.
The primary purpose of aerating a turfgrass site is to reduce soil compaction. In the short term, core removal can cause an increase in weed seed germination, particularly if done in the spring when crabgrass seed is present or in the fall on sites infested with annual bluegrass. However, in the long term, less compacted sites are not as favorable to the growth of certain weed species, such as goosegrass and prostrate knotweed.
Thatch is a buildup of tightly meshed grass stems and roots, composed of both dead and living plants. It accumulates between the zone of green vegetation and the soil surface. A thatch layer is natural, and not a problem as long as it is less than 1/2" thick. If it becomes thicker than 1/2", however, it may interfere with healthy root development and subsequently weaken the turfgrass. Seedlings that develop in thatch are more susceptible to injury from cold temperatures, traffic, and other stresses than those that develop in the soil. Thatch removal in the spring can result in greater levels of weed emergence because the soil surface has been opened to sunlight.
Herbicides are important tools for controlling weeds in turfgrass. Although repeated occurrence of weeds may reflect underlying problems that are not correctable with herbicides, the proper use of herbicides can often convert a heavily infested site into one that is weed-free.