One of the most important considerations in turfgrass culture is weed control. A weed is generally defined as any plant growing in a location where it is not desirable. A variety of grasses, sedges and broadleaf weeds continually attempt to invade most turfgrass areas. The life cycles of these weeds include annuals, biennials and perennials.
Annual plants complete their life cycle from seed in less than one year. Common annual grass weeds include the crabgrass, foxtails, barnyardgrass, goosegrass and some biotypes of annual bluegrass. A wide variety of annual broadleaf weeds are often found in turfgrass areas.
Biennial weeds require two years to complete their life cycle. The first year the weeds grow leaves and roots and store energy reserves. The second year flowers and seeds are produced and the plant dies. Wild carrot is the most common biennial broadleaf weed found in turfgrass areas.
Perennial weeds live for more than two years and may persist indefinitely. Examples of perennial weeds include quackgrass, nimblewill, nutsedge, dandelion, white clover, etc.
The incidence of weed invasions can be greatly reduced if a dense turfgrass canopy is maintained that provides continual competition to the seedling weeds. Therefore, it is important to select the proper species and cultivars of turfgrass for the site to be established. Once the turfgrass seeds germinate and the seedlings develop into established turfgrass, proper culture can have a major role in governing the weed pressures that ultimately develop. Proper mowing height, fertilizer and water usage are the most important factors in maintaining turfgrass density.
Although proper turfgrass maintenance programs culture) can greatly reduce the invasion pressures of the various turfgrass weeds, even the highest quality turfgrass areas will occasionally be invaded by one or more weed species. While a few weeds can be physically removed, considerable weed invasion may require chemical controls. Chemical control can eliminate established weeds or may be used to prevent or reduce future weed establishment. Pesticides that are used to control unwanted plants are called herbicides. The following terminology used to describe herbicide activity:
Preemergent - Products that stop the development of unwanted weed plants before they emerge from the soil.
Postemergent - Products that are applied after the weeds emerge from the soil and eliminate weeds in the early stages of growth.
Contact - kill only the plant parts which come into contact with the chemical applied. Roots are not killed and new top growth may occur.
Systemic - herbicides which are absorbed either through the foliage or below ground plant parts, translocated throughout the plant resulting in death of both the above ground and below ground plant parts.
Selective - a group of herbicides capable of affecting or controlling certain plants without affecting the desirable plants.
Nonselective - theoretically affect or control all herbaceous plants coming into contact with the herbicide.