Cultural practices are a vital component within an effective management program for turf weeds. Successfully executed cultural practices can result in a healthier, denser turf which increases the overall competitive nature of the stand and decreases the severity of many weed infestations. Some weed infestations can be significantly reduced or even eliminated with the use of appropriate and well-timed cultural practices.
Many weeds require light for germination and establishment. Increasing the mowing height within the range for the particular species or mix of species present will result in a decrease in the amount of light reaching the soil surface and can reduce weed germination and establishment. This is particularly important in the spring and early summer during the peak germination period of annual grassy weeds. Decreasing the height of cut and collecting clippings that contain seedheads during the late summer and early fall can be effective in reducing the amount of viable seed that is added to the weed seed bank.
Fertility, as with mowing height, can play a major role in the reducing the amount of light that penetrates the turf canopy and reaches the soil surface. Fertilization programs should supply adequate and balanced nutrition to yield a dense turf. Avoid high levels of fertility during the summer months when cool-season turfgrasses are less competitive. If heavy summer weed pressure has thinned turf, an application of fertilizer in late summer or early fall will support turf recovery. Fertilization after herbicide applications can aid in the filling of canopy voids left by dying weeds.
Compaction is a major contributor to thin turf and the encroachment of weeds. An effective aeration program will alleviate compaction and increase overall turf health and density. However, aeration methods that bring soil to the surface can reposition weed seeds, which were once too deep to germinate, to a location where germination and establishment are favored. The alleviation of compaction in conjunction with overseeding of desirable grasses can reduce and in some cases eliminate weeds such as goosegrass, pathrush, pineappleweed and prostrate knotweed.
Many annual weeds, including crabgrass, are warm-season species. Warm-season species are capable of growing very well during the hot, dry periods characteristic of summer. Turfgrass species utilized in the northeast are cool-season species and, without adequate moisture from rainfall or irrigation, become dormant during the summer. During periods when the growth of cool-season turfgrass species has slowed or ceased as a result of low soil moisture and high temperatures, warm-season annual weeds become very competitive in otherwise healthy, dense turf. If some weed encroachment is not acceptable, irrigation should be applied in the absence of summer rainfall to maintain turf growth and prevent summer dormancy. Special attention should be focused on areas that are prone to drought including elevated areas, south and southwest facing slopes and areas adjacent to hardscapes such as sidewalks and driveways. If there is annual weed pressure and the turf at a site is going to be allowed to go dormant during the summer months, the option also exists to apply a preemergence herbicide in the spring. If turf has thinned due to drought dormancy, overseeding with desirable turfgrass species should be done in late summer/early fall when cooler temperatures and rainfall return.
The over-watering of turf areas can also contribute to weed encroachment. Weeds such as annual bluegrass, creeping bentgrass, and yellow nutsedge can be more problematic on over-irrigated sites. On poorly-drained sites, the installation of a drainage system may be warranted to decrease naturally-existing excessive soil moisture.
Turf Renovation and Establishment
Turf renovation and establishment projects carried out in the spring and early summer without the use of a preemergence herbicide fail more often than not due to annual weed pressure. Summer annual weed pressure is minimal when turf renovation and establishment is initiated in the late summer and early fall. Warm-season annual weeds that germinate later in the season seldom reach a size that deters turf establishment and die with the onset of cold weather. Special attention should be paid to winter annual weed encroachment in such areas, however.
When planning a turf renovation, choose turfgrass species and cultivars that are best suited for the site conditions and expected turf use. Aggressive cultivars should be considered when available. Encourage rapid establishment by providing adequate fertility at seeding. Maintain good soil moisture during the germination and early establishment period. Overseeding can be a valuable tool in restoring sites where turf thinning has resulted from insufficient annual weed control. Openings in the turf as a result of insect damage, diseases and excessive wear are prone to weed encroachment. Overseeding is frequently useful to repair such damage and regain turf density.