You can look at weedy grasses in lawns in one of two ways. You can decide to get rid of the grassy weeds so you can grow strictly a bluegrass lawn, or you decide you aren't bothered by weeds as long as they are grass.
If you want to keep grass invaders out of your lawn, three actions will help: mow high, fertilize, and water every three to seven days. Mowing at two-and-one-half to three-and-one half inches allows bluegrass to shade out annual grasses such as crabgrass. A combined program of fertilizing to thicken the lawn with high mowing does reduce the amount of crabgrass.
Also helpful is avoiding daily or every other day watering. Frequent, shallow watering keeps the soil surface constantly moist, which creates ideal conditions for germinating weedy grass seeds.
While you may not eliminate all weedy grasses in one year, research shows this threefold program will steadily reduce weedy annual grasses to very small numbers over a two to three year period.
These three approaches -- mow high, water every three to seven days and fertilize regularly -- work against annual weedy grasses.
Another choice available for grassy weed management in lawns is a preemergent herbicide (a pesticide that controls weeds). Useful with annual grassy weeds such as crabgrass, preemergents act to kill weed seeds as they are germinating. Preemergents always should be used in conjunction with the threefold program described above.
In the Denver area, it is recommended that preemergents for crabgrass control be applied in April. If delayed until after May 1, crabgrass seeds have finished germinating, chemicals are needlessly introduced into the environment and the herbicide is wasted. Preemergent timing in other areas of the state differs with spring temperature variations.
While preemergent herbicides are useful tools if used correctly, don't use them routinely year after year to solve problems caused by poor lawn management. Mowing high, fertilizing, and avoiding frequent irrigations are the mainstays of crabgrass control as well as growing a healthy lawn.
A potential problem with preemergent herbicides used in non-turf landscape areas is their potential to move from bare soil into surface waters. Once applied, preemergents are trapped by microbes, plant uptake and adsorption onto the surface of soil particles. In vegetable gardens, flower and shrub beds, wheresoil particles can be washed away with irrigation or storm water, the preemergent can be carried along. In these landscape situations where bare soil is exposed, preemergents should be used with great care. Consider other weed control alternatives such as hoeing, the use of landscape fabrics, mulching, or covering the soil with other desirable plants such as ground covers.
Preliminary testing of surface waters by the USGS finds at least two commonly used preemergent herbicides in the surface waters of Cherry Creek. While it's too early to assess the significance of the amounts, it's not too early to confirm there is escape of the preemergents "off target." Since the drainage into Cherry Creek is predominantly non-agricultural, urban landscape uses of the preemergents are one suspect source. All this adds to the cautions to use these materials with care.
Weed control practices affect the environment both in terms of herbicides introduced and soil erosion from landscape areas kept bare of weeds or other plant cover.
Photograph courtesy of Judy Sedbrook.
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010