Throughout the growing season, weeds frequently trouble area lawns. The application of pre-emergent herbicides (weed killers) to prevent weed seeds from emerging is one of the best ways to control weeds in lawns. The pre-emergent herbicides will not control existing weeds.
From late spring to the first frost, crabgrass and other summer weeds can invade large portions of the lawn creating an unsightly appearance. Crabgrass can be a particularly troublesome weed because a single crabgrass plant is capable of producing up to 150,000 seeds in one season. It then dies upon the arrival of cold weather leaving large dead voids in the turf.
Crabgrass seeds begin to germinate when the soil temperature reaches 50 to 60 degrees. Pre-emergent herbicides are the optimum method of preventing these weeds from becoming established in your lawn. Once they have emerged, pre-emergent herbicides are no longer effective, and the application of post-emergent herbicides will be necessary. Pre-emergent herbicides are mostly granular formulations and are applied with a rotary spreader. These herbicides, recommended only for lawns that are well established, should not be applied to lawns that have been recently seeded or newly sodded. Make sure you follow all label directions and safety precautions when using pesticides.
The best time of the year to make the application is in early March when the air temperatures are above 60 degrees for several days in a row. A good rule of thumb for timing the application is when the forsythia shrubs are in bloom. Avoid using “weed and feed” fertilizers, which contain both the pre-emergent herbicide and fertilizer on warm season grasses. Warm season turfgrasses should be fertilized only after they have come out of dormancy, and if applied earlier, the turf may green up pre-maturely increasing its susceptibility to damage from a late season freeze.
Since crabgrass and other weeds prefer to grow in bare areas and on stressed turf, healthy lawns with dense stands of grass are less likely to be troubled with these weeds. Proper fertilization, mowing, and watering will help promote a more vigorous stand of turfgrass, thus reducing the ability of the crabgrass and other weeds from becoming established. Water the lawn deeply and less often with one or two thorough soakings weekly to promote deep root growth. Mow the turf as high as possible. This will help the grass to shade out any emerging weed seedlings.
Crabgrass and other summer weeds can be a challenge to control. Proper timing and use of pre-emergent herbicides combined with correct cultural care helps maintain a healthy lawn. This regime will reduce the infestation of troublesome unsightly weeds during the growing season.
Timothy Daly is an Agricultural and Natural Resource Extension Agent with Gwinnett County. He can be contacted at 678-377-4010 or firstname.lastname@example.org.