One of the most important lawn care tasks in early spring is the application of pre-emergent herbicides (weed killers) to prevent weed seeds from germinating. They are commonly used for controlling summer annual grasses, such as crabgrass, goosegrass, and other lawn weeds. The pre-emergents will not control established emerged 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 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 germinating when the soil temperature reaches 50 to 60 degrees. Pre-emergent herbicides, such as Scotts Halts, Pennington Crabgrass Preventer, and Hi-Yield Crabgrass Preventer, amongst others, are the optimum method of preventing these weeds from becoming established in your lawn.
Once crabgrass and other grassy weeds have emerged, control options using herbicides are few and generally have limited effectiveness. Make sure you follow all label directions and safety precautions when using chemical pesticides. Pre-emergent herbicides are mostly granular formulations and can be applied with a drop or rotary spreader. After making the application, water the turfgrass thoroughly. These herbicides are recommended only for lawns that are well established, and should not be applied to lawns that have been recently seeded or newly sodded.
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 prematurely 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 help to maintain a healthy lawn. This regime will reduce the infestation of troublesome unsightly weeds during the growing season.
Timothy Daly, MS is an Agricultural and Natural Resource agent with the Gwinnett County Cooperative Extension. He can be reached at 678-377-4010 or firstname.lastname@example.org.