Identifying Crabgrass

Crabgrass is one of the most prevalent grassy weeds in Midwest lawns. Crabgrass thrives in full sunlight and high temperatures and can easily out compete common, cool-season, desirable lawn grasses. Crabgrass is a summer annual that has a life of less than a year. It germinates in the spring, grows through the summer and dies with the first hard frost. It produces a tremendous amount of seed in the late summer that can remain dormant in the soil for years. If you have crabgrass in your lawn, it will be there next year, too. It has a light green broad-bladed leaf and a crow’s foot seed head. Homeowners who complain of crabgrass infestation in April and early May are usually identifying tall fescue, nimblewill, or quack grass.  It is important to know the difference because these are perennial weed grasses that live through the winter and will not be controlled by a preemergent application.

Cultural control

The most effective way to control crabgrass is to create a dense, healthy turf. A healthy turf will compete well with crabgrass and prevent it from establishing itself. Important cultural controls are proper mowing and irrigation practices. Mow high (3 inches) and frequently so that only 1/3rd of the blade is removed at one time. When irrigation is necessary, irrigate deeply and infrequently.

Pre-emergent chemical control

Pre-emergent treatments are preferred because they are generally more effective for crabgrass control and less injurious to the turf grass than post-emergent treatments. Pre-emergent treatments must be applied before the crabgrass germinates. In Iowa, crabgrass germination typically begins in early May when soil temperatures reach 62 degrees F. at a depth of 1 to 2 inches, or after the air temperature remains above 70 degrees F. for three consecutive days and nights. Pre-emergent chemicals form a barrier on the soil surface which is absorbed by the seedlings and stops their development. It is important not to break the barrier by core aeration or deep raking after the application.

The product may be applied as a liquid, a granular, or as a granular in combination with a turf fertilizer. Do not use pre-emergent controls on new seedlings or before seeding an area.

Post-emergent herbicides

Post-emergent herbicides control crabgrass after it has emerged and are most effective on small crabgrass plants. These products are more difficult to use than preemergent controls and it is extremely important to follow label instructions to avoid damaging the desirable grass. A second application may be needed within seven days for effective control.
Do not attempt to control crabgrass with herbicides after mid-July because crabgrass plants are usually too large to control effectively. It is better to simply tolerate the crabgrass until it dies with the first frost.

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