If you want to control crabgrass in your lawn, proper application of material and timing are critical.
Here are questions about crabgrass control with answers from horticulturists at Iowa State University Extension.
For more information, contact Hortline at 515-294-3108 or firstname.lastname@example.org.
Q: When should I apply a preemergence herbicide to my lawn to control crabgrass?
A: Preemergence herbicides must be applied before the crabgrass seeds germinate. If the material is applied too early, crabgrass seeds that germinate late in the season will not be controlled. If applied too late, some crabgrass seeds will have already germinated.
Preemergence herbicides should normally be applied in early to mid-April in southern Iowa and Illinois, mid-April to May 1 in central Iowa and Illinois and late April to early May in northern areas of the states.
The weather in late winter and early spring often varies considerably from year to year. Accordingly, gardeners should make minor adjustments in timing. If the weather in March and April is consistently warmer than normal, apply the preemergence herbicide early in the normal time period. Apply the herbicide late in the recommended time period if the area is experiencing a cold spring.
If you’re still uncertain as to when to apply the preemergence herbicide, Mother Nature provides some helpful (colorful) clues. Preemergence herbicides should be applied when the forsythia blossoms start dropping or when redbud trees begin to bloom (color). Crabgrass seed germination typically begins after these events.
To insure the herbicide is applied properly, carefully read and follow the label directions on the package. Also, make sure the spreader has been correctly calibrated and is working properly.
Q: Is there a non-chemical product that prevents the germination of crabgrass seeds?
A: Products containing corn gluten meal can be applied to lawns in spring to prevent the germination of crabgrass seeds. Corn gluten meal is a corn milling byproduct. When applied in spring, it interferes with the germination of crabgrass seeds.
Corn gluten meal is also an organic fertilizer. It contains about 10 percent nitrogen. As a preemergence herbicide, corn gluten meal becomes more effective after two or three years of repeated application. Corn gluten meal products should be applied at the same time as chemical preemergence herbicides.
Corn gluten meal products are not as widely sold as chemical preemergents, but can be found at garden centers and other retailers.
Q: Can I successfully sow grass seed after the application of a preemergence herbicide?
A: Preemergence herbicides, such as benefin, bensulide, dithiopyr, pendimethalin and prodiamine, prevent the germination of crabgrass seeds in spring. They also prevent the germination of turfgrass seeds.
If you intend to sow grass seed in spring, the only preemergence herbicides that can be used are siduron (Tupersan) and mesotrione. Siduron and mesotrione effectively control crabgrass without affecting the germination of turfgrass seeds. Siduron and mesotrione are found in starter lawn fertilizer/weed preventer products at garden centers and other retailers.
Grass seed can be successfully sown in late summer (mid-August to mid-September) after a spring application of a preemergence herbicide. By late summer, the preemergence herbicide will have broken down and not interfere with grass seed germination.