Susan J. Lawhon

February 2, 2012

Crabgrass is probably the most hated grassy weed in home lawns. Fortunately, it is an annual weed. Annual grass weeds germinate from seed, grow vegetatively, produce seed and die within a 12 month period. In addition to crabgrass, there are a number of other annual grass weeds, such as goosegrass, foxtail, barnyardgrass, fall pancium and annual bluegrass. Crabgrass and other annual grass weed normally germinate around early April. Hence, March is the perfect time to apply crabgrass preventer so that crabgrass seeds will not germinate.

To ensure crabgrass and other annual grass weeds do not establish in home lawns, both important preventive and control programs must be implemented. The invasion of crabgrass and other annual grass weeds can be prevented to a large degree by maintaining a dense, healthy stand of grass. A high quality lawn will develop a highly competitive canopy which will shade the soil surface and discourage the germination and establishment of seedling annual grass weeds.

The best way to stop crabgrass and other annual grass weeds from establishing in their lawns is through the use of pre-emergent herbicides. Pre-emergent herbicides are chemicals that prevent the germinating weeds from establishing in the lawn. These herbicides control annual grass weeds by inhibiting cell division in the young root system. The failure of the root system to develop results in the death of the young seedling weed shortly after germination. Lawns with thin stands of grass that do not provide 100% cover may require yearly applications of a pre-emergent herbicide to prevent the invasion of crabgrass and other annual grass weeds. Dense, high-quality lawns may not need yearly applications since crabgrass only occasionally establishes in lawns with good density.

There are quite a few pre-emergent products that will prevent crabgrass from germinating. The Common nems of active ingredients are Benefin, Oxadiazon, Benefin/Trifluralin, Pendimethalin, Dithiopyr, Prodiamine, and Corn Gluten. One word of caution, do not apply pre-emergent herbicide (unless the active ingredient is Siduron) if you were planning on re-seeding. Siduron is the only herbicide that will provide control of the annual grass weeds but not injure or antagonize the development of the desirable seedling turfgrasses. Siduron may be applied at the time of seeding. Refer to your product label for more information.

Pre-emergent herbicides are generally only effective if applied before the annual grass weeds emerge. Therefore, early spring applications are essential if satisfactory weed control is to be achieved. Herbicide applications should be completed and the herbicide watered-in at least 7 days prior to the initial germination date to allow time for the herbicide barrier to be established in the soil. Pre-emergent herbicide applications for annual bluegrass control should be made in late summer or early fall.

March is also a good time to fertilize your lawn. If you fertilized your lawn late last fall, you should use half of the suggested rate now. Otherwise, your lawn could turn into a hay field.

Dr. Gary Gao is the horticulture extension educator, master gardener volunteer program coordinator and an associate professor with Ohio State University Extension in Delaware County. He can be reached at 740-833-2030.