University of Illinois Extension

Crabgrass

Crabgrass is just one of many annual grassy weeds which cause problems in home lawns, said a University of Illinois Extension horticulture educator.

“Its thick, hairy blades can smother the more desirable lawn grasses during the summer,” said David Robson. “And when frost hits the plant, you’re left with large, brown patches of dead grasses. Unfortunately, in most cases it’s too late to re-seed or sod the area.”

A proactive approach offers the best opportunity to control crabgrass.

“Crabgrass germinates during the spring and early summer when soil temperatures start reaching 60 degrees F for five consecutive days,” he said. “If the temperature cools, the process has to start all over. This is one reason for the extended germination time.”

Crabgrass seeds also need light in order to sprout. Thick, dense turfgrass is the best defense, though lawns tend to be somewhat sparse in early spring, especially if they’ve had previous crabgrass problems or have been improperly fertilized the previous season.

“The primary means of controlling annual grassy weeds is by providing a vigorous, dense, competitive turf coupled with pre-emergence herbicides,” said Robson. “Pre-emergence herbicides are applied to prevent the appearance of crabgrass and other annual grassy weeds in turf areas. These herbicides can persist in the soil for several months and control annual grasses through the growing season.

“The herbicide forms a chemical barrier or blanket at the soil surface and just below that prevents grass development from germinating seeds. The new shoots and roots of germinating seeds absorb the herbicide and are killed.”

Pre-emergence chemicals should be applied one to two weeks prior to the time when soil temperatures reach 50 degrees F for three consecutive days. For this reason, in a typical year crabgrass applications should be applied by April 1 for best control in central Illinois. Add a week or two as you move farther north, and subtract a week or two going south.

“For extended crabgrass control, apply a second application of pre-emergence herbicide four to six weeks after the first,” said Robson. “Herbicides to control annual grasses normally require irrigation following application to be effective, and all turf cultivation activities should precede application.

“Remember to always read and follow label directions for safe pesticide use and effective pest control. The label should list tolerant species, controllable weeds, and application rates. Most herbicides that control crabgrass will also affect germinating grass seed, so avoid using crabgrass-preventing herbicides when seeding.”

Sound cultural practices can also help to control annual grassy weeds, he added.

“Don’t cut turf too short. Low-mowing can lead to an open turf stand in which weed seed can easily germinate,” he said. “Cut turf at a height appropriate for the turf species and season.

“Watch for natural thinning areas such as next to driveways, sidewalks, and patios. Don’t forget alleys, that can also be breeding ground for seeds.”

Turf should be watered deeply, but not frequently. This enables the soil surface to dry between irrigations. Constantly moist soil surface increases weed seed germination and seedling survival.

“Supply the appropriate fertilizers so that necessary nutrients are available for turf during the spring flush of growth,” said Robson. “Fertilizer application during late spring and early summer helps to supply nutrients to weed seedlings and plants. Large amounts of soluble nitrogen may injure turf and reduce turf density, allowing weeds to germinate. Fall applications may be better for the turf as weedy plants use less of the nutrients.

“Control diseases, insects, soil compaction, traffic, and other turfgrass stresses. Maintain a dense turf to reduce annual grassweed seed germination.”