Although the United States Patent Office imported crabgrass in 1849 as a promising feed for cattle, the annual soon escaped cultivation. Now it infests newly seeded lawns throughout the United States. Preemergent herbicides control crabgrass, but lawn seeds and herbicides rarely mix.
Wait eight to 16 weeks to plant seed after using most crabgrass preventatives. These preemergent herbicides are only effective when applied in early spring, before crabgrass seeds germinate, but create a lasting chemical layer on the soil that kills spring-sown grass seed. Apply preemergent herbicides in spring and seed your lawn in fall, avoiding the summer heat. Read the herbicide manufacturer's label for specific timing.
Siduron is a preemergent herbicide that only targets the seeds of crabgrass, foxtail and other weeds. Fescue, Kentucky bluegrass and other turf grasses are safe from this herbicide, and manufacturers often include it in starter fertilizer mixes. You may apply siduron and grass seed at similar times.
Don't use crabgrass prevention on very young lawns, even if the majority of the grass seed has sprouted. Allow the grass to grow until it has required at least three mowings. Hand-dig or individually treat existing patches of crabgrass with a targeted, wide-spectrum herbicide.