Crabgrass is a summer annual weed that begins germinating in the spring once temperatures are at 60 degrees F for three to five days, according to the Purdue University Extension. Crabgrass then flowers in July before dying at the first frost. Lawns that are not properly maintained are more likely to have dead patches where crabgrass tends to thrive. Overseeding a lawn in fall grows a dense grass that blocks out water and sunlight, which crabgrass seeds require for germination.
Apply a post-emergent herbicide to existing crabgrass to kill it off using the instructions on the herbicide packaging. Do not spray after the middle of July since crabgrass is too mature and is not easily killed by herbicides, says Purdue University.
Roll a core aerator over the lawn to remove 3- to 4-inch plugs of dirt from the lawn and reduce thatch, which is the accumulation of dead plant material. Roll the aerator in two directions across the lawn for the best coverage. Aerate 30 days before overseeding, according to the Clemson University Cooperative Extension.
Set your drop spreader to a rate of 10 lbs. per 1,000 square feet for annual ryegrass, or 5 to 15 lbs. per 1,000 square feet for perennial ryegrass.
Spread the ryegrass seed over the lawn using the drop spreader. Slightly overlap the tire tracks from the last pass to ensure coverage. Spread half the seed in a north to south direction, and the second half east to west.
Rake the seed lightly to get it under the existing grass, and to lightly cover it with 1/4 inch of soil, recommends the University of New Hampshire Cooperative Extension.
Water the seed two to three times daily until the seeds germinate in about two weeks.
Mow the grass once it reaches a height of 1 to 2 inches, and mow again once the grass reaches a height of 2 to 2 1/2 inches.
Apply a complete fertilizer at 1/2 pound of nitrogen per 1,000 square feet after the second mowing.