University of Nebraska Cooperative Extension in Lancaster County

Overseeding Cool-Season Lawns

Don Janssen, Extension Educator

Hot weather, weeds, insects, and disease often take their toll on lawns by the end of the summer. Fall is the best time to renew bluegrass and fescue lawns. Overseeding is more than scattering seed and waiting for it to germinate. Seedbed preparation is as important for overseeding as it is for establishment of new turf. So before scattering expensive seed, take time to prepare the turf and correct any problems. 

Planting certified seed assures the genetic purity and seed quality designated on the seed tag. Certified seed is planted, grown, harvested, processed and marketed following strict standards. Planting seed of questionable or unknown origin and performance is a gamble. 

The first step in renovating a lawn is to determine and correct the conditions causing the problem. If this is not done, the problem will likely occur again. Common causes of thin lawns are poor soil, thatch (over 1/2 inch), insects, disease, drainage, wrong grass for the site, old varieties and improper care. 

The best weed control is planting at the right time and maintaining a thick stand of grass. Plant in September and then fertilize, water, and mow to establish the grass before next spring when weeds invade the turf. 

Do not use crabgrass preventers before seeding or on young grass. Tupersan (siduron) is the exception. 

Do not use broadleaf weed killers (2,4-D, MCPP, or dicamba) 1 month before seeding or on new grass until it has been mowed three times. 

Roundup can be used to kill all vegetation before seeding(5-day waiting period). 

Mow the lawn short, 1-1/2 inches before overseeding to keep grass and weeds from competing with new seedlings. Short mowing helps prevent the seed from lodging in tall grass and lets light reach the new seedlings. Use a grass catcher on the mower or sweep up excess clippings. Do not scalp off all vegetation—some is needed to protect the germinating seed from wind, hot sun, and heavy rain. 

To get the seed into the soil, cut through the remaining vegetation and open the soil. High thatch levels have a detrimental effect on high maintenance turf. It is important to have less than 1/4 inch of thatch when overseeding. If thatch is 3/4 inch or more, it is easier to strip off the thatch with a sod cutter and reseed. A power rake, available at most nurseries and rental agencies, can be used. Choose a heavy-duty machine with solid, thin blades (1/16 to 1/8 inch). Thick blades damage and beat the turf, rather than slice, the soil. Set the machine to cut grooves 1/8 to 1/4 inch deep. 

Power-rake in two directions at right angles to each other. The soil should be moist, but neither too wet nor too dry for power raking. If the machine does not have a catcher, the debris will have to be raked up and removed. 

A core aerator, available from nurseries or rental agencies, pulls plugs of soil from the turf and gives the seed a place to germinate and establish. Core aerators are especially useful for clay
soil or compacted ground. Go over the turf three to five times. 

Selecting the exact grass for the right situation is necessary for a successful lawn. Overseeding is an opportunity to change the grass species or to add an improved selection. Check with the County Extension Office for the latest recommendations on improved grasses based on University research. 

September is the best time to seed. This is when nature establishes cool-season grasses; therefore, the best results with the least amount of work occur at that time. Do not delay seeding or the grass will not establish sufficiently before winter and weeds like henbit and chickweed become a problem. March and April are second choices for seeding.

Seeding rates are as follows:

Kentucky Bluegrass—2-3 pounds/1000 sq. ft. 

Tall fescue—6-8 pounds/1000 sq. ft. 

Perennial Rye—5-6 pounds/1000 sq. ft.

Use a rotary or drop-type grass seeder rather than applying the seed by hand. Rotary seeders are faster than the drop type. Be sure the seeder is set properly to apply the correct amount. 

If the soil has been furrowed with a power rake or prepared with other equipment, it will not be necessary to cover the seed after sowing. To help soil/seed contact, run the power rake over the surface after seeding. Watering will settle the seed into the soil. 

The seedbed must be continually moist during seed germination. If the germinating seeds or seedlings dry out even briefly, they will die. 

Water with a gentle spray so as not to dislodge, wash, or move the seeds around while they are germinating. Daily watering will be necessary at first, then gradually return to a normal watering schedule. 

If only a few spots need to be reseeded, they can be prepared with a hand rake. Sow the seed uniformly by
hand. A thin layer of fine soil can be spread over the seed or it can be worked in with the rake. 

New seedlings should grow as much as possible before winter. Apply a fertilizer indicated by a soil test or a complete slow release fertilizer during seeding to get the seedlings off to a rapid start. Avoid soluble fertilizers or excessive amounts of fertilizer because they will burn and kill the new seedlings. Fertilize about 4 to 6 weeks after seed germination with nitrogen to sustain growth and build up food reserves in the root system. This makes the new grass winter hardy and green-up earlier in the spring. November is the last month to fertilize. 

Mow new cool-season grass as soon as it is 3 inches tall with the mower set at 2 inches. Continue to mow at that height for the remainder of the season, including the last mowing. Soak the lawn thoroughly when the weather is dry. Also, water once or twice during dry winters when the soil is not frozen.

Back to Lancaster County Factsheet Index.  
University of Nebraska Cooperative Extension educational programs abide with the nondiscrimination policies of the University of Nebraska and the United States Department of Agriculture.