Hot, dry summer weather can be tough on Kentucky bluegrass and other cool-season grasses. Good cultural practices in late summer can help revive stressed lawns. Important late summer cultural practices include mowing, overseeding, aeration, and fertilization.
In September, lower the mowing height from 3.0 to 3.5 inches to 2.5 to 3 inches. Continue to mow the lawn until the grass stops growing in late October/early November. When mowing the lawn, never remove more than ⅓ of the total leaf area at any one time.
Mid-August through mid-September is the best time to renovate a thin lawn. Sowing grass seed in late summer has several advantages over spring seeding. Cool-season grass seeds germinate quickly in the warm soils of late summer. Once the grass germinates, the warm days and cool nights of fall promote rapid turf growth. Also, there will be less competition from weeds as few weed seeds germinate in late summer and fall.
To reduce the competition from the established turfgrass, mow the lawn at a height of 1½ to 2 inches. Successful overseeding requires good seed-to-soil contact. Core aerators, vertical mowers, and slit seeders can be used to insure good seed-to-soil contact.
After seeding, keep the upper 1 inch of soil moist with frequent, light applications of water. The seeds of most turfgrasses should germinate in 2 to 3 weeks if the seedbed is kept uniformly moist. Gradually reduce the frequency of watering, but water more deeply, when the grass seedlings reach a height of 1 to 2 inches.
Early September is the best time to aerate lawns in Iowa. Aeration relieves soil compaction, improves water and nutrient movement in the soil, and prevents thatch accumulation.
Aerate lawns with a core aerator. Core aerators have hollow metal tubes or tines that remove plugs of soil. Avoid spike-type devices that simply punch holes (compacting the soil) in the ground.
Remove soil cores that are approximately three-fourths of an inch in diameter and 3 inches long. For best results, aerate lawns when the soil is moist. Avoid aeration when soils are dry or wet. The tubes or tines will not be able to penetrate deeply when the soil is dry and may get plugged with soil when the soil is wet. Lawns that are properly aerated should have 20 to 40 holes per square foot. Since most core aerators won't remove the proper number of holes with a single pass, several passes are often necessary. After aeration, pull a drag mat or weighted piece of fencing material across the lawn to break up the soil cores on the soil surface.
Mid-September is an excellent time to fertilize lawns. An application of fertilizer in late summer stimulates turfgrass growth and encourages the grass to fill-in thin spots. Apply no more than 1 pound of actual nitrogen per 1,000 square feet.