Kentucky bluegrass and other cool-season turfgrasses thrive in the cool temperatures and frequent rains of spring. However, cool-season lawns often decline by mid-summer because of heat, drought, insects, diseases, or other factors. If your lawn has declined, late summer is an excellent time to revitalize it. Important late summer cultural practices include seeding/overseeding, aeration, and fertilization.
Mid-August to mid-September is the best time to seed new lawns and overseed existing lawns in Iowa. Late summer seeding has several advantages over spring seeding. The seeds of cool-season grasses germinate quickly in the warm soil of late summer. The warm days and cool nights of early fall promote rapid turfgrass growth. The growing grass also has less competition from weeds as few weed seeds germinate in fall.
When purchasing grass seed, choose a high quality seed mix that is best adapted to the site. In sunny areas, Kentucky bluegrass is the best choice. Select a seed mix that contains at least 2 or 3 bluegrass varieties. Use a mixture containing Kentucky bluegrass, perennial ryegrass, and fine-leaf fescues in partially shaded areas. The fine-leaf fescues (creeping red fescue, hard fescue, chewings fescue, etc.) are the best grasses for shady locations.
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.
Mid-September is an excellent time to aerate lawns growing in heavy, clay soils and those subject to heavy foot or pet traffic. 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. Core aerators are often available at rental agencies. If you prefer not to do it yourself, core aeration is a service provided by most professional lawn care companies.
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.
September is an ideal time to fertilize lawns. An application of fertilizer in late summer stimulates turfgrass growth and encourages the grass to fill-in thin spots. Apply 1 pound of actual nitrogen per 1,000 square feet.