AMES, Iowa -- Horticulturists with Iowa State University Extension and Outreach answer questions about lawn seeding, from what to look for in a grass seed mix to how frequently to water a newly seeded area. Homeowners and gardeners with lawn questions should contact horticulturists at Hortline by emailing firstname.lastname@example.org or calling 515-294-3108.
Late summer (mid-August to mid-September) is the best time to establish a new lawn from seed and to 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. Once the seeds germinate, 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, select a high quality seed mix that is best adapted to the site. Kentucky bluegrass is the best choice for sunny areas that receive at least six hours of direct sun each day. Choose a seed mix that contains at least two or three bluegrass cultivars. Because Kentucky bluegrass is slow to establish from seed, perennial ryegrass is often included in bluegrass mixes to speed establishment. The fine-leaf fescues (creeping red fescue, hard fescue, chewings fescue, etc.) are the best grasses for shady locations. In lawns that contain sun and shade, select a seed mix that is approximately 60 percent Kentucky bluegrass, 30 percent fine-leaf fescue and 10 percent perennial ryegrass. Kentucky bluegrass will be the dominant grass in the sunny areas while the fine-leaf fescues will thrive in the shaded portions of the lawn.
The first step in planting a new lawn is to establish the rough grade. Remove construction debris, then fill in low spots and level off high areas. The ground should slope away from the foundation of the house, drive and sidewalks. The rough grading should be done well in advance of seeding to allow settling to occur.
To determine soil fertility, conduct a soil test. Apply the recommended fertilizer, then incorporate it into the soil. Where a soil test has not been made, apply 10 pounds of 10-10-10 or a similar analysis fertilizer per 1,000 square feet and till it into the soil. The final step in soil preparation is hand raking the area. This is also the last opportunity to establish the final grade. Immediately prior to seeding, apply a starter lawn fertilizer. A starter lawn fertilizer is high in phosphorus.
Apply the seed with a drop-type seeder or by hand. The basic requirement is uniform distribution over the area. Sow half the seed in one direction; the remaining half should be applied at a right angle to the first application. After sowing the seed, lightly rake or drag the area. The seed should be covered to a depth of 1/8 to 1/4 inch. Roll the area lightly to ensure good contact between the seed and soil.
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. Simply throwing or broadcasting seed over the lawn typically results in poor seed germination, as much of the seed rests on the thatch layer or soil surface. Core aerators, vertical mowers and slit seeders can be used to ensure good seed-to-soil contact.
Core aerators are machines with hollow metal tubes or tines. They remove plugs of soil when run over the lawn. To prepare the site, go over the lawn three or four times with the core aerator. When finished, there should be 20 to 40 holes per square foot. Apply the seed with a drop seeder. Afterward, drag the area with a piece of chain link fence or drag mat to break up the soil cores and mix the seed into the soil.
It’s also possible to prepare the site with a vertical mower. When run over the lawn, the knife-like blades of the vertical mower slice through the thatch and penetrate into the upper 1/4 to ½ inch of soil. One or two passes should be sufficient. Afterwards, remove any dislodged debris from the lawn. Sow grass seed over the lawn with a drop seeder. Work the seed into the soil by again going over the site with the vertical mower.
Large areas also can be overseeded with a slit seeder. A slit seeder makes small grooves in the soil and deposits the seed directly into the slits.
Core aerators, vertical mowers and slit seeders can be rented at many garden centers and rental agencies. If you would rather not do the work yourself, many professional lawn care companies can overseed your lawn.
After seeding, keep the upper 1 inch of soil moist by watering once or twice a day. With adequate moisture and warm soil temperatures, the seeds of most turfgrasses should germinate in two to three weeks. When the grass seedlings reach a height of 1 to 2 inches, gradually reduce the frequency of watering, but water more deeply.