Cool-season grasses, like the tall fescue and Kentucky bluegrass lawns common to many parts of Kansas, have had about enough of summer.
The state’s high temperatures – 100 degrees and higher in many parts – and sporadic rainfall often cause cool-season varieties to become thin by summer’s end. September typically signals a good time to replenish those stands, said Kansas State University horticulture expert Matt McKernan.
“When overseeding, start by mowing the grass short (1 to 1 ½ inches) and removing the clippings,” he said. “This will make it easier to achieve good seed-soil contact and increase the amount of light that will reach the young seedlings.”
Then, he says, check the lawn’s layer of thatch – an organic layer of dead and living shoots, stems and roots that develops between the zone of green vegetation and the soil surface.
“Normally, we’d want ¼ inch of thatch or less when overseeding,” McKernan said. “If the thatch layer is ¾ inch or more, it is usually easier to use a sod cutter to remove it and start over with a new lawn. A power rake can be used to reduce a thatch layer that is less than ¾ inch, but more than ¼ inch.”
Preparing the soil
Once thatch is under control, McKernan said homeowners can use a hand rake to roughen up soil in small areas. For larger areas, he suggests either a verticutter, slit seeder or core aerator machine.
“A verticutter machine has solid vertical blades that can be set to cut furrows into the soil that seed can later be broadcast into,” he said. “A slit seeder is a verticutter machine with a seed hopper added so the soil prep and seeding operation are combined.”
By contrast, a core aerator literally punches holes in the soil, depositing the cores on the surface of the ground.
“Each hole produces an excellent environment for seed germination and growth,” McKernan said. “Make 3-4 passes with the core aerator to ensure enough holes for the seed, with holes ideally spaced approximately every 3 inches.”
A core aerator also reduces the amount of water needed to germinate the seed. Aeration increases the water infiltration rate, decreases compaction and increases the amount of oxygen in the soil.
“Of the three methods (for large areas), the slit seeder is favored for obtaining good seed to soil contact,” McKernan said. “However, if watering is difficult or soil conditions are poor, core aeration may be a better option.”
McKernan noted that fertilizer should be applied to the lawn at rates suggested on the bag, regardless of which overseeding method is used.
Alicia Boor is an Agriculture and Natural Resources agent in the Cottonwood District (which includes Barton and Ellis counties) for K-State Extension. You can contact her by e-mail at aboor@ksu.edu or calling 620-793-1910.
K-State Extension is an equal opportunity employer and provider. Kansas State University, County Extension Councils, Extension Districts, and U.S. Department of Agriculture Cooperating.