Late summer or early fall is the best time to sow grass seed, according to a University of Illinois Extension horticulture educator.

"Why? One of the main reasons is weed competition," Matt Kostelnick said. "When planting grass seed in the spring, many weed seeds will be germinating around the same time, thus competing with the new grass seedlings for water, nutrients, light and space. This makes establishing new turf difficult — especially since weed control is difficult with emerging grass seedlings."

Plus, for the home gardener, grass seeding in late summer or early fall means one less chore to have to do in the spring. It is also one that can help fill the late dog days of summer.

"There are two main reasons for sowing grass seed," Kostelnick said. "One reason is to start a new lawn from scratch on bare seedbed. The other is to fill in bare spots and/or thinning areas of the lawn. This is overseeding.”

In northern Illinois, both practices are best done in mid-August to the first week of September.

Most herbicides are too strong to be applied when grass seedlings are young, Kostelnick said. Conventional pre-emergent herbicides, for example, would not work because they would prohibit weed seeds from germinating as well as the desirable grass seeds from germinating. Also, keep in mind that these are seedlings, so they are weak and vulnerable.

"Soil and air temperatures are ideal in late summer and early fall, which encourage rapid seed germination and root growth," he added. "A disadvantage of sowing grass seed in spring is summer heat creeping up on the young grass seedlings. Summer heat slows the root and shoot growth of cool-season grasses, evaporates water quickly and can foster rapid weed growth.

"It is particularly important for the grass to develop a deep root system in order to handle summer heat stress. A shallow, undeveloped root system will cause significant stress to grass during the summer,” Kostelnick said. “With late summer/early fall plantings, summer heat will be waning, and the grass will have plenty of time to develop a strong root system before the following year's summer heat."

Establishing a strong turf stand is important in developing a healthy lawn, he said.

"If a decent lawn is developed in the fall, the following spring will be started with a healthy, established lawn," Kostelnick added. "Additionally, since the grass is established, it will be developed enough to handle normal pesticide applications such as crabgrass pre-emergent herbicides in spring as well as all other regular lawn practices. This is not the case with spring-sowed turfgrass."