Hort Q&A: When to plant grass seed

Carla Smith, horticulture educator
Pottawatomie County Extension Service

When should I plant grass seed for the fall?

The period mid-September through early October in Oklahoma typically has near-ideal day/night temperature combinations for germination of cool-season grasses. So, let the tall fescue, perennial ryegrass and Kentucky bluegrass seeding begin (if you have access to water)! Sodding of these grasses is also appropriate at this time. (Note: seed Bermuda in the spring when soil temperatures warm up.) The best temperatures for germination are when we experience a mid-80s day and upper 50s/low 60s night. You might be asking, is it possible that we will get fooled and the temperatures will shoot back up. Sure, anything is possible in Oklahoma, but what is key to remember is the nighttime lows are what’s important. When you see evening temperatures from the upper 50s to mid-70s, it’s time to seed cool-season lawns. So even if a few day-time highs slip back in the mid to upper 90s, (and it will happen) our day-time lows are looking great!

Fact sheet HLA-6418 covers turfgrass selection, while HLA-6419 covers the establishment (planting method) and HLA-6420 covers the mainstream long-term maintenance practices (mowing, fertilization, irrigation, etc.). A newer fact sheet, HLA-6608, addresses managing turfgrass in the shade.

There are many satisfactory performing tall fescues. These include, but are not limited to Crossfire II, Houndog V, Millenium, Rembrandt, Plantation to name just a few. There are dozens of good performers. A blend is a combination of two or more varieties within the same species. A mix is two or more species combined. Blends and mixes are beneficial in cool-season lawns as they broaden the genetic diversity present. In theory, this decreases the likelihood that your lawn will be completely wiped out by a single disease or single insect infestation.

Most importantly, if turf-type quality is expected, choose a turf-type rather than a forage type tall fescue. Forage type fescues include Fawn and Alta. General purpose soil stabilizer types include the old K-31, Kentucky 31, KY 31, they get used as a forage and as a lawn, but these variations on Kentucky 31 are not true turf-type tall fescue despite what the marketing message on the seed bag might say. Turf-types are selected for improved color, texture, density, slower vertical leaf expansion rate and other important characteristics for lawn use.

Tall fescues are best in medium to light shade. There are no hard and fast rules for “hours of sunlight” required. There are no perfect solutions to dense shade where grasses fail repeatedly, year-in and year-out. It is best to take a hint if grass is failing in a shaded site many years, it’s time to move on to mulches, shade tolerant perennial ground covers, hardscape elements, etc.; a list of alternate shade tolerant plants can also be found in fact sheet HLA-6608. Sometimes grass does not die exclusively from shade, but rather the combination of shade and tree root competition for nutrients and water in combination with added disease pressure due to less air movement and more grass canopy moisture caused by less air movement in a “tight and mature” landscape.

In lightly shaded areas, mixtures of tall fescue and Kentucky bluegrass can sometimes work best. While Kentucky bluegrass is generally not as shade tolerant as tall fescue, it still has some shade tolerance, and it has improved brown patch disease and Rhizoctonia blight resistance over that of tall fescue. Brown patch is usually the most serious disease of tall fescue. These mixtures will often have Kentucky bluegrass present at 5 to 10% by weight and tall fescue at 90 to 95%. There are 10 times as many bluegrass seeds in a pound of bluegrass as there are tall fescue seeds present in a pound of fescue, so we use about 10 times less bluegrass seed to get to a 50/50 species count. Never use a 100% stand of Kentucky bluegrass in most areas of Oklahoma because pure stands of Kentucky bluegrass in most of Oklahoma can get summer patch disease. Also, older Kentucky bluegrasses such as Park, Newport, South Dakota Common (SD Common), Kenblue and variety not stated (VNS = when there is no variety name stated) really don’t bring any value to the cool-season mix. So, if these are the only ones available locally, you might as well use 100% tall fescue. Most other varieties of Kentucky bluegrass that you might encounter (there are hundreds nationally, and yet few repeatedly available in Oklahoma from year to year) are improvements and will benefit the mix!

There is seldom any benefit and there is often detriment created by mixes of cool-season perennial grasses with annual or Italian ryegrass. Yet, if you scout the store shelves, you will find these mixes. Annual ryegrass simply competes with the cool-season perennial grasses in the mix in the cool portion of the year when good growth can take place and then annual ryegrass, having taken its fair share of the lawn, dies out in the heat. This leaves uninformed consumers in a panic at worst and with unsightly dead areas in their remaining cool-season perennial lawn at best. Avoid mixes of annual ryegrass with the desirable cool-season perennials like tall fescue, perennial ryegrass, and Kentucky bluegrass.

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