I frequently have sites that are under construction through the summer and then ready for planting in the fall. A fall planting means that certain kinds of seeds, if planted when the rest of the planting is done, will not germinate until spring. This can alter the type of grass that I specify or how it is planted and it can alter the way in which other seedings are done as well.
For a fall grass seeding, I can either ensure that a cool season grass is planted or that the planting regimen is modified to meet the demands of the season. Cool season grasses for American lawns generally include fescue, bluegrass, bentgrass, and perennial ryegrass. For most of the country now is a good time to plant them. They will generally germinate at temperatures in the mid 70's and go dormant at temperatures below 50. So, time your planting accordingly. Immature seedlings can be damaged or killed by freezing temperatures. Perennial ryegrass will take five to ten days to germinate, fescue - seven to twelve, bentgrass - ten to fourteen days, and bluegrass fourteen to thirty.
There are cases though where the site really dictates that I use a warm season grass for the long term cover. Ideally, warm season grasses should be planted in the late spring or early summer. For a fall planting, they should be planted at least sixty days prior to the anticipated first frost. Under extreme circumstances, I will specify the planting of a bermudagrass in the late fall with the intention that it not germinate until spring. In this case a late planting can occur as long as the seed in unhulled and a good cover of clean grain straw is used.
With somewhat off season timing of seeding or with the planting of a bermudagrass in the fall, I can get a nice instant green lawn with the use of ryegrass seed (Lolium multiflorum). This can be planted with the bermuda or other grass seed and will germinate in five to ten days. It literally is the solution to instant green. Later the intended grass can fill in. This is an annual grass so the plants will not live for a second season, but it is being planted in this case as a temporary plant. It will need to be mowed throughout the winter and can produce seed that will germinate the following year if mowing is not done in a timely fashion, but you will reap the benefit of a green lawn that holds the soil in place until the intended grass has a chance to germinate.
For those lawns that are warm season and plugged or sprigged, annual ryegrass can help to hold the soil until the holes have filled in. It can also be planted when those lawns go dormant to keep a green lawn throughout the winter.
I often have other areas that are seeded on a site that are not grass. These might be wildflower plantings or seed mixes intended for a specific use like that used in the bowl of a bioretention area or mixes used for dry sites that will not support a good lawn. Most of the seed in these mixes will not germinate in the fall, but often fall planting is desirable. For many of the seed used, being subjected to the freezing and thawing of winter actually triggers germination. For others, fall seeding might be desirable simply because of timing. For these fall seed mix areas, I recommend that a seeding of rye grain (Secale cereale) be spread either with the seed mix or on top of it. Rye grain, sometimes called cereal rye, will germinate in temperatures as low as 34 degrees and will help to hold the seed and soil in place until spring. It also will act to enhance the soil by capturing nitrogen and recycling potassium. Ryegrass should never be used in these situations because of the possibility of re-seeding. Make sure that you get the right kind of seed, or you may end up with a grass that becomes a pest.
Regardless of what options you take, fall seeding is possible and even in many cases desirable. Just make sure that you keep in mind the ultimate goal of the seeding, and that you utilize the seed that will provide you with that need.
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