Grass seed germination occurs when the seed absorbs enough moisture to develop roots and start sprouting. The soil temperature regulates how quickly this process occurs. If the environment changes too much once the seeds begin to sprout, the seed can die.
Plant grass in the spring, after the danger of frost has passed, or in the fall, before the frost will set back in. Cool-season grass does best with a soil temperature between 10 and 18.3 degrees Celsius. Warm-season grass seed must have warmer soil to germinate--between 18.3 and 21.1C.
To ensure the proper germination temperature, sow warm-season grass seed in the spring if possible. If you must plant varieties like Bermuda grass and oysia grass in the fall, make sure it is no later than 90 days before the first expected frost if you live in northern states. In the south, you can push that to 60 days before the first frost. This gives the seed time to develop stronger roots before the soil cools down too much.
Grass seed can handle some temperature fluctuations when it is germinating, as long as they are not extreme. Cool season grass can handle soil temperatures as low as 7.22 to 12.8 degrees Celsius. Warm season grass needs a soil temperature of 12.7 to 18.3C. However, both types do better in slightly warmer conditions. But, remember that, according to the Garden Counselor, soil temperatures are usually cooler than air temperatures. And soil takes longer to warm up than the air.