Extremely hot or cold temperatures will not kill grass seeds. However, temperatures that continue to switch between freezing cold and above freezing are likely to kill grass seed. This is because the freeze-and-thaw cycle attracts mold and moisture that destroys grass seeds. Additionally, grass seed in these conditions may sprout during a brief warm period. Then, the new grass seedlings can be killed by a sudden frost. To avoid killing your grass seed, it’s best to seed your lawn when there is no chance of frost for at least the next 6 weeks.
Is it OK for Grass Seed to Freeze?
It is perfectly fine for your grass seed to freeze as long as it has not been spread on your lawn. You can store grass seed in a garage or garden shed in winter without any negative effects. Grass seed is biologically designed to remain dormant during cold weather, then sprout once daytime temperatures rise well above freezing.
- Grass seed stored in bags or other containers will not be damaged by freezing cold.
- Seeds remain dormant so they can ride out cold weather.
- Seedlings that have just sprouted are more vulnerable to cold weather than unsprouted grass seeds.
While some cool-season grasses can survive winter seeding, every grass seed is vulnerable to cold temperatures when they first sprout. To increase the percentage of grass seeds that grow into new grass plants, do not seed in spring until the danger of frost is past. If you are seeding in the fall, spread your grass seed at least 6 weeks before the first average fall frost.
What Temperature is Bad for Grass Seed?
The worst conditions for grass seed are when the temperature is shifting between nighttime frosts and warm days. Frost brings a layer of moisture to the ground, then thaws it during the day. This can cause mold to form on seeds, killing them.
- Weather that switches between frost and warm spells is the worst condition for grass seeds.
- Grass seeds exposed to freeze-and-thaw cycles can be destroyed by mold.
- If your seedlings have just sprouted, a frost can kill them overnight.
A few days of warm weather followed by a cold snap is the worst possible situation for grass seed. After a few days of warm weather, the seeds will germinate and send out their first roots. If a frost arrives at this time, it will freeze the top layer of soil. This freezes the baby grass roots and prevents them from pulling in nutrients or water. A single hard frost can wipe out large quantities of grass seedlings. If your seedlings have just sprouted and frost is on the way, take these steps to protect new grass from frost.
Can You Plant Grass Seed in 40-Degree Weather?
Daytime highs of 40℉ (4℃) are too cold for sowing grass seed. On average, soil temperatures are 10 degrees Fahrenheit (5 degrees Celsius) colder than air temperatures. So, your soil may still be at or below freezing. This can place grass seed in the danger zone where they are partially frozen and thawed. This weather will attract mold that destroys grass seed before it sprouts.
Can You Plant Grass Seed in 50-Degree Weather?
Daytime air temperatures around 50℉ (10℃) are still not warm enough for grass seeding. Cool-season grass seeds will sprout in temperatures between 70–80℉ (21–27℃). Warm-season grass seeds germinate when daytime highs are at or above 80℉ (27℃). Because new grass seedlings are very delicate and can easily be killed, it’s best to spread them in the optimal temperature range. This will ensure more of your seeds grow into lush, green grass.
At What Temperature Should You Overseed Your Lawn?
Sow warm-season grass seeds in spring when daytime temperatures first reach a high of 80 (27℃). For an even more precise gauge, use this online map to plan your grass seeding once the average soil temperature reaches 70℉ (21℃). This is the best choice for the following types of grass: Bermuda, Centipede, St. Augustine, and Zoysia.
- Warm season grass should be overseed in spring when daytime temperatures first reach 80℉ (27℃).
- Cool season grass respond bests when it is seeded in fall once daytime highs drop to 75℉ (24℃).
If you are seeding cool-season grass, spread your grass seed in the fall when daytime temperatures are around 75℉ (24℃). An average soil temperature of 60–70℉ (16–21℃) is best for cool-season grass. If you’re not sure if your type of grass is in the cool-season family, the most common examples are Kentucky Bluegrass, Fine Fescue, Tall Fescue, and Perennial Ryegrass.
Will Cold Temperatures Kill Grass Seed?
Extreme cold won’t kill grass seed but alternating hot-and-cold will. Keep these rules in mind when planning your grass seeding:
- Grass seed that has not sprouted can survive temperatures well below freezing.
- Grass seed is most vulnerable to deadly mold when temperatures are alternating between frosty nights and warm days.
- New grass seedlings that have just sprouted are easily killed by nighttime frosts.
- Sow warm season grass seed in spring when daytime temperatures first reach 80℉ (27℃).
- Spread cool season grass seed in fall when temperatures first fall to 75℉ (24℃).
- If you are seeding grass in fall, seed at least 6 weeks before the first average fall frost.
By avoiding cooler temperatures, you will seed your grass in conditions where it grows fastest. At the same time, seeding during warmer temperatures helps you avoid out-of-season frosts that can kill newly sprouted grass.