When overseeding or establishing a new lawn, it is crucial to plant grass seeds when daytime temperatures are warm enough to support germination.
Planting grass seeds when soil temperatures are too cold will cause them to go dormant and possibly rot.
Warm-season grasses are planted in the spring because they require warmer temperatures to germinate.
However, cool-season grasses are planted in the fall since they grow best in cooler temperatures.
So, how cold is too cold for planting grass seed?
It is too cold to plant grass seeds when soil temperatures are below 50° degrees Fahrenheit (10° C). In the fall, soil temperatures may be 10° degrees cooler than air temperatures because of the gradual decrease in sunlight. Never plant grass seeds when there is a risk of frost in the weather forecast.
If the ambient air temperature is consistently lower than 60° degrees Fahrenheit (16° C), it is likely too cold to plant grass seed.
Even the hardiest cool-season grass will not sprout in low winter temperatures.
Keep reading for more information about planting grass seeds in cold weather.
Table of Contents
How Cold Will Grass Seed Germinate?
The lowest soil temperature for grass seed germination is 50° degrees Fahrenheit (10° C), but the ideal temperature range varies according to grass type.
Warm-season grasses germinate in soil temperatures ranging from 65 to 80° degrees Fahrenheit (18-27° C), so it is best to plant this type of grass in the spring.
As a rule of thumb, cool-season grass seeds must be planted at least six weeks before the first frost.
This allows plenty of time for germination, so the grass becomes established enough to withstand harsh weather in the winter months.
Check your local weather forecast for frost before planting your grass seeds.
The following table shows the optimal germination temperatures for the most common varieties of cool-season grasses.
|Ideal Daytime Air Temperatures for Germination In Fahrenheit (Celsius)
|59-86° F (15-30° C)
|59-86° (15-30° C)
|68-86° F (20-30° C)
|59-77° F (15-25° C)
|68-86° F (20-30° C)
Will Snow Kill Grass Seed?
Snow will not kill unsprouted grass seeds but will cause them to go dormant.
The sudden drop in temperature from a layer of snow will halt grass seed germination until the snow has melted and ground temperatures recover.
If your grass seeds have already sprouted, a thick layer of snow may cause damage.
When the snow has a chance to melt and refreeze, it will form a layer of ice.
This ice layer will cause your tender grass blades to become brittle and prone to tearing, and your lawn may not be able to recover fully.
Avoid walking on grass covered by snow or ice to prevent further damage to new and established lawns.
You may also consider covering your newly planted grass seedlings with a tarp at night to protect your grass from frost.
In established cool-season grass lawns, snow may be both helpful and harmful.
A heavy layer of snow on your lawn will act as an insulator to protect your grass from frigid temperatures and retain moisture.
Snow also contains nitrogen, released into the soil as it melts to encourage healthy grass root growth.
However, the excess moisture from heavy snow may cause a fungus known as snow mold.
Snow mold thrives on poorly drained soil, especially when there is a lot of leaf litter on the lawn.
Winter lawn damage from snow mold is not typically seen until the snow has melted, leaving you with patches of dead, bleached, or matted grass.
How Long Does It Take for Grass Seed To Germinate?
The time for germination depends on the variety of grass seeds and the weather conditions.
Cool-season types of grass tend to germinate more quickly than warm-season grass.
Kentucky bluegrass is the exception to this rule, which grows much slower than other cool-season grass species.
With the optimal temperature range and moisture levels, it will take 5 to 30 days for most cool-season grasses to germinate.
The table below shows the average germination time for common varieties of cool-season grasses.
|Average Time Range for Germination
Further Reading: Speed up grass growth through pre-germination
Why Are Soil and Air Temperatures So Different?
Soil is more insulating than air because it has pore space to trap air and moisture.
This allows warm soil to retain heat on cold days and cold soil to stay cool in warm weather.
Soil temperature may be 10° degrees warmer or cooler than the surrounding air temperature.
Factors affecting soil temperature include:
- Soil moisture levels
- Pore space
- Soil depth
- Plant cover
Wet soil stays cooler and experiences fewer temperature fluctuations than dry, sandy soil.
As the layer of moisture evaporates, the soil gradually becomes warmer.
When wet soil freezes, the frost layer causes the ground to expand and blocks the pore spaces.
As a result, frozen soil becomes very rigid, making it difficult for nutrients or plant roots to penetrate.
Compacted soil lacks adequate pore space for storing air and moisture, so it is less insulating than well-aerated soil.
Temperatures in the upper layers of soil will vary more than in the deeper layers.
Measuring at least 4” inches deep when taking soil temperatures is recommended to get an accurate reading for planting.
Deeper soil layers take much longer to respond to temperature changes.
Once you reach a depth of around 30” inches, temperatures remain more constant.
Bare soil is also more responsive to temperature changes because it lacks plant cover to provide insulation from sunlight, moisture, and frost.
Further Reading: Top layer planted grass seed and how it grows
What Is Dormant Seeding?
When you plant your grass seeds after the grass has gone dormant (post-regular growing season), this is known as dormant seeding.
Winter seeding allows the grass seedlings to stay dormant until temperatures are warmer.
The advantages of dormant seeding are less maintenance after planting and increased grass growth in the spring.
The ideal time for winter sowing is when the ground is frozen, so the grass seedlings stay dormant.
A warm spell after dormant seeding could cause the seeds to begin germinating.
There is a risk of the seeds rotting in the ground when the weather gets cold enough for them to become dormant again.