Grass Seed Germination Temperature? Air Temperature Vs Soil Temperature

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When planting grass seed it is important to know that if the temperature is too cold or too hot the seed will fail to germinate or to germinate properly. So, what is the right grass seed germination temperature?

This will largely depend on the type and variety of grass seed that you are growing. Some grass varieties only grow in warmer temperatures and these are known as warm-season grasses. Other varieties grow in much cooler conditions and won’t grow below and above certain temperatures and these are known as cool-season grasses.

If you’re planting cool-season grass, the soil temperature should be above 45°F, however, root development may tolerate temperatures to as low as 40°F^[1]. Cool-season grasses also go dormant where temperatures are above 90°F. Warm-season grasses need soil temperatures between 60° and 65°F to germinate.^[2]

Optimal Grass Seed Germination Temperature Chart

Grass Species	Soil Temperatures (°F)
Kentucky bluegrass	59 – 77
Rough bluegrass	59 – 77
Annual bluegrass	59 – 77
Chewings fescue	59 – 77
Creeping red fescue	59 – 77
Sheep fescue	59 – 77
Tall fescue	59 – 77
Red fescue	59-77
Perennial ryegrass	59 – 77
Annual ryegrass	59 – 77
Bermudagrass	68 – 86
St. Augustine	68 – 86
Centipede grass	68 – 86
Zoysiagrass	86-96[3]
Bahiagrass	65-70

At What Temperature Does Grass Seed Germinate

Temperature is by far the most essential factor in determining grass seed germination. Grass seeds require the soil temperature to be within a certain range in order for germination to occur. This range depends on the type of grass seed that you are planting, either cool-season grass or warm-season grass.

Air Temperature vs Soil Temperature

When we talk about the temperature it is important not to get confused with air temperature. Whilst the daytime temperature impacts the temperature of the soil, the soil itself retains heat far longer than the air does.^[4] Daytime and night temperatures can vary considerably whereas the soil’s will remain fairly constant. 

It is also significant to note that soil temperature can take time to catch up to that of air. If you have had a cold spell and the daytime temperature suddenly increases, you will likely find that the ground temperature can be much colder. You could have a situation where there is a brief warm weather period, such as with a ‘false spring’, where the soil will not become warm enough for the seed to germinate as expected as the soil is too cold. 

It is therefore not unusual to find the temperature of the soil is different from that of the air. So, whilst climatic temperature can give you an idea when it comes to planting grass seed it is best to determine the soil temperature using a soil thermometer.

Warm-Season Grasses

Warm-season grass should be planted in warmer climatic conditions. They begin to germinate and show root activity when temperatures reach 60°F to 65°F. Below this temperature, the grass remains dormant and the seeds won’t germinate. Its peak growing temperature for leaf growth is between 80° to 95°F

When it comes to planting warm-season grass, late spring is often considered the best time. You should look to plant when the soil temperature reaches 65° to 75°F. The grass then should then be already established so that when the heat of summer arrives top growth can thrive fully establishing your lawn.

Cool-Season Grasses

Cool-season grasses thrive in more temperate conditions and can struggle in warmer climates. Root development can be triggered with temperatures as low as 40° to 45°F, although the optimum temperature range for leaf growth is between 65° to 75°F. 

However, cool-season grasses struggle in warmer temperatures. Once the temperature exceeds 75°F growth slows down, with it becoming dormant when temperatures exceed 90°F.

In terms of planting cool-season grass varieties, the best germination rates are achieved when temperatures range from 50°F to 65°F. Fall is often considered the best time to plant cool-season grasses. You need to leave a window where there are at least eight weeks before temperatures drop to below 45°F to give the grass enough time to establish itself as grass seedlings are vulnerable to frost and freezing weather.

Grass Seed Mixtures

If you are in a transitional zone it is not uncommon to look to use grass seed mixtures to provide a better lawn that can adapt to transitional conditions. In terms of germinating the sweet spot would be between 60° to 70°F to ensure both the warm and cool season seeds can germinate in unison.

Spring is likely to meet this criterion best as this will allow the cool season grass to establish itself before the hot weather. Although it would also be possible to plant in late summer or early fall other criteria, such as daylengths^[5], could impact on warm-season grass seedlings growth.

Other Factors that Affect Germination

Temperature is just one of a number of factors that affect germination. Whilst warm soil is a pre-requisite of germination other factors such as sunlight, daylengths, and adequate moisture can determine the germination rate. 

It is temperature and moisture that trigger germination. Once you have planted your seed you must keep the soil moist so that they germinate. However, you have to ensure that your watering regime is on point as excess water, noticeable if water pools, can reduce germination rates through the seeds rotting. 

Summary: What Is the Right Grass Seed Germination Temperature?

As we have seen it is soil temperature and not air temperature that is important when it comes to the point at which grass seeds will germinate or remain dormant.  

Cool-season grass seeds will germinate at temperatures as low as 40°F but they germinate at the fastest rate between 50°F and 65°F. Warm season grass seeds,  on the other hand, need the soil temperature to be at least 60°F to germinate, but germinate at their fastest rate when the temperature is between 65° to 75°F.

Ultimately, the best time to plant grass seed is when the grass type lines up with the optimal temperatures for the fastest germination as it will allow grass seedlings will appear sooner and the lawn can be established faster.

Notes: 

1. Pennsylvania State University, Peter Landschoot, Professor of Turfgrass Science: The Cool-Season Turfgrasses: Basic Structures, Growth and Development
2. Oregon State University, Department of Crop and Soil Science: Cool-season or Warm-season Grasses
3. Prudue University, Aaron J. Patton: A Guide to Establishing Seeded Zoysiagrass In the Transition Zone
4. Pennsylvania State University, Jason Reed: Soil Temperature and Seed Germination
5. Oregon State University, Department of Crop and Soil Science: Cool-season or Warm-season Grasses