When Does Grass Seed Germinate? [Air Temp Vs Soil Temp]

Written by Nate Bahler | May 24, 2021 3:20:00 PM

Coming into Spring, soil temperature dictates just about everything we do in lawn care - especially in the Northeast where we have a lot of cool season grasses.

What temperature does my soil need to be for seeds to germinate?

Spring seeding and dormant seedings won't germinate until soil temperatures are above 50 degrees Fahrenheit. A range of 50-65 degrees is optimal (measured at a depth of 2 inches). Once we reach this soil temperature, the process of germination will begin.

What most people don’t realize is that just because the air is 75 degrees, it doesn’t mean the soil is 75 degrees. What we need is for air temps to be sustained at about 10 degrees warmer than what we need our soil temp to be.

So, for example, if we need soil to reach 50-65, we need consistent air temperatures of about 60-75 degrees for an extended period of time.

Why are soil temperatures and air temperatures so different?

Soil acts as an insulator. Soil’s pore space is made up of either air or water which are both great insulators.

These pore spaces either keep temps very cold when they reach a cold level or keep temps very hot when they reach a high level. Because of these insulating qualities, it can take a long time for soil to make this shift from warm to cold or cold to warm.

Soil Temperature can be affected by:

Frost
Water levels
The amount of pore space (water and air that acts as insulation) in your soil
Plant cover
Soil depth

For Cool Season Grass Seed Germination:

Need Soil temps between 50-65 F
Need sustained air temps about 10 degrees above desired soil temp. (65 F-75 F).
Seeds need adequate moisture.
Soil needs adequate oxygen. A seed will have a hard time establishing itself in compacted soil. If your soil is compacted and you have a thick thatch layer, consideration Core Aeration.
Seed to soil contact.