Grass seed that fails to germinate cannot grow into part of a lush green lawn, so it's important to sow seed when conditions are right for germination. Soil temperature, depth of planting and soil moisture are all key factors. Ensure the correct conditions when you seed your lawn for the best results.
Grass seed germinates when the soil temperature is 18.3 degrees C and above. It's important to understand that it is the soil temperature, and not the surrounding air temperature, that is the trigger for seed germination. In early spring days, outside temperatures can reach 70 or more, but the soil, which heats and cools more slowly, may still be frozen in places. Likewise, in the fall, the air temperature may drop below freezing, but the soil might still be relatively warm.
Check Soil Temperature
To measure the soil temperatures, place the sensor end of any thermometer 3 inches into the soil and leave it in place for at least a minute. Special soil testing thermometers are available, but aren't necessary. If you have another type of thermometer, it will work, too, as long as it can measure temperatures between 0.555 and 29.4 degrees C. If the initial soil temperature measurement is only a degree or two above 18.3 degrees C, test the temperature at several other locations, especially in areas that get less sun. Don't plant grass seed until the soil temperature is at least 18.3 degrees C throughout the area to be seeded.
Soil and Moisture
Grass seed must also be covered by soil in order to germinate, and must have adequate moisture. To ensure both optimal soil moisture and soil temperature, sow grass seed in the spring or in the fall. Grass seed sowed in midsummer is likely to dry out before it can germinate because of high temperatures and relatively dry conditions.
Rake the area to be seeded with a garden rake before you sow the seeds so that the top 1/2 inch or so of soil is soft, loose and somewhat ridged. The grass seeds will settle into the ridges and be covered over with soil when you water or when it rains, which will also help increase the germination rate.
Germination is a biochemical process, and like other chemical processes, it is impeded or facilitated by the surrounding temperature. Different plant seeds have different temperature requirements. Some seeds will not germinate if they have not first been exposed to cold temperatures. Grass seed needs a minimum temperature of 18.3 degrees C and will germinate faster at higher soil temperatures as long as the increased heat doesn't dry out the seeds and soil.