Grass does not care how warm the afternoon feels. Roots and seed respond to the soil, and the soil runs on a slower calendar. After the snow melts it lags weeks behind the air; in fall it holds summer heat long after the evenings turn cold.
Which soil temperatures matter for a lawn?
These are the ranges the What a Green Lawn app uses for cool-season lawns, with soil at about 10 cm:
| Soil temperature | What happens | What to do |
|---|---|---|
| Below about 6°C | Seed will not germinate, grass barely grows | Leave it alone |
| 6 to 8°C | Seed germinates very slowly | Too early for seed |
| 8 to 12°C | Seed germinates steadily | Good for dethatching |
| 12 to 18°C | Fast, even germination | Best window for overseeding |
| 18 to 24°C | Warm, seedlings dry out fast | Keep seed and lawn watered |
| Above 24°C | Heat and drought stress | Hold off on seeding and dethatching |
For dethatching the app uses a slightly different range: good from about 8 to 15°C, fine up to 22°C if you do it in the morning.
When is the soil warm enough to feed?
Roots start taking up nutrients as the soil warms, so the first spring feeding pays off once the soil has passed about 8°C and the grass is visibly growing. Fertilizer on cold soil right after snowmelt mostly sits there or runs off. The app’s fertilizer score uses air temperature, rain and season rather than the soil estimate, but they usually agree.
How do you measure soil temperature?
Push a soil thermometer 5 to 10 cm into the lawn in the morning, when the soil is coolest. Check a sunny spot and a shaded one, and watch the trend for three to five days.
The app estimates soil temperature for your location every day from the air temperature and the month:
Below about 3°C it reads too cold, below 8°C cool, up to 18°C ideal, up to 24°C warm with a reminder to keep the lawn watered, and above that hot. It is an estimate, not a measurement, calibrated against Danish soil data at 10 cm, so treat it as a guide.
How do you compare your own readings with the estimate?
Log thermometer readings in the Log tab, and the ground temperature card charts them against the app’s estimate for today:
These example readings are model values, not a thermometer in a Toronto lawn, but they show that measured soil can sit above or below the estimate. If yours differs day after day, trust your thermometer.
Why is late summer the best time to seed?
Because the soil is still warm. From late August through September the soil across southern Canada usually sits between 12 and 18°C while the air cools, which is exactly what seed wants. See when to overseed your lawn and when to dethatch.
The overseeding and dethatching scores in What a Green Lawn use this soil estimate every day, together with rain and the season.