The best time to overseed a lawn in Ontario is mid-August through mid-September — 85 to 95% germination success. Spring overseeding works as a backup. Full process and seed blend recommendations.
The best time to overseed in Ontario is late August to mid-September — specifically when soil temperatures drop below 20°C but air temperatures are still warm enough for germination (above 10°C overnight). This window gives seed 6 to 8 weeks of warm-soil germination before the first hard frost. In the GTA, this typically means targeting the last two weeks of August through the third week of September.
Avoid foot traffic for the first 21 days. Footsteps press seed too deep and damage emerging seedlings. After grass reaches 2 inches and you have mowed it once, normal use is fine.
First mow when new grass reaches 3 inches — typically 3 to 5 weeks after seeding. Use a sharp blade and remove no more than one-third of the blade height. Bag clippings for the first 2 mows so you do not smother young grass.
Yes — apply a starter fertilizer high in phosphorus, such as formulations like 10-20-10 or 18-24-12, at seeding to support root development. Apply a regular nitrogen fertilizer 6 weeks after germination once the new lawn is established.
Yes — annual overseeding is a maintenance practice, not just a repair tactic. Even healthy lawns lose 3 to 5 percent of grass plants per year to age, traffic, and stress. Adding fresh seed each fall keeps lawn density high and naturally crowds out weeds.