Overseeding spreads new grass seed over an existing lawn to fill thin patches, repair damage, and keep the turf dense enough to crowd out weeds. Done right, it's the cheapest way to a thicker lawn.
Time it right
Early fall is ideal for the cool-season grasses common in much of Canada — warm soil, cool air, and fewer weeds give seed the best start. Spring is a second choice. Overseeding into summer heat usually fails.
Prep the ground
Mow a bit shorter than usual and rake out thatch and debris so seed reaches the soil — seed sitting on thatch or grass blades won't germinate. Aerating first helps seed make soil contact, and topdressing with a thin layer of compost improves results.
Seed and water
Spread seed at the recommended rate (more for bare repair, less for general thickening), rake lightly so it contacts the soil, and keep it consistently moist with light, frequent watering until it germinates and establishes. Hold off on weed treatments that would harm new seedlings.