Over time, soil compacts and a layer of thatch builds up, choking off the water, air, and nutrients grass roots need. Aerating relieves that and is one of the best things you can do for a tired lawn.
Use a core aerator
A core (plug) aerator pulls small plugs of soil out of the lawn, which relieves compaction far better than spike aerators that just poke holes and can compact further. You can rent one or hire it out for a larger lawn.
Timing
Aerate during the growing season when the lawn can recover quickly — early fall is ideal for cool-season grasses common in much of Canada, with spring as a second option. Water the lawn a day before so the aerator penetrates.
Follow up
Leave the soil plugs to break down on their own. Aeration pairs perfectly with overseeding and fertilizing — the seed and nutrients reach the soil through the holes, thickening the lawn and crowding out weeds.