Aerate and overseed: when to do it and why it works

If your lawn looks thin, has bare patches or turns to concrete in July, it probably needs two jobs done together: core aeration and overseeding. Either one helps on its own. Together they are the most useful thing you can do for a tired lawn.

Here is what each one does, when to do it, and how to look after the lawn afterward.

What aeration actually does

A core aerator pulls small plugs of soil out of the lawn, usually about 2 cm wide and 5 to 8 cm deep. The holes let air, water and nutrients reach the roots, and they give roots room to grow. Spiking the lawn with a pitchfork or spiked shoes is not the same thing. It pushes the soil sideways and can make compaction worse.

Aeration matters most on clay soil, on new-build lawns with thin topsoil, and anywhere people, kids or pets walk a lot.

Why you overseed at the same time

Seed needs contact with soil to sprout. Right after aeration the surface is covered with open holes and loose soil, which is the ideal seedbed. The seed drops into the holes, stays moist and gets going before the weeds do.

Overseeding also thickens the lawn. A thick lawn shades the soil and crowds out weeds without any chemicals.

Crew tip

mow the lawn a little shorter than usual, and bag the clippings, the day before you aerate. Seed reaches the soil more easily.

The best time in the GTA

Early September to mid-October is the window. Soil is still warm, nights are cool, and autumn rain does half the watering for you. Spring works too, from late April to mid-May, but the new grass has to survive a hot summer, and crabgrass seeds are germinating at the same time.

If you are planning to use a crabgrass preventer in spring, remember that it also stops grass seed from sprouting. Choose one job or the other for the same patch.

Choosing the seed

Check the bag for the grass types. For sunny GTA lawns, Kentucky bluegrass with perennial ryegrass is a good choice. For shade under mature trees, which is common across Toronto and Oakville, choose a mix with a lot of fine fescue. Skip the cheapest bags; they often contain a lot of filler and weed seed.

“A thick lawn is the best weed control there is.”

How to do it

1. Water the lawn a day or two before, so the soil is moist but not soaked.
2. Make two passes with the aerator in different directions. Leave the plugs on the lawn.
3. Spread the seed with a spreader, following the rate on the bag. Overseed bare patches a little heavier.
4. Rake lightly so seed settles into the holes, and apply a starter fertilizer.
5. Water lightly every day, long enough to keep the top 1 cm damp, until the seed sprouts.

Look after the new grass

New seed usually sprouts within seven to fourteen days. Keep watering lightly each day until the seedlings are about 5 cm tall, then switch to deeper, less frequent watering. Stay off the lawn as much as you can for about four weeks. The first cut should wait until the grass is around 8 cm, with a sharp blade and no more than a third taken off.

Do not worry if the lawn looks messy at first. The plugs break down in a couple of weeks, and the seedlings fill in over the next month.

How often?

Most lawns benefit from aeration once a year. A healthy lawn on sandy soil can go two or three years between visits. If the screwdriver test fails (a screwdriver will not push easily into moist soil), you need it now.

Common mistakes

Three things go wrong most often. The first is seed that dries out: a few hot, windy days can undo a week of watering, so check the soil every day for the first two weeks. The second is mowing too soon and too short, which tears out seedlings. The third is putting down weed killer too close to seeding, which can stop the new grass as well as the weeds. Read the label for how long to wait.

How long it takes

On an average lawn the aerating and seeding itself is a couple of hours of work, with a bit more for feeding. The part that takes patience is the daily light watering for the first two weeks. Put a reminder on your phone, because that is the step that most often gets missed.

Want a hand?

Aeration and overseeding are part of our Complete and Estate lawn care programmes, and we also book them as a one-off. If you are not sure whether your lawn needs it, ask for a free visit through the contact page and we will check the soil and tell you.