Spring feels like the season for lawn repair. It is not. If you only do this job once, do it in the first three weeks of September.
Why the timing wins
Grass seed germinates on soil temperature, and cool-season grasses want soil between 10 and 18 °C. Early September gives you exactly that: soil still holding summer warmth, air cooling off, and reliable moisture. Seed germinates in seven to fourteen days instead of the three weeks a cold April gives you.
More importantly, the new grass then gets two growing windows before its first summer: the rest of the autumn, and the following spring. Spring-seeded grass gets a few weeks before July heat arrives with a root system that is nowhere near ready.
The other advantage is quieter. Crabgrass germinates in spring and dies with the first frost. Seed in September and your new grass establishes with essentially no competition, on ground crabgrass has just vacated.
Aeration first, and why it is core aeration or nothing
Core aeration pulls plugs of soil out of the lawn, roughly 50 to 75 mm deep, and drops them on the surface. Those holes relieve compaction, let air and water reach the root zone, and give seed a protected pocket to land in.
Spike aerators, including the shoe attachments, do the opposite of what you want: they push soil sideways and compact the walls of every hole. If it does not remove a plug, it is not aeration.
Leave the plugs. They look untidy for about ten days and then break down in the rain, returning soil and microbes to the surface. Raking them off throws away half the benefit.
Aerating bone-dry soil gets you short, crumbly plugs and shallow holes. A good soak the previous day means the tines pull full-depth cores. Do not aerate saturated ground either; you want damp, not muddy.
The sequence
- Mow slightly shorter than usual, about 50 mm, so seed can reach the soil.
- Core aerate, two passes in different directions on compacted areas. You want holes roughly every 50 to 75 mm.
- Spread seed immediately, while the holes are open. A tall fescue and perennial rye blend for sun and traffic; add fine fescue for shade under maples.
- Top-dress lightly with compost if the soil is poor, 5 to 10 mm, raked in. Optional, but it is the single best thing you can do for a tired lawn.
- Apply a starter fertiliser, higher in phosphorus, to feed root development rather than leaf growth.
- Keep it damp for three weeks. Light watering once or twice a day until germination, then taper toward deep, infrequent soakings.
What not to do afterwards
- No pre-emergent weed control, in autumn or the following spring. It cannot distinguish between crabgrass seed and your grass seed. You are choosing to skip it for one cycle.
- Do not stop mowing. Keep cutting through autumn as growth continues, dropping to about 60 mm for the last cut of the season. Long grass going into winter mats down under snow and invites snow mould.
- Stay off it while it establishes as much as you reasonably can, particularly with a dog.
When spring is the right answer anyway
If a section is bare enough that erosion or mud is the immediate problem, patch it in spring rather than waiting five months. Just treat it as a holding measure, and plan the proper renovation for September.
September books out early
Aeration and overseeding runs on a tight seasonal window. Get on the schedule before the calendar fills.
