Overseeding is sowing grass seed over an existing lawn to fill thin patches, improve density, and introduce stronger grass varieties.

When should I overseed?

September is the best time in the UK. The soil is warm, autumn rain helps germination, and there is less weed competition. Spring is the second-best option.

Do I need to prepare the lawn first?

Yes. Mow short, scarify to remove thatch, and rake to expose soil. Seed needs soil contact to germinate. Top dress with a thin layer of compost after seeding.

How long until I can mow after overseeding?

Wait until new grass is at least 5 cm tall (usually 3-4 weeks). Set mower to a high cut for the first few mows.

Should I use topsoil or compost after overseeding?

A thin layer (5mm) of fine compost or lawn top dressing helps seed-to-soil contact and retains moisture. Do not bury the seed more than 5mm deep.

GardenCalcs

Overseeding Calculator — Thicken Your Lawn

Calculate seed rates for patching and thickening existing lawns.

Lawn Area

Lawn Condition

Seed Type

How We Calculate This

Overseeding rates are a fraction of new lawn rates, depending on lawn condition.

Light overseeding (minor thinning): 30% of new lawn rate. Moderate: 50%. Heavy renovation: 70%.

Combined with the seed type rate (General 35g/m², Fine 25g/m², etc.) this gives the grams per m².

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.