Short answer: Overseed a thin or patchy lawn in early autumn (September) or spring (April–May), when the soil is above about 8°C. Mow short, rake or scarify to expose the soil, spread grass seed evenly, lightly cover it, and keep it moist until it germinates — usually 1–3 weeks. Overseeding thickens the lawn, fills bare patches, and crowds out moss and weeds.

Overseeding just means sowing new grass seed into an existing lawn. It's the simplest way to rescue a tired, patchy lawn without digging it up and starting again.

The best time to overseed

Early autumn is best. The soil is still warm from summer, rainfall is more reliable, and there's less competition from weeds — so seed germinates fast and establishes before winter. Spring (April–May) is the next-best window once the soil has warmed. Avoid the height of summer (heat and drought) and winter (cold soil), when germination is poor. See the month-by-month lawn care calendar for how this fits with your other jobs.

What you'll need

  • Grass seed suited to your lawn and its use (choose a mix that matches your grass type and light levels)
  • A rake or, for bigger lawns, a scarifier
  • A spreader (or a steady hand) for even coverage
  • Optional: a thin top-dressing or sieved compost

Step-by-step

  1. Mow the lawn short. Cut lower than usual so light and seed can reach the soil, and remove the clippings.
  2. Rake or scarify. Rake out moss, thatch and dead grass to expose bare soil — seed needs soil contact to germinate, not a mat of debris.
  3. Loosen bare patches. Lightly scratch up the surface of bare areas so seed can settle into the soil.
  4. Spread the seed evenly. Follow the rate on the pack — usually a heavier rate on bare patches and a lighter rate over thin areas. Even coverage avoids clumps and gaps.
  5. Cover the seed lightly. Rake it in gently or scatter a thin top-dressing so seed is in contact with the soil and less visible to birds. Don't bury it deep.
  6. Water gently and keep it moist. Water in lightly, then keep the surface consistently moist — not waterlogged — until germination. This is the opposite of the usual deep-and-infrequent rule; see how often to water your lawn.
  7. Wait to mow. Let the new grass reach about 5–7cm before its first, high cut, then ease back into your normal routine.

Helping new seed establish

  • Keep off it. Minimise foot traffic until the new grass is established.
  • Don't let it dry out. Young seedlings die fast if the surface dries; light, frequent watering is key for the first few weeks.
  • Hold off on weed killers. Most lawn weed treatments harm new seedlings — wait until the new grass has been mown a few times.
  • Feed once established. A gentle feed helps the new grass thicken up; see when to feed your lawn.
Timing overseeding to warm soil and coming rain makes all the difference. YardIQ watches your local forecast and tells you when conditions are right to sow — then reminds you to keep it watered until it takes.

Frequently asked questions

When is the best time to overseed a lawn in the UK?

Early autumn (September) is best, because the soil is still warm and rainfall is more reliable, so seed germinates quickly and establishes before winter. Spring (April–May) is the second-best window once the soil has warmed to around 8°C.

Can I just throw grass seed on my lawn?

You can, but results are poor. Seed needs contact with soil to germinate, so scattering it over an uncut, thatchy lawn mostly feeds the birds. Mow short, rake to expose soil, sow evenly, cover lightly and keep it moist for much better results.

How long does grass seed take to germinate?

Usually 1–3 weeks, depending on the seed mix and soil temperature. Warm, moist conditions speed it up; cold or dry soil slows it right down, which is why autumn and spring are the best times to sow.

When can I mow after overseeding?

Wait until the new grass reaches about 5–7cm, then give it a first cut with the blades set high, removing no more than a third of the height. Mowing too soon can pull up seedlings that haven't rooted yet.

Sources