When to overseed a UK lawn for the best germination odds

Overseeding means sowing new grass seed into an existing lawn, to thicken it up or fill the thin patches a dry summer leaves behind. The seed is cheap. Whether it takes comes down to timing.

Early autumn is the main window

For most UK lawns, the best time to overseed is late summer into early autumn. Three things line up then:

The soil is still warm from summer, so seed germinates quickly.
Rain is more regular, so the seedbed stays moist without constant watering.
Weeds are slowing down, so new grass has less competition for light and space.

The limit is the cold. Seed sown too late germinates slowly, and young grass needs several weeks of growth to establish before the first frosts.

Spring is the second option. It works, but the soil takes longer to warm, dry spells in late spring are harder on seedlings, and weed seeds are germinating at the same time as yours.

Soil temperature, not the calendar

Seeds respond to soil temperature. Turf Alert's pre-emergent alert is built on exactly that: it waits for the soil to hold at 8°C or above for three days running, the point in spring when weed and moss seeds start germinating in earnest.

Grass seed wants the same conditions: warm, moist soil. Cold soil slows it right down. In spring, that means waiting for the soil to warm up properly rather than sowing on the first sunny weekend in March. In early autumn the soil is already warm, and the question is how many weeks of it you have left.

Don't sow into dry soil

A drought is often the reason a lawn needs overseeding in the first place. But sowing into soil that is still dry wastes seed, unless you can water every day until it establishes.

Turf Alert tracks this as a soil moisture deficit: a running total of water lost to evaporation, minus rainfall. When the deficit reaches 60mm, the lawn is under drought stress and the watering alert fires. If your lawn is still in a drought episode, wait for autumn rain to bring the deficit back down before you sow. How much rain is enough? explains the deficit in more detail.

How to do it

Mow a little shorter than usual and clear the clippings.
Rake or scarify to pull out thatch and moss and expose the soil. Seed that lands on a mat of dead grass won't take.
Spread the seed at the rate on the pack. Two passes at right angles to each other give more even cover.
Rake lightly so the seed is just covered, then firm it in.
Keep the surface moist until the new grass is established. Little and often works better than an occasional soaking.
Hold off on weed and moss killers. Most will damage seedlings, so check the label for how long to wait.
Make the first cut once the new grass is a few centimetres tall, on a high setting.

Check how things look near you on the lawn forecast, which shows whether your region is currently in a drought-stress episode, or see how the model works.