How to overseed a lawn, step by step

Decide whether overseeding is the right job

Seed will not fix a problem it cannot survive. If grass is thin because of shade, compaction, waterlogging or a thick mat of thatch, new seed will thin out for the same reason. General guidance: deal with the cause first, then re-sow. The RHS lists compacted or waterlogged soil, low fertility and too little light among the reasons grass struggles in shade, for example, and each needs its own fix.

Overseeding suits a lawn that is sparse rather than dead: lots of grass, but gaps, thin areas and a weak sward. For true bare patches the RHS repair guidance applies; for a lawn that is mostly moss or weeds you probably need a bigger reset, which is what lawn restoration is for.

When to sow in the south-east

The RHS says the best times to sow a lawn from seed are autumn (September to November) or mid-spring (March to April), and that early autumn is the better choice if you have one. Its repair advice is similar: spring and autumn are the ideal times, when the weather is damp and cool.

Soil temperature matters. The RHS October lawn page says that for good germination the soil temperature should be at least 7°C, and that cooler areas of the country are likely too late for seeding in October. The RHS does not name East Kent specifically, so treat the window as a starting point. Late September and early October are the safest bet; if a cold November arrives, waiting until March or April is the sensible fallback. A cheap soil thermometer removes the guessing.

Summer is the one to avoid. Seed needs steady moisture, and hot dry weather will undo it. The RHS notes that the autumn after a dry summer is the ideal time to renovate and repair a lawn, and that sparsely grassed areas should be re-sown then, ideally with a drought-tolerant mix.

Step 1: prepare the surface

Seed needs contact with soil, not a layer of old grass clippings and thatch. That means cutting the lawn short, removing debris, and opening the surface. For lawns with visible thatch, scarifying is the usual first step: the RHS says to scarify in autumn, with a spring-tined rake used vigorously without uprooting the grass, if you cannot see the soil between the blades.

If the ground is hard, aerate as well, either with a fork or a hollow-tined machine; our guide to hollow-tine or spiking explains which to use. The RHS says to scarify when the soil is not waterlogged and the weather is not very wet, very dry or frosty.

On genuinely bare patches, the RHS repair method is to lightly fork or dig over the soil, work in topsoil or garden compost, and firm it gently with your feet to leave a level surface.

Step 2: choose and measure the seed

Pick a mix that matches the conditions: sun, shade, or wear. Our guide to choosing lawn seed goes through this. The RHS says to use the weight per square metre printed on the packet, because rates vary with the mix.

For patching bare areas, the RHS says the packet rate is usually about 15-25g per sq m (½-¾oz per sq yard). The RHS pages I read do not give a separate figure for whole-lawn overseeding, so follow the packet for the mix you buy and weigh it out rather than guessing. More is not better: dense, crowded seedlings compete and are more likely to fail.

For larger areas the RHS notes that you can increase the sowing rate by 50 per cent to offset losses to birds, though that advice is given for new lawns rather than thin ones.

Step 3: sow evenly

Split the seed into two equal batches. The RHS says to sow one half walking up and down and the other half side to side, which overlaps the coverage and reduces stripes. Small gardens can be sown by hand; a hand-held spreader is more even on a larger lawn.

Then rake lightly so that most seeds are covered. For bare patches the RHS suggests a light sprinkling of topsoil or compost over the seed, which also hides it from birds. It also says fleece pinned at the edges, or bird tape on larger areas, will protect seed.

Step 4: water carefully

Newly sown grass dries out easily until it is well rooted, according to the RHS. Water gently with a fine rose or light spray so that seeds are not washed into heaps. Once seedlings appear, keep watering regularly if it does not rain.

The RHS pages differ a little on frequency. The lawns-from-seed page says to water every few days if necessary when no rain is forecast, while its spring and summer page says that watering a new lawn every seven to 10 days is normally enough, or less often if the weather is damp. In practice, check the surface: it should never dry to a crust during germination. Do not saturate it.

Step 5: the first mow and early use

When seedlings reach 5-7.5cm (2-3in), the RHS says to lightly refirm the soil with a roller or by treading, and two or three days later cut off only about one-third of the grass height. It recommends a sharp cylinder mower with the front roller removed for new lawns. If you only have a rotary mower, keep the blades sharp and the height high.

In October the RHS suggests setting the cutting height to at least 5cm (2in) for newly sown areas. An autumn-sown lawn usually needs no more mowing until spring, while a spring-sown one can be cut roughly weekly, lowering the blades gradually.

Keep traffic off. For fully new lawns the RHS says to limit use for about eight months. For an overseeded patch, general guidance is to keep feet and pets off the sown area until the new grass has been mown a few times.

Common failures

Most overseeding failures trace back to one of these rather than to bad seed.

• Seed sown on thatch or compacted soil, so roots never reach the ground. Prepare the surface first.
• Sowing too late in autumn. If the soil has dropped below 7°C, germination is poor.
• Letting the seedbed dry out for a few days in a dry spell.
• Birds eating exposed seed. Cover lightly or use fleece.
• Heavy feeding with summer lawn feed. The RHS warns it is high in nitrogen and encourages disease-prone growth in autumn.
• Shade not addressed. Choose a shade-tolerant mix and raise the mowing height.

Aftercare

Hand-weed any low-growing perennial weeds as they show. The RHS says to top-dress any dips in September with a thin layer of sieved potting compost, and to apply lawn feed in spring. If you want a hand with the whole sequence, our overseeding service and the annual plan cover it.