- A thin, worn lawn after summer is not finished, it is just short of plants, and sowing fresh seed into the existing grass now is the cheapest way to thicken it.
- The trick is getting the seed down onto bare soil, not onto thatch or long grass: mow low for once, rake hard, scatter seed at about 25 to 35 grams per square metre, then cover and keep it damp.
- Sow while the soil is still above 7°C (45°F) and the new grass roots before winter, so the lawn knits together thick and crowds out moss and weeds by spring.
The lawn is thin, not dead, and autumn is the window to fix it
A summer of heat, foot traffic, kids and the odd dry spell leaves most lawns looking tired: thin in places, worn along the paths everyone walks, with soil showing through where the grass has given up. The reaction is often to wait and hope it fills back in on its own. It rarely does. Thin grass stays thin, and the gaps fill with moss and weeds instead. The fix is overseeding, sowing fresh grass seed straight into the lawn you already have to thicken it up, and early autumn is the best window of the whole year to do it.
Overseeding is not the same job as laying a new lawn or patching a bare spot. You are adding young plants in among established grass across the whole thin area. Get the method right and you walk onto a thick, even lawn in spring. Get it wrong and you feed the local birds an expensive breakfast. Here is how to do it properly.
Why early autumn beats spring for this
Grass seed needs three things to germinate: contact with soil, moisture, and warmth. Autumn hands you all three at once. The soil still holds its summer warmth, so it sits comfortably above the 7°C (45°F) that grass seed needs to come up, and at that temperature seedlings push through in about seven to ten days. The rain has returned, so the ground stays damp without you standing over it with a hose. And the weeds that race new seedlings for space in spring are slowing down for the year, so the young grass gets a clear run. Sow now and the plants root before winter and are established enough to thicken fast when spring comes.
Leave it too late and the window shuts. Once the soil drops below 7°C the seed simply sits in the cold, where it rots or gets eaten rather than growing. In cooler regions that usually means getting the job done by mid to late autumn rather than putting it off.
How to overseed a thin lawn, step by step
Start by mowing the lawn shorter than usual for this one cut, down to around 2.5cm (1in). The point is to let light reach the new seedlings and to make it easier to get seed down to the soil. Then rake the whole area hard with a spring-tine rake, or run a scarifier over a larger lawn, to pull out the dead thatch and moss and expose soil between the grass plants. This step decides the whole job: seed that lands on a mat of thatch or long grass never reaches the soil, dries out and fails, which is why scattering seed over an untouched lawn almost never works. A spring-tine rake costs about £8 to £15 (roughly $10 to $20) from B&Q or Screwfix; scarifier hire runs about £30 to £50 a day (around $40 to $65). If the ground is hard and compacted, push a garden fork in across the area first, spacing the holes about 10 to 15cm (4 to 6in) apart, which lets air, water and roots back down (our guide to aerating a compacted lawn covers this in full).
Now scatter the seed. For overseeding an existing lawn, aim for about 25 to 35 grams per square metre, which is roughly a generous handful to the square yard, and is around half the rate you would use starting a lawn from bare soil. Match the seed to the spot: a hard-wearing mix heavy in perennial ryegrass for family lawns and play areas, or a fine fescue or dedicated shade mix for grass under trees and on the north side of the house. A one kilogram box of a decent mix costs about £10 to £20 (roughly $13 to $26) and covers around 30 square metres at the overseeding rate, from B&Q, Amazon, Home Depot or Lowe’s. Sow half your seed walking up and down and the other half walking across, so the coverage is even rather than striped.
It pays to match the seed to how the lawn is used rather than grabbing the first box on the shelf. Perennial ryegrass germinates fast and takes hard wear, which is why it dominates family-lawn mixes, but it is coarse and clumps if sown on its own. Fine fescues are softer and finer and cope with shade and poorer soil, so a mix leaning on them suits ornamental lawns and the gloomy strip under a tree where ryegrass sulks. A sensible rule is to pick a blend built for your conditions, play lawn or shade, and to use the same type as the grass already there so the thickened lawn looks of a piece rather than patched with a different texture.
Cover the seed so it holds moisture and hides from birds. The tidiest way is a light top dressing brushed over the surface: a mix of roughly three parts sandy loam, six parts sharp sand and one part well-rotted compost, applied at about 2 to 3kg per square metre (4 to 7lb per 10 square feet) and worked into the surface with the back of a rake. A ready-bagged lawn top dressing, about £8 to £12 for a 15 to 20kg bag (around $10 to $16), saves mixing. If you skip the dressing, at least rake the seed in lightly so it sits in contact with soil rather than on top of the grass. Firm it down with the back of the rake or a light roll, then water with a fine spray if no rain is due.
The first few weeks, and the payoff
The one rule that cannot be broken is moisture. From sowing until the new grass is established, the surface must not dry out, as a germinating seed that dries even once is dead. In autumn the rain usually does this for you, but water with a fine spray in any dry spell. Keep off the area as much as you can while the seedlings are soft, and keep children and pets off too. Try to sow into a settled mild spell rather than the day before a hard frost or a downpour that would wash the seed into drifts, and if birds are stripping an open patch faster than it can cover, a light net or some horticultural fleece pegged down for a week or two sees it through.
Leave the mower in the shed until the new grass reaches about 5cm (2in). The first cut takes the top off only, on the highest setting the mower has, removing no more than a third of the height, with a sharp blade so it slices the tender seedlings rather than ripping them out by the root. Mowing too soon or too low is the quickest way to undo all the work. Once the new grass is growing with the old, give the whole lawn an autumn feed that is high in potassium and phosphorus rather than nitrogen, which steadies the young plants for winter instead of pushing soft growth that disease attacks (here is how to feed a lawn in autumn). Do all of this now and the thin, worn lawn you started with comes through winter knitting together, and greens up in spring as the thick, even surface that crowds moss and weeds out for good.
