When should I sow grass seed?

Late summer to early autumn, or mid to late spring — whenever the soil is warm and the moisture is reliable. Air temperature is the wrong thing to watch.

What germination needs

Three conditions, all at once and for long enough.

Warm soil. Grass seed germinates from the soil, and soil lags air by weeks. A warm week in March does not mean a warm seedbed.

Continuous moisture. This is the one that fails. Seed that has taken up water and then dries out is dead seed, not dormant seed. The surface layer where the seed sits is the fastest part of a garden to dry.

Contact. Seed lying on top of thatch or a crust never reaches soil. Raking in, or a thin covering of a topsoil or a sand mix, is what makes the difference between a thin result and a thick one.

Why autumn usually beats spring

Soil is at its warmest of the year after a summer, rain becomes more reliable, and the weed seed that competes for an open seedbed is far less active. The one constraint is leaving enough season for the seedlings to establish before growth stops.

Spring works, and it works later than people try it — once the soil has genuinely warmed, not at the first mild weekend.

What goes with it

An open surface. Seed sown onto an established lawn without scarifying or raking first mostly does not take, which is why overseeding follows scarifying rather than replacing it.

And patience about mowing: a new sward gets its first cut when it is tall enough to be topped rather than pulled, with a sharp blade and the height set high.

How this site handles it

The seed shelf here prints the coverage figure as grams or kilograms per square metre, which unlike a machine’s stated area is a physical quantity with a real derivation. Pages state the mixture type as the maker describes it — hard-wearing, shade, drought, fine ornamental — and treat those as the maker’s categories rather than as defined classes, because no document defines them.