Overseeding sows new grass seed into an existing field to thicken up thin, bare or weedy areas without ploughing and starting again. It’s for paddocks worn out by winter grazing, poaching or years of heavy use.
The existing grass is opened up first — usually with tined harrows or a scarifier — so the seed can reach the soil. The seed is then sown with a harrow seeder that scratches and broadcasts in one pass, or with a slot drill that cuts seed into the ground.
Rolling after sowing presses the seed into firm contact with the soil, which is what makes it germinate. Seed that sits on top of a thatch of dead grass mostly fails.
The seed mix matters. Mixes sold for horse paddocks usually contain hard-wearing grasses and less ryegrass than mixes for dairy cattle; your contractor may supply seed or sow seed you have bought.
Late summer to early autumn, around mid-August to September, is often the best window: the soil is still warm and there’s usually more moisture, and weeds are growing less vigorously. Spring, around April to May, also works if the ground isn’t too dry afterwards.
New seedlings need moisture for several weeks. Sowing into dry soil before a dry spell is the most common reason overseeding fails, so the timing is often decided by the weather forecast as much as the calendar.