Done now it has time to grow strong before winter.
September is an ideal time to sow grass seed while the soil temperature is still warm and hopefully moistened by the first of the autumn rains. Sowing now can be done to create a new lawn from scratch or to repair damaged areas in an existing lawn.
The main advantage of sowing now is the seed will have plenty of time to germinate and grow before the onset of winter. As well as producing plenty of top growth, more importantly the new grass will be able to establish a strong root system, to get it through winter. Come next spring the lawn will grow away to produce a thick, healthy lawn. When creating a new lawn, the area needs preparing properly to remove any perennial weeds and this can be done by thoroughly forking them out, or you can spot treat with weed killer. The ground can then be cultivated by forking the soil over or rotavating, before raking, treading and a final rake to produce a firm, fine seed bed to sow the seed on. When repairing worn areas, use a fork or rake to loosen the surface where the grass is thin or damaged to expose some soil to sow the grass seed onto. Sown now in moist, warm soil, the seed should be through in 10-14 days.