The optimal time for planting grass seed is in the late spring or early summer. Planting seed in the autumn or during the early spring increases the risk of frost damage to the seed.
If grass seedlings have not had enough time to harden off -- to become accustomed to full outdoor exposure -- before frost occurs, they may be severely damaged or killed. Grass seed is in danger of freeze-drying over the winter if exposed to harsh temperatures.
The best time to plant grass seed to protect it from frost damage is at least 60 days before the first frost in southern regions of the UK and 90 days before the first frost in northern regions. This will allow the grass to achieve sufficient maturity to survive the dormant state required for winter.
Northern areas may require frost protection to ensure the seed does not freeze-dry over winter, and will remain in a dormant state if planted in the late summer or early autumn. Apply mulch in a layer 1.25 cm (1/2 inch) deep, and allow the mulch to remain throughout the spring.