Grass seed can be sown anytime between March and early October in normal weather conditions. Where you live in the UK will have an effect on the timing too, in the South there are more months in the year when you can sow grass seed than in the North. However to improve your chances of success, wherever you live, it is advisable to follow a few guidelines.
Avoid extremes! If it is very cold in March/April or there is a run of frosts or snow it is best to avoid sowing. Low ground temperatures prevent germination and frost can kill freshly emerging shoots. It is better to wait for the weather to warm up and for the sun to heat the ground.
It is worth remembering that in sowing terms, the North of the country may be four weeks behind the South due to lower temperatures.
If you have avoided the cold weather it is equally important to avoid high temperatures. Periods of hot sun and warm wind will dry out and kill emerging shoots even if they have been watered the evening before. So if possible, pick a time when it is more likely to be mild and damp, in our climate this is usually more often than we would like!
Too much is as bad as too little. The ground should be damp when you sow but not saturated so do not sow after prolonged rain. Drought is to be avoided as the ground is too dry and there is no regular rainfall to water the grass, particularly when it is just emerging from the ground.
It is very important that the new shoots are kept damp. Rain is ideal, otherwise a fine spray should be applied in the evening to reduce water loss from evaporation. This also allows the water to soak down into the soil which encourages deeper rooting grass. If the weather becomes very hot it is possible that the young shoots will be scorched and die even if they have been watered.
Sow grass seed when the weather is mild and the ground has had a chance to warm up. Avoid extremes of temperature and rainfall. Late summer and early autumn often provide the ideal conditions when the ground is warm and there is frequent rain.