Grass will grow as long as you have moisture and temperatures that are not freezing. The best time to plant seed is in early fall after all the extreme heat of summer and before the freezing winter. Make sure you keep the seed moist after planteing and it is a good idea to aerate your lawn before seeding, especially in the fall. You can however plant grass in the summer but if you are in a hot climate then be prepared to use alot more water. There is also no need to cover your lawn with hay unless you have lots of birds eating up your seed.