Golf courses over-seed in the Spring and Fall. You need only over-seed in the Fall to replace any grasses that have died over the year.
Nature does its own over-seeding at the end of summer when the grasses drop their seed. However your lawn is cut regularly so that the grasses do not go to seed. You must perform this renewal for your lawn or it will slowly depopulate.
Over seeding when done in conjuction with Aeration gives you the best results.
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