Lawn seed sprouts when temperatures reach 55 degrees. In the Pacific Northwest we usually begin to experience 55 degrees during the day in late March or early April. It is ideal to spread seed as soon as the temperatures reach 55 degrees at night as well as the day. This may not happen until later April or early May. Seed also requires constant moisture to germinate so we like to take advantage of spring rains and 55 degrees (or above) to spread seed. Your seeds should sprout within 7-14 days. After sprouting you want to wait at least 3 weeks before you mow or disturb your new grass. You also need to water thoroughly for 3 weeks if it isn’t raining enough to keep the soil moist.