Like most seeds, cosmos require moisture and warmth to sprout. Maintaining a soil temperature of 65 degrees Fahrenheit encourages quick and even germination. The plants don't require light until after they sprout, so you can keep the pots in a warm but dark room until they begin to emerge. Evenly moist soil further aids germination. Watering the soil prior to seeding then keeping the pots covered with a plastic bag conserves the soil moisture so the seedlings won't require watering until after they sprout.