Fill the seed trays full of seed starter. Water the seed starter until it is moist but not soaking.
Press the rainbow chrysanthemum seeds in the seed starter about two to four weeks before the last frost in your area. Keep the starter moist at all times.
Place the seed trays in a location with a temperature of 70 degrees Fahrenheit. The seeds will germinate in six to 10 days.
Plant the rainbow chrysanthemum seedlings outdoors once the air temperature is at least 68 degrees Fahrenheit. Dig a hole for the transplants as deep as the root ball and twice as wide. Space the holes 18 to 24 inches apart. Place the chrysanthemums into the holes and fill the holes with soil.