Sow marigold seed directly in the ground and cover with a thin layer of soil (about 1/8 inch deep). Water thoroughly. Thin to 8-18 inches apart after seedlings have sprouted. Marigolds can also be started early indoors under grow lights for transplanting outdoors about six to eight weeks before the last frost date. Depending on the variety, marigolds can grow 8 to 40 inches tall, and develop blooms that can range in size from 1 to 5 inches.