Marigolds (Tagetes spp) are a group of annual flowers that are grouped into three categories: African marigolds (Tagetes erecta), French marigolds (Tagetes patula) and triploid marigolds. Both the African and French marigolds are indigenous to Mexico. The triploid varieties are the result of a cross between the African marigold and the French marigold. French marigolds grow to between 6 and 12 inches tall, African marigolds can reach up to 3 feet tall and triploid varieties can grow to just over a foot tall. For maximum flowering, plant marigolds in full sun.
Fill up planting cells to within 1/2 inch from the top with a good-quality seed-raising soil mix. Pat down the soil in each of the planting cells until it's well firmed down.
Pour water into the irrigation tray so it has about 1 inch of water in it. Set the planting cells into the watering tray so they can soak up the water. Pat the soil down one last time. Then, drain out any remaining water after the soil in the planting cells is well moistened.
Place the marigold seeds into the planting cells by pinching a few seeds between your thumb and forefinger and scattering them across the soil. Try to plant about four to five marigold seeds per individual cell. Sprinkle the equivalent of 1/4 inch of seed-raising mix over the marigold seeds. Press the soil down in the cells gently. Moisten the surface of the soil over the seeds with a fine mist of water.
Transfer the tray of planting cells into a warm and well-lit area in your home. The temperature should be between 65 and 75 degrees F. Ideally, between eight and 10 hours of light daily is best. If you can't provide a good source of light, you can put the tray 4 to 6 inches away from a fluorescent light for 10 to 12 hours a day. Marigold seedlings will begin to sprout in seven to 14 days.
Thin out the marigold seedlings once they've got a pair of leaves. Snip off at the base of its stem all but the hardiest-appearing seedling in each cell.
Transplant your marigold seedlings once they've grown to about 2 or 3 inches tall. Make sure you don't plant them outside until no further chance of frost is expected in your growing region.