African marigolds (Tagetes erecta) are the giants of the marigold world. The double flowers can measure up to 5 inches across, and these annual plants often grow up to 3 feet tall. Like all marigold varieties, the African type flowers from spring through fall, supplying an ongoing supply of yellow or orange blooms. Starting them from seeds indoors four to six weeks before the last spring frost ensures they are ready to flower soon after transplanting.
Sow two seeds in a 3-inch diameter pot filled with sterile soil, planting the seeds about 1/4-inch deep. Water the soil until it is evenly moistened throughout the depth of the pot. Empty any water that collects in the drip tray beneath the pot after watering.
Cover the pot with a plastic bag to retain soil moisture during germination. Set the pot in a warm 70 to 75 degree Fahrenheit location. African marigolds usually sprout within seven to 14 days.
Remove the bag after the sprouts emerge and move the pot to an area that receives six hours or more of daily sunlight. Pinch out the weaker seedling after the stronger seedling grows its second set of leaves, so only one seedling remains in each pot.
Water the marigolds when the soil surface feels dry. Provide water until the excess just begins to drip from the bottom of the pot, then empty the drip tray after watering.
Transplant the African marigolds to a well-drained, full sun garden bed after frost danger has passed. Plant the marigolds at the same depth they were in their pots and space them 12 to 18 inches apart in all directions.
Water the garden marigolds about once a week, or when the soil begins to dry to a 2-inch depth. Supply 1 inch of water, or enough so the top 6 inches of soil is thoroughly moistened. Cover the bed with a 2-inch-thick layer of mulch to help conserve soil moisture.
Fertilize the marigolds monthly with a soluble all-purpose fertilizer, such as a 24-8-16 blend. Dissolve the fertilizer in water, according to the recommended ratio on the package directions. Water the marigolds with the solution.
Pinch off the old flowers as soon as they begin to wilt. Cut back the entire plant by up to one-third of its height if it becomes leggy and begins to flower poorly. Deadheading and cutting back prevents seed formation, which encourages the plant to produce more blooms.
Monitor the foliage for mites, which are extremely small crawling insects that feed on the sap in the leaves. Rinse the marigolds with a sharp spray of water to wash away the pests, or spray the plants with a ready-to-use insecticidal soap at three-day intervals until the mites are gone.