The African marigold (Tagetes erecta) has the largest flowers of all the marigold varieties. The flowers are commonly 3 to 4 inches in diameter; when compared to the 2-inch flowers of French marigolds and some hybrids, the African marigold is a giant variety. Other common names include Aztec marigold and big marigold, according to the Missouri Botanical Gardens. Though the name suggests a different origin, African marigolds are native to Mexico. Grow these large annuals from seed each year to infuse your garden with giant yellow flowers.
Purchase seeds in early spring. Some yellow African marigold varieties to try are Antigua primrose, Marvel yellow, Antigua yellow and Inca yellow.
Start giant yellow marigold seeds indoors two to three weeks before winter frosts leave your area. Use a seed tray, 2-inch seed-starting cells or individual 2-inch pots. Plastic, biodegradable peat or cardboard or ceramic are all suitable pot materials to use.
Make your own potting mix using equal parts peat moss, vermiculite and perlite or purchase a ready-mixed seed-starting formula at a local garden centre. Put the seed-starting mix into a bucket and mix it with water until it is damp and crumbly.
Place the seeds 1 inch apart in the seed flat or one seed per cell or pot when using 2-inch containers. Cover the giant yellow marigold seeds with 1/4 inch of soil.
Keep the pots containing the giant yellow marigold seeds at 21.1 to 23.9 degrees Celsius. The marigold seeds will germinate in three to five days.
row the giant yellow marigolds indoors until they are 2 to 3 inches tall and until the ground thaws and the last frosts are gone.
Transplant the marigold seedlings outdoors into a sunny area where the ground drains well. Dig holes 2 inches wide and the same depth as the seedling's root ball. Slide the marigold seedlings out of the containers. Space the marigold seedlings 12 to 18 inches apart.
Water marigolds once a week during mild weather and twice a week during hot, dry weather. As long as the soil is still damp 1 inch below the surface, the roots are getting enough water.