Studies have concluded they are not effective in reducing insect damage on vegetable crops. French types may be useful for root-knot nematode control in soil. These marigolds have large, double flowers from midsummer to frost. Flowers may measure up to 5 inches across. They can grow as tall as 36 inches.
African marigolds are excellent bedding plants.
Tall varieties can be used as background plantings. African marigolds are often called American marigolds. Actually, all marigolds are native to subtropical America, and have been cultivated in Mexico for over 2,000 years. Marigold cultivars are usually sold as part of a series with similar growth characteristics and a wide range of color. Antigua' series marigolds grow 12 to 16 inches tall.