The genus Tagetes contains 40 species. All are native to the New World with the greatest diversity being found in south central Mexico. Three species have been chosen for research and cultivation as ornamental plants. They are:
- T. erecta – commonly called African or American.
- T. patula – commonly called French marigolds in spite of American origin.
- T. tenuifolia – Signet marigolds, unusual lacy leaves with small single flowers.
A fourth species, T. filifolia or Irish Lace, is not widely grown or known. It is planted for the foliage rather than flowers.
Marigold 'First Lady' is an African marigold. This species is characterized by larger leaf size and larger flowers than French marigolds. The flower forms are either semidouble or fully double and the colors are always solid. No bicolors occur in this species. The color range is from white and cream to primrose, yellow, gold and orange. There are many F1 hybrid T. erecta cultivars available to home gardeners. Since they are hybrids, they express hybrid vigor by producing numerous flowers on uniform plants with a long flowering season.
African marigolds grow to heights of 9 - 28 inches, for dwarf varieties, to 38 to 40 inches , for the longer stemmed varieties.
T. erecta species are day length sensitive. Each cultivar varies in the response to day length. If a home gardener is growing T. erecta from seed sown after March 1 and wants earlier flowering plants, a short day treatment can be applied. Just cover small seedlings with a light proof cover at 4:00 p.m. and remove at 8:00 a.m. This treatment can be applied for two weeks.
Source: National Garden Bureau