This group consists of twenty-seven deciduous, tender, tuberous-rooted perennials, which are native to Mexico. Dahlias may be grown anywhere, but are only hardy year-round in warm climates; they can simply be grown as annuals or dug up and stored in cold-winter regions. Dahlias may also be grown in containers or tubs, depending upon the size of the variety. From these Mexican species, hybridizers have developed a multitude of forms of varying heights, as well as distinct flower shapes, sizes and colors. Dahlias range in height from about 1 foot to 7 feet. They may produce button-sized flowers or blossoms as large as a dinner plate, in most every shade except blue. Dahlias bloom from early summer to fall and their flowers will last up to a week when cut. The flowers may be single or double, some with twisted petals. Collarette Dahlias have a collar of petals growing around a central disk. Ball-shaped Dahlias form compact globes of petals. Cactus-flowered Dahlia's blooms are quite large with numerous showy petals. Dahlia merckii is a medium-sized plant with lilac ray florets, which are pointed at the tips. Dahlia coccinea produces scarlet, single flowers with brilliant yellow disks. These two species were the forerunners of the single and Collarette hybrids. Dahlia imperialis is a tall species with white flowers and reddish central disks. These species are especially adapted for growing in the sweltering, humid atmosphere of the South.

Dahlias can be grown as perennials from zones 8 to 11. They can be grown as annuals or dug up and stored in regions where temperatures fall to 10º F. Dahlias should be grown in acid to neutral, well-drained, but moist, organic soil. Plenty of material rich in humus (leaf mold, manure, etc.) can be mixed in prior to planting. Mulches of old manure, compost, or lawn mowings will be beneficial. The main thing is to prevent dryness at the roots. Choose a sunny location with some midday shade if grown in hot climates. In the spring, plant the tuberous roots with their crowns 3 inches below the soil's surface. As you plant, set strong stakes next to the roots of medium to tall varieties to avoid damaging them later. As the shoots grow, add one to three inches more of soil. The shoots of Dahlias should be pinched back twice to ensure a bushy plant. The first time should be after three sets of leaves have grown. Pinch the growing tip just above the third pair of leaves. They should be pinched again a few weeks later. Taller varieties should be tied to their stakes with strips of cloth or nylon stockings. Dahlias should be watered well and given a low-nitrogen fertilizer during the growing season. If they're fertilized with too much nitrogen, they will grow lots of foliage with few flowers. In climates where winters are cold, clumps or individual tubers may be dug up and stored in the fall. Don't wash the tubers as this may cause them to rot. Instead, gently brush off the soil, taking care not to break or cut the fragile tubers. Dry the tubers for a day and store them in shallow trays of dry sand or vermiculite in a temperature of about 40º F. If they are too dry or too warm during storage, the tubers will shrivel. Check the tubers occasionally throughout the winter.

The clumps of tubers may be divided in the spring. Make sure to include a piece of the old stem in each division. Seeds may be sown indoors in a 70º F temperature, 12 to 16 weeks before it is safe to transplant outdoors. They should be planted in flats of sifted sandy loam. Cover the flats with glass and then with paper to maintain a constant temperature and moisture. When the seedlings reach 1 or 2 inches, they are potted individually in small pots filled with loam and leaf mold. When they've filled up those pots with roots, they may be transplanted to larger ones; eventually they are hardened off, prior to planting outdoors. Seeds may also be sown directly outdoors as soon as it is warm. Seeds of dwarf Dahlias are usually available from garden shops in the spring.

Dahlias are classified according to their formation and type -
Single Dahlias; Mignon Dahlias; Orchid-flowering Dahlias; Anemone Dahlias; Collarette Dahlias; Peony Dahlias; Incurved Dahlias; Straight Cactus Dahlias; Semi-Cactus Dahlias; Formal Decorative Dahlias; Informal Decorative Dahlias; Ball Dahlias; Miniature Dahlias; Pompon Dahlias; Dwarf Dahlias.
 
Some of the Mexican Species:
Dahlia merckii;
Dahlia coccinea;
Dahlia imperialis.