There are hundreds of varieties of Dahlia to choose from. Below are some of the more common classes of Dahlia, based on the size and form of their flowerheads.
For more variety information: American Dahlia Society
Ball-flowered cultivars have rounded, dense, 2- to 8-inch-wide blooms composed of wide, slightly curled petals. The flowers sometimes have flattened tops. Plants grow 3 to 4.5 feet tall.
Pompom-flowered cultivars have rounded, 2- to 4-inch-wide, globular blooms composed of wide, rounded petals. The outer petals are wholly curled, the inner petals are often gently curled. Plants grow 2.5 to 4 feet tall.
Collarette cultivars have unusual 2- to 6-inch-wide blooms that consist of an outer ring of nearly flat, large petals, and an inner ring of short, small petals that surround the central disk as a "collar." Plants grow 2.5 to 4 feet tall.
Fimbriated cultivars have large, 6- to 8-inch-wide blooms with split petals that give the flowers a fringed look. Plants grow 4 to 5.5 feet tall.
Formal decorative cultivars have large, 2- to 10-inch-wide flattened blooms composed of wide, flat petals. Plants grow 4 to 5.5 feet tall.
Informal decorative cultivars have 2- to 10-inch-wide (or larger) blooms comprised of wide petals that may be curled or twisted to give the overall bloom a casual, full look. Plants grow 4 to 5.5 feet tall.
Some groups have flowers that resemble other species:
Anemone-flowered cultivars have blooms that consist of a ring of flat outer petals and an inner ring of tubular inner petals. The blooms resemble anemone blooms, hence the common name. Plants grow 2 to 4 feet tall.
Orchid-flowered cultivars have exotic and unusual, 2- to 4-inch-wide, star-like blooms composed of a single row of tightly curled petals. Plants grow 3.5 to 4.5 feet tall.
Peony-flowered cultivars have 2- to 6-inch-wide blooms composed of two to many rows of wide, rounded petals that often gently curve inward. Plants grow 3 to 4 feet tall.
Single-flowered cultivars bear daisy-like, 1- to 6-inch-wide blooms. The flowers consist of a single row of petals around a central disk. Plants grow 1.5 to 2 feet tall.
Waterlily cultivars bear 4- to 8-inch-wide blooms that consist of flat, cupped, wide petals that resemble water lilies. Plants grow 3 to 4.5 feet tall.
Some groups have flowers that resemble cactus flowers:
Straight cactus-flowered cultivars have 2- to 8-inch wide blooms composed of downward curled petals that are arranged in either a flat or recurved manner. Plants grow 4 to 5.5 feet tall.
Semi-cactus-flowered cultivars have 2- to 10-inch-wide (or larger) blooms composed of petals that are curled for up to one half their total length. The petals may be arranged in a flat, incurved, or recurved manner. Plants grow 4 to 5.5 feet tall.
Incurved cactus-flowered cultivars have 2- to 8-inch-wide blooms composed of curled, nearly tubular petals that curve inward. The outer petals are only gently incurved, and the inner petals are increasingly incurved as they near the center. Plants grow 4 to 5.5 feet tall.