Dahlias are classified according to flower shape and arrangement of petals. Single-flowering dahlias have one row of petals, generally grow to 3 ft or less, and have flowers 4 inches or less in diameter. Types of single-flowering dahlias include: singles, orchid-flowering, anemone, and collaretts. Single-flowering dahlias have a ring of evenly spaced petals. Orchid-flowering dahlias are similar to single-flowering types, but the ray florets curl up along the edges. Anemone flowers have an extra ring of small petals.

Double-flowering types have multiple rows of petals, grow taller, and have large flowers. Double-flowering types are also classified according to flower size and shape. Cactus dahlias have somewhat tubular shaped petals that curve backwards for over one-half of their length. Semi-cactus dahlias resemble cactus types but the petals are broad at their base and curve for less than half their length. Incurved cactus cultivars have petals that curl strongly towards the center of the flower. There are two types of decorative dahlias: formal and semiformal. Formal decorative types have broad, regularly arranged, pointed or rounded petals that curve back towards the stem. Informal decorative dahlias have long, irregularly arranged, often twisted petals. Ball dahlia flowers are usually 3 1/2 inches in diameter and ball shaped or slightly flattened. The quilled petals are blunt or rounded at the tip. Pompons are smaller version of ball dahlias with flowers less than 2 inches in diameter.