Dahlias are easy-to-grow, perennial flowering plants that bloom from mid-summer through fall. There are countless dahlia species and cultivated varieties, with more cultivars created each year. Dahlia flowers may be white, yellow, orange, pink, purple, red or scarlet. Dahlia types are classified according to the characteristics of their blooms. The flower shape and petal arrangement determine in which category each dahlia belongs.
Single-flowered dahlias have a single ring of petals on their flowers. The flower petals can be rounded or pointed-tipped and may overlap, but are generally evenly-spaced. Single-flowered dahlias usually grow to 3 feet or shorter and have smaller flowers that are 4 inches wide or less. Orchid dahlias fall into this category, but their petals curl upward along the edges and overlap each other for at least two-thirds of their length. Some common single-flowered dahlias are the 'Moonfire,' 'Imp,' 'Red Dwarf' and 'Princess Marie Jose.'
Anemone-flowered dahlias have one or more rings of evenly-spaced petals that are flattened. The petals of the inner ring are elongated, creating a dense grouping and showing no central disc of the flower. Anemone-flowered dahlias include the 'Scarlet Comet,' 'Honey' and 'Purpinka.'
Collarette dahlias have one outer ring of flat petals that surround the central disc and an inner ring of small petals that form a collar around the disc. Collarette dahlia cultivars include 'Mariposa,' 'Christmas Carol' and 'Elizabeth Snowden.'
Cactus and Semi-Cactus
Cactus dahlias have fully double blooms that show no central disc. The petals are pointed, usually narrow, straight and rolled for half their length. Cactus dahlias' petals are rolled their entire length and curve upward and toward the center of the flower. Semi-cactus dahlias have petals that are flat at the base and rolled for less than half their length. 'Kiwi Gloria' is a cactus-type dahlia, while 'Reginald Keene' is a semi-cactus dahlia.
Peony-type dahlias have two or more rings of petals surrounding a central disc. These are open-centered flowers with petals that are flattened and may be irregularly-shaped. 'Bishop of Llandaff' is a peony-flowered dahlia cultivar.
Decorative dahlias have fully double blooms with no central disc showing and broad, flattened, blunt petals. Informal decorative types have an irregular petal formation, while formal decorative dahlias have evenly-spaced petals. 'Almand's Climax' is an example of a formal decorative dahlia.
Ball and Pompom
Ball dahlias have ball-shaped or sometimes slightly flattened, fully double, 3 ½-inch flowers with blunt, rounded or indented petals. The petals are arranged in a spiral formation and are rolled inward along more than half their length. Pompom dahlias' flowers are similar to ball types, but they are more spherical in shape and smaller in size, usually less than 2 inches in diameter. Also, pompom dahlia flower petals are rolled inward along the entire length of the petal. 'Riisa' is a ball dahlia, while 'Hallmark' is a pompom variety.