Although often referred to as a "bulb" plant, dahlias are actually grown from tubers, which are sections of dormant roots. Dahlia tubers are planted in the spring, and the shoots usually emerge in a few weeks.
Dahlia plants grow up to 12 feet tall, depending on the variety. The colourful flowers are classified by the shape and arrangement of the petals. Single dahlias have one row of petals, and resemble daisies, while peony dahlias have two to five rows of petals, and resemble peonies. Anemone dahlias have a single row of petals around a central disk of smaller tubular petals, and collarettes have a single row of petals around a row of smaller petals that are usually a different colour. The petals of orchid-flowering dahlias turn inward and are tubular. Double-flowering dahlias include ball dahlias, which are ball shaped or slightly flattened, with the petals in a spiral. The flowers are 3 1/2 inches across or larger. Miniature ball dahlias are 2 to 3 1/2 inches across, and pom-pom dahlias are smaller than 2 inches across. Formal decorative dahlias have broad, rounded or pointed petals that are arranged regularly, while informal decorative dahlias have long petals that are sometimes twisted. The petals of cactus dahlias curve backward and are tubular.
Dahlias grow best in slightly acidic, sandy loam with at least six hours of sun per day. Plant the tubers after the last predicted date of frost in the spring in a hole 6 to 8 inches deep. Insert a stake into the soil next to the tuber, and cover it with 2 to 3 inches of soil. As new growth appears, add soil until it is level or slightly mounded. When foliage is 12 to 20 inches tall, break the centre stem just above the top pair of leaves to encourage more stems for a bushier plant.
Dahlia tubers are not cold hardy, and have to be dug up and stored in the winter. Gently dig up the tubers in the fall, and pack them in sawdust or peat. Store the tubers in a cool, dry place, where the temperature does not drop below freezing. Before planting in the spring, use a sharp knife to divide each tuber into several pieces with an eye or stem on each piece.