Few flowering plants can top dahlias (Dahlia spp.) for showy, colorful flowers. These plants grow from brown tubers that resemble oval potatoes, and most bloom from the middle of summer until weather turns cool in fall. Dahlias are native to warm regions and grow outdoors year-round in U.S. Department of Agriculture plant hardiness zones 7 through 10, but they can also be lifted in fall in colder areas and stored indoors until the following spring, or you can grow them as annuals. Dahlia flowers come in dozens of shapes, colors and sizes, on plants that range from extra-tall to petite and usually spread to cover an area of 1 to 3 feet, depending on the variety.
Tall and Impressive Varieties
Dahlias are classified into 10 different groups, with some of these divided into sub-categories, so gardeners can choose from hundreds of flower types and plant sizes. Generally, most of the large-flowered varieties also tend to be tall, although there are some exceptions to this rule.
Cactus-flowered dahlias get their name from their spiky, tapered petals that end in sharp points; some types have straight petals, while others have petals that curve inward -- called incurved cactus types. Cactus-flowered dahlias produce flowers that range in width, with the largest 8 to 10 inches wide. These plants generally become 5 to 5 1/2 feet tall. Examples include the cultivar 'Encore,' with 10-inch wide golden flowers, and 'Myrtle's Folly,' which has 10-inch flowers that feature red petals tipped in yellow. Both plants grow about 4 feet tall.
Another type of dahlia called decorative is subdivided into formal and informal types. The latter has flat, slightly rolled petals irregularly arranged, while the formal type features evenly spaced, completely flat petals. These blossoms can be up to 10 inches wide, although some are smaller, as tiny as 2 inches wide. Decorative dahlias are usually tall, between 4 and 5 1/2 feet high. 'Clyde's Choice' is an example of a formal decorative plant, with bronze-colored, 12-inch-wide flowers on a 4 1/2-foot-tall plant.
Many types of dahlias have medium-sized flowers, usually on plants that are modest in height. For example, a type of dahlia called ball-flowered gets its name from its rounded, densely-petaled flowers that range in size from 2 to 8 inches wide. The plants are usually about 3 feet tall; 'White Nettie' is an example of this type that features white ball-shaped, 2 1/2-inch wide flowers on a 3 1/2-foot- tall plant.
Anemone-flowered dahlias feature flowers made up of a ring of flat petals surrounding an inner cluster of tube-shaped petals; these are usually on modest-sized plants, 2 or 3 feet tall, although some can be taller. Examples include 'Totally Tangerine,' with soft orange, 3-inch wide flowers on a 3-foot-tall plant, and 'Gitty Up,' with orange petals surrounding a deep red center -- for a 3-inch wide blossom -- on a 3 1/2-foot tall plant.
Dwarf Dahlia Choices
Many dahlia varieties are petite, making them effective choices for the front of a border or bed. Some dwarf plants produce small flowers, but others have larger flowers. For example, 'Baby Red' features single, red-petaled flowers with yellow central buttons 1 inch wide on a 1-foot-tall plant, while 'Mz. Bee Haven' has 3-inch wide flower called a collarette -- with one row of flat red petals surrounding a central orange disc -- on a 2-foot-tall plant. 'Red Pygmy' is a cactus-flowered plant with red, 4-inch-wde flowers on a 2-foot tall plant.
For best results, plant dahlias in full sun and well-drained soil. Keep plants evenly moist, watering whenever the top few inches of soil feels dry. For taller types, use a strong stake driven into the ground for support, but set this in at planting to avoid damaging roots. In cold-winter areas, dig tubers after frost arrives and store them in a cool dry spot until re-planting them the next spring.