Dahlia flowers (Dahlia spp.) range in color from deep purples to striking yellows to variegated pinks, with diverse shapes and patterns. These perennials came originally from Central America but have been hybridized extensively. Choose from more than 20,000 cultivars, which range in size from 1 to 6 feet tall when mature. Although they're perennials in some climates, you can grow them almost everywhere with the right care.
Perennial or Annual
Dahlias generally grow as perennials in the warm climates of U.S. Department of Agriculture plant hardiness zones 8 through 10, but are treated as annuals or dug up each year in cooler climates. Due to extensive dahlia hybridization, you may find cultivars you can leave in the ground to grow as perennials year-round in most climates. When choosing dahlias, pick varieties rated for your USDA zone to grow them as perennials. If the one you love isn't hardy in your climate, dig it up in fall and overwinter the tuber indoors, or simply treat it as an annual and buy new plants next year.
Digging Up Dahlias
You can grow dahlia varieties anywhere and dig them up at the end of summer. At the end of the growing season, allow the first frost to kill the plant. After three or four days, cut the stem back to about 4 inches tall, then dig up the tubers. Rinse the tubers with a garden hose and spread them out in a single layer to dry for 24 hours. Store the dried tubers in a paper bag in a cool, dark area for winter. Do not store them in plastic, which leads to rot. If you wish, place a layer of sawdust in the bag followed by a layer of tubers then another layer of sawdust, alternating until the bag is about one-half full. Check the condition of the tubers once a month while they're in storage, and remove any that are rotten. If they appear shriveled, lightly spritz the sawdust with water. In spring, plant the tubers in pots with drain holes four to six weeks before the last frost, or directly plant them in the garden once it has warmed to 60 degrees Fahrenheit.
When growing dahlias as perennials in warm climates, they do not need to be lifted from the ground in fall and overwintered indoors, but they may benefit from it. To overwinter them in the ground, cut back the foliage to 4 inches tall when the plants stop producing flowers. Dahlias that grow in USDA zones 8 through 10 can be left in the ground. In USDA zone 7, they may survive if you place a 10-inch layer of organic mulch over them. Plants in USDA zone 8 may also benefit from mulching. Tubers can be dug and divided in fall to make more plants, or if you have dug and stored them, divide the tubers in the spring, just before planting.
Sun or Shade
Dahlias are grouped into classes by their petal shape and bloom size, but all have the same cultural requirements. Dahlias prefer full sun for at least five to six hours each day in northern zones, but filtered shade during hot afternoons in warm climates. Dahlias are easy to grow in well-draining, rich soil with a pH between 6.5 to 7.0. Keep the soil moist but not soggy.