Dahlia flowers are showstoppers in the sun-filled garden and the flower vase. Available in a wide array of colors and flower forms, dahlias bloom from midsummer to frost, adding wonderful color to the late-season garden. They also are long-lasting as cut flowers. Dahlia tubers are planted in the spring, after the last frost date in your area. They prefer well-drained soil and full sun. Because some dahlias grow quite tall and produce flowers up to 12 inches across, they may require staking. In zones 3-7, dahlia bulbs must be lifted, or dug up, in the fall and stored indoors over winter. When selecting dahlia varieties for sale online, first consider flower form, such as ball, decorative, dinnerplate or cactus. Then select the color.