Dahlias offer flamboyant flowers on lush plants from summer through fall, right up to the first frost. These hybrids of species native to Mexico and Colombia may overwinter with a thick blanket of mulch. Gardeners in colder zones can get a head start by planting tubers in pots 4 to 6 weeks before the last spring frost date.
Plant several inches deep in a light, soilless potting mix and water sparingly until new growth appears, then more freely. Place pots in a sunny window or under grow lights, and then plant outside after danger of frost has passed. Taller varieties will need staking, with stakes placed carefully so as not to injure the roots. If planting, either indoors or in the ground, is delayed, store the tubers in their bags in a cool, dry, dark location.
Dahlias are at their best when grown in full sun in the North, afternoon shade in the South. Do not water until growth appears above the ground; once plants are established, a deep watering twice a week will get them through summer dry periods.
Dahlias make excellent cut flowers; to achieve nice stems for cutting and bushier, compact plants, pinch out the center shoot just above the third set of leaves. These shoots can be rooted, if desired. To get the most out of your cut flowers, place them in very hot water (160 degrees F) until it cools.