Dahlia variabilis are the dahlia species commonly found in garden plots. They are closely related to Dahlia pinnata, which are native to Mexico. Dahlias prefer warm climates, well draining soil and moisture. They are classified as tender perennials and can survive winters outdoors in U.S. Hardiness Zones 8 through 11. Zone 8 growers provide plants with mulch for added winter cover. Even in marginal zones, like 7b, overwintering dahlias outdoors is risky. Gardeners in cool climates overwinter dahlias indoors.
In cool climates dahlias act as annuals and die back during the first frost. Gardeners make use of this first die back to allow the flower's tuberous roots to achieve dormancy. Once dormant, gardeners cut back the dead stems to within 6 inches of the ground and then gently unearth the tubers with a spade or pitchfork. Gardeners delicately loosen the dirt from the tubers and allow the tubers to dry, a process that takes roughly 24 hours. During this time the tubers need to be in above freezing temperatures or they will incur damage. Stems are again cut back to roughly an inch from the root. To avoid parasitic growth gardeners remove small hairlike root stems. Tubers are then placed in sand or any other sterile, highly porous medium. The medium is kept slightly moist and the containerised roots are kept within 10 to 18.3 degrees Celsius. These roots, if properly overwintered, are planted in spring when they yield new shoots and, ultimately, new flowers.
Even in warm climates, when the growing season has concluded gardeners leaving dahlia root systems in the ground protect their ornamental crops by laying a loose layer of mulch over the planting beds. Leaves, shredded newspaper or wood chips are adequate cover. Take care not to compress the mulch as this will allow moisture to build up and potentially damage the interred tubers.
In warm climates where dahlias never die back completely tubers continue to grow and yield new shoots and flowers. This method of procreation is termed "vegetative." Dahlias also reproduce via seed but since dahlias are often cross bred the seeds yield flowers quite unlike the mother plant. Most gardeners plant new dahlias from tubers to ensure a uniform appearance. In spring perennial dahlias are uprooted and the root systems divided. The offspring of the divisions are replanted in other areas of the garden or given away.
Dahlias are available in a wide range of shapes and colours. From white to deep maroon, to daisy-like petal configurations to tightly concentric mum-like pom poms, gardeners easily find dahlia cultivars that are well-suited for their particular garden design. The flowers are succulent stemmed and often used for cut flower arrangements. Because they are tall -- some reaching heights up to 6 feet -- gardeners must provide them with structural support. Not all gardeners find dahlias low maintenance garden additions. Between providing them with support structures, maintaining their shape and keeping up with their overwintering requirements, most find the flowers are well suited for advanced gardeners or those with extra time to spend in the dirt.