The question of whether the dahlia grows from a bulb or a tuber is a common point of confusion. While both are underground structures that store energy, dahlias are definitively not bulbs. The plant produces its impressive blooms from specialized, fleshy roots known as tubers. This botanical distinction influences how the plant must be handled, planted, and preserved across seasons.
The Botanical Distinction: Tubers and Bulbs
The primary difference between a tuber and a true bulb lies in its anatomical structure and origin within the plant. A true bulb, such as a tulip or onion, is a modified stem with layers of fleshy scales that are actually specialized leaves surrounding a central embryonic shoot. These layers contain the stored food reserves and are attached to a flat basal plate at the bottom, which is where the roots emerge.
A dahlia tuber, in contrast, is a modified, swollen root that functions purely as a storage organ for starches and water. The tuber body itself is made of undifferentiated tissue and does not contain the layered structure found in bulbs. New growth, known as “eyes” or buds, is concentrated on the crown, the narrow area where the tuber connects to the main stem of the plant.
This structural difference dictates how new plants grow. A bulb’s growth is directed from the central basal plate, while a dahlia tuber must have a viable eye on its crown to produce a new shoot. Confusion arises because both structures are underground parts that allow the plant to survive dormancy.
Planting and Growing Dahlia Tubers
Understanding the tuber’s anatomy directly impacts the correct planting technique. Dahlia tubers should be planted outside after the last expected frost, when the soil temperature has warmed to at least 60°F (15°C). The general planting depth should be shallow, typically placing the tuber about 4 to 6 inches deep in the soil.
The tuber must be laid horizontally in the planting hole with the crown, where the eyes are located, facing upward. Planting too deep can cause the tuber’s stored energy to be used up before the shoot breaks the surface, leading to rot. Avoid watering the tuber immediately after planting, as excess moisture before the plant is actively growing can cause the fleshy root to rot. Once the plant has sprouted, a regular watering schedule and the addition of support, such as staking, become necessary.
Essential Care: Overwintering Dahlia Tubers
Dahlias are tender, meaning their tubers cannot survive freezing temperatures in the ground. Gardeners in cold climates, generally USDA Hardiness Zones 7 and below, must lift the tubers for winter storage. This process begins after the first hard frost has blackened the foliage, signaling the plant has gone dormant.
The tuber clump is gently dug up, excess soil is brushed off, and the clump is allowed to cure for a day or two to dry the skin and allow any moisture to drain. The goal of storage is to find a balance between a cool temperature and slight humidity. Tubers should be stored in a dark, frost-free location, ideally maintaining a temperature range between 40 and 50°F (4–10°C).
Storage media like peat moss, wood shavings, or newspaper help maintain the necessary humidity to prevent the tubers from shriveling and drying out. Periodic inspection throughout the winter is necessary to check for signs of rot, which indicates too much moisture, or excessive wrinkling, which suggests the storage conditions are too dry. Proper storage ensures the tuber’s viability for the next season’s growth.