Dahlias are popular for their dramatic, vibrant blooms in summer and autumn gardens. The question of whether these plants “come back” each year is common, and the answer depends entirely on where they are grown. While dahlias are botanically classified as perennial plants, their ability to survive the winter is conditional. Most gardeners find that successfully bringing their dahlias back requires specific intervention to protect them from cold weather.
Understanding the Dahlia’s Tuberous Structure
The dahlia returns via its tuberous root structure, often mistakenly called a bulb. The tuber is a swollen root that stores energy and starches to fuel the plant’s growth the following season. Unlike a true bulb, the dahlia tuber lacks protective layers. New growth emerges from small, dormant buds known as “eyes,” located on the crown where the tuber connects to the old stem. For successful regeneration, a tuber must be divided to include at least one viable eye attached to a portion of the tuber.
Climate Determines Whether Dahlias Return
The survival of a dahlia tuber is directly tied to the soil temperature during the winter months. Dahlias are native to regions with mild winters and cannot tolerate freezing temperatures. A sustained, hard freeze causes the water-filled tubers to freeze, resulting in a dead, mushy tuber when the ground thaws.
Gardeners in USDA Hardiness Zones 8 through 10, or equivalent mild climates, can generally leave their tubers in the ground year-round. The soil provides enough insulation to protect them from freezing. In colder zones, particularly Zone 7 and below, the risk of the ground freezing is too high for survival without protection. A guaranteed return in these areas requires digging up and storing the tubers indoors.
The Process of Lifting and Storing Tubers
For those in colder climates, ensuring the dahlia’s return involves carefully “lifting” the tubers and storing them indoors for the winter. The first step is to wait until the foliage has been killed by the first hard frost, signaling the plant is dormant and has finished storing energy. After the frost, leave the tubers in the ground for one to two weeks to allow the skin to toughen, or “cure,” before digging.
When lifting, cut the foliage back to a stem of four to six inches. Use a pitchfork to gently loosen the soil at least a foot away from the stem, minimizing damage to the brittle tubers. Once the clump is out, carefully brush or wash off excess soil to prevent rot during storage.
The tubers must be dried completely in a cool, dark, and well-ventilated location for a few days to a week. After drying, they are packed in a storage medium to maintain moderate humidity and prevent shriveling. Suitable materials include peat moss, vermiculite, or wood shavings, used to line a ventilated container like a cardboard box or plastic crate.
The storage location must maintain a consistent temperature between 40 and 50 degrees Fahrenheit and remain frost-free. Periodically check the tubers throughout the winter. Shriveled tubers indicate conditions are too dry and need misting, while mold or rot suggests excess moisture and poor ventilation.
Reawakening and Spring Management
The dahlia’s annual cycle completes when tubers are prepared for replanting in the spring. Clumps should be divided beforehand, which is easier when the eyes swell and become visible. Each division must have at least one viable eye and a piece of the tuber body to ensure growth. Dividing clumps increases plant stock and improves health by preventing overcrowding.
Tubers can be started indoors in containers a few weeks before the last expected frost to get a head start. Alternatively, plant them directly into the garden after all frost danger has passed and the soil temperature reaches about 60 degrees Fahrenheit. Gardeners in mild zones who left tubers in the ground should check for new sprouts and remove any protective mulch applied in the fall.