Dahlias are popular, tuber-forming plants that bloom from mid-summer until the first hard frost. They are botanically classified as tender perennials, meaning they can live for multiple years only if the climate prevents their underground tubers from freezing. Given Michigan’s cold, protracted winters, which place the state primarily in USDA Plant Hardiness Zones 4 through 6, dahlias cannot reliably survive the winter outdoors. While technically perennial plants, they will not naturally return year after year in Michigan without intervention.
Defining Perennials in Michigan’s Climate
The term “perennial” generally refers to a plant that lives for more than two years and persists through the winter. Dahlias are native to the mountainous regions of Mexico and Central America, where temperatures rarely drop below freezing. This origin makes them ill-equipped to handle the severe cold of a Michigan winter, where soil temperatures can plunge below 32°F for extended periods.
Michigan’s hardiness zones, which range from 4a to 6b, mean the average annual minimum winter temperature can fall between -30°F and -5°F. These conditions are lethal to the fleshy dahlia tubers, causing them to freeze and turn to mush. Gardeners in zones 7 and below, which includes all of Michigan, must actively preserve the plants to ensure their return. This need for intervention is why dahlias are often referred to as annuals in cold climates.
Winter Storage for Dahlia Tubers
To successfully grow dahlias as returning plants in Michigan, gardeners must lift the tubers after the first hard frost kills the foliage. This process should begin immediately after the leaves turn black, signaling that the plant has entered dormancy. After carefully digging up the clumps, gently remove the excess soil. Tubers should not be washed unless heavily caked with mud, as excess moisture promotes rot.
The clumps must then be cured in a cool, dark, and frost-free location for several days to a week, allowing the skin to toughen and any surface moisture to evaporate. Once cured, the tubers require a precise environment for dormancy: a cool temperature range between 38°F and 50°F is considered ideal. Temperatures above this range may cause the tubers to sprout prematurely, while temperatures below it risk freezing and destruction.
Humidity is just as important as temperature, requiring 75% to 85% relative humidity to prevent the tubers from dehydrating and shriveling. Common storage mediums include dry peat moss, vermiculite, wood shavings, or newspaper, which help maintain a stable, high-humidity environment. Store the tubers in ventilated containers like cardboard boxes or crates to ensure adequate airflow and prevent mold. Check stored tubers periodically throughout the winter for signs of shriveling or mold.
Replanting and Dividing in Spring
Once the risk of a hard freeze has passed, typically in mid to late May for most of Michigan, the stored tubers can be prepared for replanting. Before planting, each stored clump should be carefully inspected for soft, rotted areas or excessive dehydration and shriveling, and any non-viable parts must be removed. This inspection is a good time to divide the tubers, which is necessary to ensure vigorous growth and increase the number of plants.
Division requires separating the tubers so that each new section has at least one visible “eye” or growth bud, which is located on the crown near the stem. Without an eye, the tuber will not sprout a new plant, even if it appears healthy. The divided tubers are then planted in the garden after the soil has warmed sufficiently, typically to about 60°F. Planting the tubers back into the ground completes the annual cycle, treating the dahlia as a preserved perennial that is re-established each spring.