The best method for winterizing your dahlia plants (Dahlia spp.) depends on where you live. Dahlias grow in U.S. Department of Agriculture plant hardiness zones 7 through 10. Below USDA zone 7, the tubers -- sometimes called bulbs -- rarely survive winter if left in the ground, and they must be lifted and stored indoors. In USDA zones 7 and 8 dahlia tubers in the ground may be at risk of snap frosts and are safer lifted and stored. In USDA zones 9 and 10, dahlia tubers can overwinter in the ground.
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Lifting Dahlia Bulbs
In areas that experience hard frosts, lift dahlia tubers after the first frost. Freezing temperatures cause dahlia stems and leaves to die back. In areas that rarely experience frosts, lift dahlia tubers in fall after the plants have stop flowering and growing. Before lifting dahlias, prune the stems to 3 to 4 inches above the soil surface.
Sterilize pruning shear blades by wiping them with a cloth soaked in rubbing alcohol before and after pruning dahlias.
Push a garden fork into the soil 6 inches from the edge of the dahlia plant base. Lever the fork upward to loosen the dahlia tubers in the soil.
Push the fork into the soil and lever it upward around the dahlia base, until the tuber lifts out of the soil.
Brush off loose soil from the dahlia tuber.
Place the tuber in a dry area, upside down.
Break off the remaining soil when the soil is dry, which is usually after one or two days.
Winterizing Dahlia Tubers
Packing dahlia tubers in moist peat, sawdust or vermiculite, and storing them in a cool place, helps them survive over winter.
Check the dahlia tubers for signs of disease or decay, such as soft patches, discolored areas or a bad smell. Throw out diseased and decaying dahlia tubers as they may spread the problem to other tubers.
Spread a layer of moist peat, sawdust or vermiculite 2 or 3 inches deep on the bottom of a wooden or cardboard box or a basket.
To moisten peat, sawdust or vermiculite, sprinkle it with water while turning it over constantly, until the material feels damp to the touch, but not wet.
Place the dahlia tubers on top of the peat, sawdust or vermiculite, and add more of the same packing material until the tubers are buried.
Place the filled box or basket in a dry place at 40 to 55 degrees Fahrenheit over winter.
Check the tubers every three or four weeks for signs of disease or decay. Remove and throw out any diseased and decaying tubers.
Winterizing Dahlias in Containers
Small dahlia varieties grow well in containers with drainage holes. In USDA zones 8 and lower, move the containers to a dry, frost-free area, such as a garage or shed, when the dahlias have stopped growing in fall. Water the potting soil occasionally during winter so it stays just moist.
Protecting In-Ground Dahlia Tubers
In USDA zones 9 and 10, dahlia tubers don't need protection when left in the garden over winter, but in USDA zones 7 and 8, spreading a layer of mulch helps protect against snap frosts. Spread a layer of straw, compost or other organic mulch 3 to 4 inches thick over the dahlia bases when the plants have stopped flowering and growing in fall.