Dahlia come from the warm areas of South and Central America and can be grown and left in the ground during the winter in U.S. Department of Agriculture plant hardiness zones 8 through 10. However if you live in an area of zone 8 that near zone 7b or you in an isolated cold spot in zone 8 that drops below freezing in the winter, you may need to dig your dahlia tubers up in the early fall and store them for the winter.
When to Dig
There is no specific calendar date to tell you when to dig dahlia tubers. They do not mature until late in the summer or early fall. Tubers that remain in the ground longer develop more fully and are more likely to survive storage over the winter. A light frost, when the temperature drops between 29 to 32 degrees Fahrenheit, may ruin partially opened blooms, but the plant may continue to grow. Many areas may experience a light frost followed later by a severe or killing frost when the temperature drops below 24 F. The American Dahlia Society recommends that gardeners on the Pacific Coast wait until a severe frost to dig up tubers.
Selecting Tubes to Dig
Before digging up dahlia tubers, go through your garden and remove and dispose of weak plants that appear diseased or that yielded inferior blooms.
Dahlia stems should be cut 2 to 4 inches above the ground after the first severe frost. Clumps of tubers are best dug with a spading fork beginning one foot away from the plant’s center. After you brush large chunks of soil away, allow the clumps to dry for few days in a ventilated, shaded area then remove the rest of the soil. The tubers are best packed upside down in perlite or vermiculite in a basket or ventilated box for winter storage.
Dahlia tubers do best when stored in temperatures between 35 and 50 degrees Fahrenheit. They should not be exposed either to dry or damp conditions or to hot or cold drafts. If they begin to shrivel, sprinkle them lightly with water.
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