Answer: You'll want to wait until frost kills back the tops of your dahlias, then cut the stems at soil level and dig the tubers for winter storage. Allowing the tops to die down naturally sends the message to the storage organs that it's time to rest. After digging the tubers, brush off excess soil and store them in dry sawdust or peat moss in a cool, dark place with good air circulation. (A garage is a good place.) In the spring, when the soil warms, you can replant the tubers in a sunny location in your garden.
Dahlia tubers usually grow in a group clustered around a single stem, looking something like the spokes of a wheel. Before replanting you'll want to separate the tubers by cutting them from the old stem. You should see a bud or an "eye" at the top of each tuber. This will be the point of new growth so be careful not to cut that part of the tuber off when separating them from the stem. Plant the tubers 6-8" beneath the soil surface with the "eye" up, then set a wooden stake in the planting hole for future support of the dahlia. Some dahlias can grow 6-8" in height and the flowers get quite heavy so staking ensures the plants will remain upright even in wet weather. Space your tubers 2-3" apart so each plant has plenty of room to grow.
Best wishes with your dahlias!
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