Dahlias die back after the first heavy fall frost but store energy in tubers over the winter in preparation for new growth in the spring. Freezing harms the tubers, so if you live where the ground freezes, you'll need to help your dahlias survive the winter by storing the tubers in a more friendly environment. "The most important considerations for a good storage area are humidity and temperature," notes Bill McClaren in "Encyclopedia of Dahlias."
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Things you need
- Garden pruners
- Shovel or garden fork
- Hose with sprayer
- Root cellar, refrigerator or other cool, humid area
Trim the tops of the dahlias to 4 to 6 inches with garden pruners after the first frost but before the ground freezes in the fall. Dig up the dahlia tubers with a shovel or garden fork, working carefully so you don't break or cut them.
Wash the dirt off the tubers with spray from a hose. Cut off any long, thin roots and trim the stems to 1 inch long with garden pruners. If you'd like to propagate your dahlias, divide clumps of tubers with a sharp knife, leaving at least one eye on each one
Place the tubers in a humid, dark location that will stay cool but not get below freezing. From 4.44 to 10 degrees Celsius with 90 per cent humidity is ideal. Some possible storage locations are in a root cellar, in a refrigerator or packed in a container of damp sand in the coolest part of a heated garage or basement. You can also dig a hole in the garden deeper than the ground freezes, lay the tubers in it surrounded with straw or leaves and bury them over the winter.
Check the tubers during the winter if you're not sure the storage area is optimum. If they're shrivelled, the area is too dry. Mist them with water and increase the humidity. If some have rotted, discard the bad ones and lower the humidity. If you see mould on them, keep them both cooler and drier.
In the spring, remove the tubers from storage, divide them if you didn't already divide them in the fall and plant them in the garden again at the same depth from which you dug them up.
Tips and warnings
- Dig up each variety separately and mark the tubers with a code letter or number, using a permanent marker, so you'll know which variety is which in the spring.
- If the humidity is under 50 per cent, dig the tubers from the ground and move them to a humid storage area the same day so they won't dry out.
- To increase the humidity in storage, pack the tubers in damp sand, sawdust, peat or vermiculite inside a plastic bag with a few holes punched in it.
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