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It is very risky to overwinter dahlias outside
Dahlias hibernate outdoors rarely works
Dahlias are very sensitive to frost. In winter, they must be protected from sub-zero temperatures. Ideally, the tubers are dug up and stored in the house. In sheltered locations and very mild winters, wintering can also succeed outdoors.
Winterizing dahlias outside
The problem is that you do not know in advance if the winter will be very cold. An attempt to overwinter dahlias outdoors is therefore only worthwhile if the planting site is located in a protected location.
Even wetness can be the tubers doom. If the winter is very humid, the moisture can not escape through the thick cover. This rots the roots.
Tips & Tricks
In addition to frost and too much wetness, voles are a danger for dahlia onions that overwinter outside. They eat up the tubers so that they will wait in vain for new shoots next spring.