Why Dig?

• Digging up dahlia tubers will protect them from hard frosts and prevent them from freezing. It allows you to divide bulbs to maintain a consistent size among bulbs and flowers. The more offshoots the tubers send out, the smaller the next season's flowers will be. Digging and dividing allows you to inspect the tubers and "rejuvenate" them, according to the Snohomish County Dahlia Society.
  
  

When to Dig

• After a killing frost, the stems will begin to turn black. Cut the stems down to a height of 6 inches and leave them in the ground for at least another two weeks before digging. The Oregonian recommends leaving the bulbs in the ground as long as possible to allow them to form the thick outer skins that prevent them from drying out in storage.
  
  

When to Leave Tubers in the Ground

• According to the Oregonian and the Snohomish County Dahlia Society, if you live in an area that doesn't experience harsh winters, you may be able to leave your dahlia tubers in the ground. The risks you face include a winter colder than expected, which might kill the tubers, and waterlogged soil, which will rot them.
  
  After the foliage is killed, cut the stems down below the surface of the ground and cover the area with black plastic, then add mulch. Lift the mulch in the spring and watch your dahlias re-emerge.