Posted: 18/10/2014 at 11:34
Rob, we used to live in Hants with very sandy soil and not much rain. I didn't lift my dahlias and only lost one after a nasty winter.
We now live in North Devon on heavy clay and I'd not dream of leaving them in the ground. I have to lift all 200 odd plants and store them in the garage over the winter.
I'd advise you to carry on dead-heading as you rightly point out they'll waste energy forming seeds.