I lost all my dahlia tubers this year. I had stored them in the garage, but they rotted away, a combination of damp and cold. Sue at Green Lane Allotments commented on my post about it. She has a plot in North Yorkshire and gardens on heavy clay like me, so I was encouraged to hear that they now leave their dahlias in the ground. She gave me a link to a post about how they deal with Dahlias, which is to mulch them well with straw and then cover with black plastic. The straw keeps them warm and the plastic prevents the soil from becoming so wet that the tubers rot away. I’m going to try this on my allotment this year.