Hi Tony, You can dig up dahlia tubers after the first frost has turned the leaves brown. Don't leave in the ground after this, they are tender. Allow the dirt to dry and brush off excess, don't wash. Leave a 6 inch stem and turn upside down for one week to drain excess moisture. Discard any damaged or rotten tubers.
Store in peat moss or saw dust to prevent the tubers from drying out in a cool area, not somewhere where they will freeze.
Did you like your Dahlias? What kind did you grow? I grew them for the first time this year (moved to a new house with enough sunshine for dahlias) and some I liked and some I did not. I saw a purple bush the other day while out driving and had to turn around to find out what it was. It was a HUGE purple dahlia! Just beautiful! How about yours?
Bye, Diane