At its best:
Jul, Aug, Sep
Dig all around the dahlia clump and lift the tubers with a fork, taking great care not to damage them with the spikes. Shake off as much soil as you can. Then, using secateurs, cut down the dahlia stems and compost the leaves.
Rinse off the soil thoroughly, then stand the tubers upside down to dry.
Pack the tubers in a box or pot and cover them with dry compost. Store them in a frost-free place, such as under the greenhouse staging or a dry shed.