Lift and store dahlia tubers as soon as the frost blackens the stems. In milder areas with free-draining soil, you can risk leaving them outdoors over winter, as dahlias can cope with a little frost. Even so, it’s always a good idea to pile up a thick mulch of dry spring leaves or straw over the tubers, held in place with a square of hessian, so any frost is kept off the tubers. But in cold areas with soggy clay soil they will really suffer: as soon as the ground freezes solid, or (worse) becomes waterlogged in the relentless rain of winter, the tubers begin to rot and eventually die.
Trim back the stems to about 5cm, then carefully ease the entire cluster of tubers out of the soil with a fork, damaging them as little as possible. Bring them indoors, into a cool greenhouse, conservatory or table in front of a window in an unheated spare room, and turn them upside down so the moisture can drain out.
After a few days, gently brush off the dry soil and any other debris and pack into boxes, layered with plenty of shredded newspaper, sand or spent compost. Keep them somewhere cool, dry and dark for winter: under the bed is traditional, but if you don’t fancy that the shed or greenhouse will do just as well!
Then next spring, in around February, pot the tubers up again into multi-purpose potting compost so the stems stick out above soil level, and place them in a frost-free greenhouse, conservatory or bright windowsill to start growing again. Keep them well-watered and you should start to see new growth within a few weeks. Plant outside once the last frost has passed and you’ll soon be enjoying these flamboyant stars of the summer border all over again.