Lift dahlia tubers as soon as the first frost strikes to store them safely, ready for you to plant them again next year and enjoy their spectacular flowers for another season.
Dahlias cannot tolerate prolonged low temperatures or – worse – soggy, cold soil, so they need to spend winter somewhere dry and frost-free. However don’t be too quick to take them out of the ground, as they continue to flower for as long as temperatures stay above freezing. Wait until the foliage and stems have been blackened by frost and you know they have finished for the year.
Then cut back the top growth and with a fork, gently prise each plant out of the soil. Leave them to dry off for a few hours, then brush off loose soil, trim back the roots, and cut back the stems to 15cm.
Lay out your tubers on a slatted shelf, or a rack or grill resting on bricks, with the stalks hanging downwards to help any excess moisture drain out. Leave them to dry like this for about two weeks, then place your tubers in trays (right way up again) and pack them with potting compost or dry sand, covering the tubers but leaving the crowns exposed. Store in a dry, cool but frost-free place – the garden shed is often a little too cold, so cover your trays in a few layers of horticultural fleece if a hard frost threatens.
Check your tubers regularly to make sure none are rotting, discarding any you find which are turning soft. They should stay in suspended animation until spring, when you can pot up the tubers in fresh compost and start to water as normal. They resprout within a week or two, ready to plant back out in the garden as soon as the last threat of frost has passed.