How to overwinter dahlia tubers

There are two ways of overwintering dahlias, and which you choose depends on what role you want them to play in your garden. 

In mild winters dahlias can be left in the ground, but to be avoid losing them you can easily lift them and bring them indoors. 

Overwintering dahlias in single beds

If, like me, you grow dahlias for cutting in big beds, you can leave them in the ground over winter.

Mulch your dahlias in late autumn under several inches of mushroom compost or similar and just clear this away once the worst of the frosts are over in the spring. Overwintering dahlias like this means you do not need to lift the tubers.

Overwintering dahlias in a mixed bed

If you grow dahlias for late summer and autumn colour in a mixed border, you're probably best lifting them. Pip Morrison (a great friend and garden designer who introduced me to Dahlia 'Admiral Rawlings'), advises you are best to lift them as winter begins. Overwintering dahlias left in their beds will become overshadowed by spring and summer growth.

In an intensive and colourful bed, you also want to showcase earlier performers, such as a teepee of sweet peas or a big drift of a tall, impressive annual bedding plant like the invaluable Ammi majus. Dahlias can be slotted in to replace these, but in these circumstances, lifting it must be. 

To be sure of conserving your plants for next year, dig them up after the tops have been frosted once or twice in the autumn. Cut them down to 15cm (6in) before you do so. Knock off the surplus soil and, with a small piece of stick, scoop out the loose soil between the tubers – but leave enough to hold them in place. Do not clean the tubers under a tap; to get water on a tuber at this time of year often spells disaster. Let them dry, hanging upside down from a dried stalk, leaving them there for a couple of weeks. Pack them away in a storage box filled with dry potting compost or sand. Store them in a cool, dry, frost-free place – a garden shed or cellar is perfect.

In March plant them up individually into pots, place them somewhere sunny and water them to bring them into growth. Using big pots – 3 litres or above – means they can grow to a decent size before planting out. Then whatever is flowering in the early summer can perform until the end of June, when the dahlias can go in to replace them, already impressive and almost in flower.

You'll also need to stake and support all dahlias. Bar the ''container-sized'' varieties such as Dahlia 'Bishop of Oxford' and Dahlia 'Roxy', all the ones I grow need canes and string to hold them up in wind and rain.

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