Dahlias: to dig or not to dig
At this time of year there’s always a flurry of worried messages about dahlias and what to do with them over winter. Do you dig the tubers up, or do you take a punt and leave them in the ground to see if they will survive another year? The answer of course depends on where you live and the type of soil you garden on. While dahlia stems and flowers are easily blackened and destroyed by frost, the tubers themselves are surprisingly hardy, surviving short periods of harsh temperatures as long as they aren’t waterlogged at the same time. I had big clumps of ‘Rip City’ dahlias on my last allotment and can remember temperatures reaching as low as -17C about ten years ago - but because we were on free-draining chalk they survived.
Labyrinth
Chat Noir
Preference
If you garden on heavy clay, or have very damp or waterlogged soil, then lifting your dahlias is a good idea. If you have a lighter soil, then you are probably OK, but when you cut back your plants in November, give them a thick mulch of coarse compost - or sheets of newspaper if you are less worried about aesthetics - which will help to protect them over winter. My approach now is to lift half and leave the rest in the soil - although I daresay that if I left them all in, they would be absolutely fine given the mild winters we’ve had in the south of the UK over the past few years. I dig up those that I think are in the wrong place, or those that are particularly precious, and keep the tubers in a box of straw in the shed over winter. I make more work for myself by digging them up, but I actually love the whole process of dividing and potting up those tubers in spring, and watching their phenomenal growth before finally planting them out again in early summer. Lifting them also means that you can start increasing your stock by dividing the clumps before you replant them, or later by taking cuttings.
Dahlia tubers just planted in spring
Dahlia blooms on the auricula theatre
New shoots for taking cuttings
If you decide to lift your dahlias, clean the tubers by brushing them free of soil, and leave them somewhere to dry out. You can either hang them up by string or put them on a slatted bench in a greenhouse for a week or two, before finally putting them to bed in a box of straw, sand or dry compost. Leave them be until early spring and then one sunny day bring out the boxes and plant up the tubers in large pots which should be kept in a greenhouse or cold frame until late spring. At this stage you can divide them to make more plants, by pulling them apart or snipping off tuber offsets. And you can make even more plants later on in spring by taking root cuttings from your potted dahlias.
So to sum up. If you’re in the south of the UK and have reasonably light, free-draining soil, then you probably don’t need to lift your dahlias - but always mulch them over winter as a precaution. Further north, in particularly cold, exposed microclimates, or on heavy soil that is prone to waterlogging, then it’s better to lift. Or compromise, as I do, and do half in half to be safe.
