Q: Now that it’s late autumn, my dahlias are starting to die back. Can I leave the plants in the ground or do I need I lift the tubers and store them in my shed for the winter? I’d much prefer the first option if possible! MD, Kilkenny
A: Ah, the million-dollar question. For Irish gardeners growing their dahlias in milder parts of the country and on a free-draining soil, it’s usually fine to simply cut the stems back down to almost ground level, and then leave your dahlia tubers in the ground after covering them with a deep protective blanket of straw, or bracken, or well-rotted garden compost. But for those living in colder parts of the country, or with gardens where the soil is prone to winter waterlogging, it’s advisable to cut back the plants and then use a garden fork to carefully dig up the tubers.
Shake or rub off any big clumps of soil, but leave a light layer on to protect them from excessively drying out, label them (use string to firmly tie each one securely in place) before placing them in shallow, open trays in a cool, dark, frost-free garden shed until next spring.
It’s important to add that dahlia tubers are much more likely to be killed by sitting in wet than cold soil. In wet gardens, the soft growth that emerges from the tubers in late spring is also more likely to suffer slug and snail damage than it would if the plants were first grown on in large pots or trays under cover of a glasshouse or polytunnel, before being planted out into their final positions.
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Another thing to bear in mind is that the size of the tuber will also have a bearing on its ability to safely overwinter outside in the garden or allotment (the larger the tuber, the better the odds). My experience is that certain varieties of dahlias are also naturally much more vulnerable to winter damage, with the large dinner-plate types least likely to survive in this country.
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Finally, even the most organised gardener can still be caught out by the kind of once-in-a-generation extreme weather event as experienced, for example, during the Beast from the East in 2018. In that case, many Irish gardeners lost dahlias even when they had diligently covered them with a protective mulch that would have been more than sufficient in a normal Irish winter. Sadly, climate change has shown us that the new normal is to expect the unexpected. In this case, the safest bet is to lift your tubers and store them under cover, no matter how dry and sheltered your garden typically is in winter.
