How to overwinter dahlias the Monty Don way – the best methods to ensure successful blooms next year

Dahlias are among the most rewarding flowering plants out there: they're prolific bloomers, producing beautiful displays from summer all the way into the autumn. But to help the blooms return each year, you'll need to learn how to overwinter dahlias.

Whether you've learned how to grow dahlias in pots for the first time or you've mastered the art of deadheading dahlias this year, brushing up on overwintering techniques will prolong the life of these flowers.

But there's one long-standing dahlia debate: to lift or not to lift? Well, according to gardening guru Monty Don, you should lift the tubers – and we'll get into the how and why shortly. But we're also covering the other overwintering methods you can use, whether you've been growing your dahlias in pots or borders.

How to overwinter dahlias

We know, we know: once you've overwintered one tender plant, surely you've overwintered them all?

Wrong! Unlike the process for overwintering geraniums, learning how to overwinter dahlias doesn't involve rushing your precious flowers indoors and coddling them at the first sign of autumn. Far from it, in fact.

If you're wondering when to lift dahlia tubers, you should actually wait for the first frost to arrive before taking action.

'Frost reduces dahlias to blackened tatters so it will be time to bring them in [then],' explains Monty Don in his popular gardening blog. 'However the tubers will not be harmed unless the ground freezes, so do not panic.'

Monty's preferred method of overwintering dahlias? Lifting them.

'Wait until the top has fully died back and then cut back the top growth to six inches whilst they are still in the ground and carefully dig up the tubers, removing as much soil as possible,' he says.

'Stand them upside down for a few days to drain any moisture from the hollow stems and to let the tubers dry a little and then store them in a tray or pot packed with old potting compost, vermiculite, sharp sand or sawdust.'

Successful overwintering will depend on the conditions you keep the tubers in.

'The idea is to keep them cool but frost-free, dark and dry but not to let them dry out completely or else the tubers will shrivel,' Monty says.

'The ideal storage spot is somewhere that stays between 4°C and 10°C, like a basement or an unheated closet,' adds David Denyer, two-time Florist of the Year, six-time Chelsea Gold Medalist and in-house floral expert at Eflorist.

From there, you'll want to check the tubers every so often. 'I lightly water mine after layering them into large pots or crates and then check them every month to see if any are mouldy or shrivelling up,' says Monty.

Of course, if you really want to learn how to overwinter dahlias like an absolute pro, there are other methods you can try. Look this way, then, for more...

What you will need

If you're hoping to learn how to overwinter dahlias properly, you will need...

• A good pair of secateurs, like the Spear & Jackson Razorsharp Geared Anvil Secateurs from Amazon
• A garden fork, like the Draper Carbon Steel Border Fork from Amazon
• An unheated shed or garage
• Labels, like these plant labels from Amazon
• A cardboard box lined with newspaper/brown paper bag
• Soil, compost or dry sand like this Horticultural Sharp Sand from B&Q

How to overwinter dahlias in pots

If you've been busily growing dahlias in pots this year, then you're in luck: it's incredibly easy to overwinter these little beauties.

Like other perennials, you can simply relocate dahlias in pots for the winter. 'Bring the pot into a dark, unheated garage or frost-free shed,' says Morris Hankinson, director of Hopes Grove Nurseries.

'Let the compost dry out completely over the winter, and remember that it’s a good idea to cut back the top growth by about half (not down to ground level).'

Just make sure your garage doesn't get too cold during the winter. 'If the garage dips below freezing, your tubers will likely be damaged,' says David from Eflorist. 'If you’re worried about that, consider insulating them by wrapping them in something like bubble wrap or storing them in an insulated container.'

Morris Hankinson

Morris Hankinson is the founder and managing director of Hopes Grove Nurseries Ltd, the UK’s only specialist grower-retailer of hedging plants, which he established after graduating with a Commercial Horticulture Degree from Writtle College, Essex in 1992.

In the spring, as temperatures rise, you should start to see some new growth from the base. This is the time to bring the plants back into the light and start watering them, but still work hard to protect them from late frosts.

'You might like to repot them into a larger size, or at least tease out some of the old compost and freshen the pot up with some new compost around the roots,' says Morris.

'This is also the time to trim those old stems back to make them tidy, and then you should be set for another summer’s enjoyment of these lovely plants.'

How to overwinter dahlias in garden borders

If you've been growing your dahlias in garden borders (or anywhere in your garden, to be honest), don't despair: there is, of course, still a way to overwinter them – but you'll need to get overwintering dahlia tubers, specifically.

'These colourful plants overwinter from one year to the next by means of their fleshy tubers underneath the soil,' says Morris.

'The key to successfully keeping these plants each year is to preserve the tuber by protecting it from rotting or becoming frozen.' For that, you'll need to learn how to protect plants from frost.

Wait until the foliage on your dahlias has been blackened by the first frost. Then, you'll need to take out the secateurs.

'Cut flowering stems a few centimetres from the base, then use a fork to prise the tubers out of the ground,' says Andrew White, gardening expert at Rhino Greenhouses Direct.

Use your hands to brush the soil away from the tubers, taking care not to bruise or damage them, then hang the healthiest-looking ones upside-down in a cool place to dry off.

The final step in overwintering dahlias is to transfer your tubers to that aforementioned cardboard box, cover them in dry sand, soil or compost, and leave them alone in your garage or shed (hint: it's a good idea to label them, so Future You knows what they're dealing with).

When spring rolls around, all you have to do is dig out the tubers, give them a short soak in warm water to wake them up, and plant them in warm spring soil (ideally, you want it to be around 15°C).

Alternatively, you can start them indoors in pots and then plant them out when they are in growth.

FAQs

How do you store dahlia tubers in an unheated garage?

An unheated garage can be an ideal place to store dahlia tubers for overwintering, provided that the conditions are right. We've already mentioned that dahlia tubers shouldn't be stored in a garage that gets especially cold during the winter (unless you provide frost protection, of course), but it's also important to make sure it doesn't get too warm.

'Garages can sometimes get too warm or too humid,' explains David from Eflorist. 'If it’s too warm, the tubers can dry out, but too much moisture can cause rot. So, make sure there’s good airflow and check the storage medium (like peat moss or vermiculite) to see if it’s keeping the right moisture balance.

'If your garage tends to fluctuate a lot in temperature or humidity, I’d suggest moving the tubers to a more stable location like a basement.'

Do you have to wash dahlia tubers before storing?

It's generally better to brush the dirt away from your dahlia tubers instead of washing them to keep excess moisture at bay before storing them.

However, some gardeners do choose to wash dahlia tubers before storing them – and if you do, just make sure you hang the tubers somewhere to dry before burying them for the winter.

Learning how to overwinter dahlias is easier than it looks, right? Follow these tips and you'll be rewarded with another round of beautiful blooms next summer.