How to overwinter dahlias so that they keep blooming year after year

 Garden Basket of Dahlias - stock photo.
Garden Basket of Dahlias - stock photo.

Mastering how to overwinter dahlias will grant you almost magical powers: you'll be able to keep your dahlias alive over the cold winter months. But how do we best go about making this garden idea a reality?

Well, while it might take a little time and effort, we can promise you this: it's 100% worth it to see those bold bright blooms return year after year – no small thing, especially when you've spent all that time learning how to deadhead dahlias already.

Here's how to get started, then, on this seriously clever garden trend...

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.

Close up of rich purple dahlias in bloom
Close up of rich purple dahlias in bloom

'Dahlias and cannas can be left in situ until the first signs of frost damage have affected them,' advises Monty Don via his popular gardening blog.

'Then they can be cut back, dug up and bought indoors to check over before storing in old potting compost in a cool, dark place where they will stay alive but not grow over winter.'

Which is... well, which is all very well and good, but it's a little succinct, Monty. Thankfully, we've consulted a team of experts and put together a guide on how to overwinter dahlias, just for you. Because, as you've likely guessed, there's more than one way to do it properly.

What you will need

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

  • A good pair of secateurs

  • A garden fork

  • An unheated shed or garage

  • Labels

  • A cardboard box lined with newspaper/brown paper bag

  • Dry sand, soil or compost

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.

Dahlia in container
Dahlia in container

'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).'

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 working 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 fresh new compost around the roots,' says Morris.

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

How to overwinter dahlias in garden borders

How to keep dahlias blooming: close-up image of a red dahlia in New Forest, UK
How to keep dahlias blooming: close-up image of a red dahlia in New Forest, UK

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.

'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 from one year to the next is to preserve the tuber by protecting it from rotting or becoming frozen.'

Mature dahlia tubers being dug up for overwintering
Mature dahlia tubers being dug up for overwintering

Wait until the foliage on your dahlias has been blackened by the first frost. Then, cut off the old flowering stems about 5cm from the base (chuck them in your hot compost bin when you're done), and use a fork to prise the plants out of the soil.

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

Finally, 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 their dealing with).

Dahlia flowers growing in pots and window boxes on a balcony
Dahlia flowers growing in pots and window boxes on a balcony

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 out when they are in growth.

Sounds almost too easy, right? We're off to give it a go, if you'd care to join us...

What is the best way to overwinter dahlias?

There are two methods when it comes to overwintering dahlias: bringing your (cut back) potted flowers into a garden shed, or carefully digging up the tubers, drying them off, and storing them in a cool, dry, dark place that doesn't freeze over the winter months.

'The garage is a suitable location for this,' says Chris Bonnett, founder of GardeningExpress.com. 'Be sure to avoid storing them in extremely cold or excessively warm locations.'

Can I leave dahlias in pots over winter?

With a little TLC, you can absolutely leave dahlias in pots over winter. All you need to do is cut the top growth back by about half, and move them inside to a cool, dry, darkened place (again, a basement or garage works, so long as they don't dip below freezing).