Preparing dahlias for winter | A guide to digging & storing dahlia tubers

This week’s words of gardening wisdom (and photos) are from our garden expert, Elly Keen.

In late autumn, the daylight hours have shortened and many of us have experienced our first frosts. This transition signals the end for our beloved dahlias and gently reminds us to prepare for their wintertime slumber. 

Dahlias are a perennial flower that go completely dormant over the colder months, with the  foliage above ground dying off – what’s left is a clump of tuberous roots underground. These tubers are a swollen stem that is used by the plant for food storage during their growth phase. 

In the wintertime, when the foliage dies back, these tubers can be subject to rotting if left in boggy, clay soils. For gardeners that are growing in clay or any area that is not particularly well draining, you may choose to lift your tubers and store them indoors over winter instead. 

If you are growing your dahlias in pots, raised beds or elevated gardens with free draining soils, you may choose to simply cut back any blackened foliage now and apply a thick layer of mulch like pea straw or wood chips to protect from frost and rain. 

Lifting and storing dahlias can be a daunting task especially if you have amassed a collection over the years, but if you are worried about losing special varieties then stowing away can give you a  sense of security.

How to dig dahlias 

Dig all around the clump with a shade or fork. Leave a 10cm wide margin to ensure you do not damage any of the tuberous roots.  

In heavy clay soils, it can be difficult to lift the tubers without damaging them. Especially if the soil is wet from recent rains. Go slowly and dig around clumps as carefully as  possible with your hands if needed. 

Lift the clump with care and gently shake off or manually remove excess soil. 

You may choose to use the hose to wash off the remaining soil from the clump. I like to  leave a little soil on my clumps as this can help prevent the tubers from completely drying out in storage. So instead, I will leave tubers out in the sun for a day or two to dry naturally and then I will brush off any big pieces of dirt stuck to the clumps.

Blackened dahlia foliage ready to be cut back
Dahlias cut back before digging
Freshly dug dahlia clump

Tuber with Crown Gall

Check for signs of disease:

Washing the tubers does help when trying to identify disease.  

Dahlias can be prone to an issue called Crown Gall (Agrobacterium tumefaciens). This is a bacterial infection that can cause an excess of plant hormones that control growth resulting in a white cauliflower-like deformity around the crown or neck of the tubers.  

When lifting your dahlias, check for any signs of Crown Gall. If you identify this issue, bin  immediately and do not compost.

How to store dahlia tubers 

Now that we have removed our dahlias from the ground it is time to store. Some people choose to divide tubers in autumn upon lifting, when the eyes are clearly visible – but I prefer to divide in the spring. I find that tubers seem to fare better when stored in clumps, rather than individual tubers which tend to dry out quicker. 

Regulate temperature 

Dahlias are cold sensitive so it is imperative that you do not allow the tubers to be subject to freezing temperatures or frost as this will damage them. Storing your tubers in an environment which is dry and warm is ideal. 

Allow tubers to breathe 

It is best to keep your tubers in a crate or cardboard box rather than an airtight container.  Dahlia tubers will sweat and continue to release moisture, and are prone to rotting if  condensation is allowed to build up. 

Keep off concrete 

Concrete is said to draw moisture out of the tubers so if you are storing, make sure to place the box or crate on top of a shelf or table instead of the garage floor. 

Constant monitoring 

Finally, you should continue to monitor your tubers throughout winter. As time progresses you may find you need to add mixes to lengthen the storage capacity depending on your situation. 

For tubers that appear quite shrivelled – add dampened potting mix to elevate moisture levels

For tubers that are growing mouldadd vermiculite to help draw out excess moisture that can lead to rotting. 

Individual dahlia tuber with eye

Elly’s dahlia garden
‘Brown Sugar’ dahlia
‘Genova’ dahlia

What do I do with the garden bed after I lift my dahlia tubers? 

After you lift your dahlia tubers, you may be left with an area of unused garden bed. Instead of allowing the weeds to grow all winter you may choose to make use of the area until Labour Weekend, when the time comes to plant again. 

Planting out a green crop, like broad beans, can be a helpful option for fixing nitrogen in the soil. Alternatively, tulip bulbs or a mix of hardy annual flowers like poppies, calendula, cornflower, phacelia and borage would provide a beautiful display come early spring, and can simply be cut back when the time comes to replanting tubers.