As dahlia providers from the heart of the Dutch flower bulb region, we get this question very often: are dahlias hardy? The short answer is no, but the story behind it is essential if you want to enjoy an overwhelming sea of flowers in your garden year after year. In this guide, we will explain exactly how the dahlia works and how you can safely help these beautiful tubers through the winter.
What does hardiness actually mean?
When we talk about hardiness in the gardening world, we are referring to a plant's ability to survive cold, frost, and harsh winter conditions without dying. Hardy plants enter a dormant phase and wake up in spring as soon as temperatures rise. The dahlia, however, falls into the category of non-hardy summer bulbs. This is due to their origin; dahlias descend from wild species from the high plains of Mexico. There, winters are dry and mild, which is not at all comparable to the wet and unpredictable cold we experience in Northern Europe.
The structure of a dahlia tuber also explains its vulnerability. A tuber consists largely of moisture. As soon as the ground freezes, the water in the tuber's cells also freezes. Because water expands when it freezes, the cell walls simply burst. The result is a soft, mushy tuber that will no longer sprout in spring but will rot in the ground.
Why dahlias do not survive winter in the open ground
The biggest danger to your dahlias is not just the temperature below zero, but especially the combination of cold and moisture. In our region, there is often a lot of precipitation during the winter months. A dahlia that remains in cold, soaking wet clay or sandy soil will face fungi and rot. Even in a winter with little frost, there is a high chance the tuber will lose the battle against excess water.
Sometimes you see dahlias survive a mild winter if they are planted deep enough in the ground. However, this is a risk we advise against as enthusiasts. The chance of failure is enormous and the plants that do survive often start much later and with less vigour than tubers that you have overwintered frost-free.
Lifting dahlias: the step-by-step plan for a safe rest period
To preserve your dahlias year after year, you must lift them. This process usually begins after the first night frost. You will see the foliage turn black and limp. This is the sign for the plant to withdraw all energy into the tuber. Wait a week after that first frost, cut the stems to about ten centimetres above the ground and carefully dig up the tubers with a garden fork. Be careful not to damage the tubers, as every wound is an entry point for diseases.
Next, shake off the excess soil and let them dry upside down for a few days in a sheltered spot. This allows moisture to drain from the hollow stems. Then store them in a cool, dark and especially frost-free place, such as a cellar or a dry garage. A box with some dry peat moss or newspaper works perfectly to prevent the tubers from drying out completely while still allowing them to breathe.
The exception to the rule: dahlias in pots
Do you have dahlias in pots? Then you are in luck, as this makes overwintering a lot easier. Because pots freeze much faster than the open ground, you do have to intervene sooner. As soon as the nights get colder, you can move the pots indoors in their entirety. An unheated shed where it does not freeze is ideal. You do not need to give them any water during the winter; the tuber goes into deep dormancy and only needs attention again when the first spring sun appears in March or April.
Tips for a flying start in spring
When winter is over and the chance of night frost in May has passed, the tubers can go back into the ground. A well-kept secret we love to share with you is to start the tubers indoors in pots during April. This gives them a head start and you will see them begin to bloom much earlier in the season. Moreover, the young shoots will be a bit stronger and less attractive to slugs, which absolutely love the fresh green of dahlias.
Dahlias as a sustainable investment for your garden
Although lifting dahlias requires some work, it is more than worth the effort. A dahlia tuber actually grows larger every year. After a few seasons, you can even carefully divide or split the tubers, giving you new plants for free! This way, your collection from the Dutch Bollenstreek grows every year, as long as you provide them with the protection they need during the cold months.
Do you have questions about storing specific varieties like the impressive Dinnerplate dahlias or the refined Pompon dahlias? We are always ready with practical advice.