Dahlias are arguably one of the most beautiful cut flowers to grow, but they can be one of the trickiest to overwinter in cold climates, such as my zone 3 garden in the Canadian Prairies. If not stored properly, dahlia tubers can either rot or dry out–destroying your precious dahlias!! And let’s be real, a Cafe au Lait dahlia tuber is not cheap and you don’t want that baby to die on you. In this post, I’ll show you how to take care of your dahlias over winter, so they come back again next Spring.
P.S. A special thanks to Chantelle of City Girl Flower Farm for letting me photograph her beautiful dahlias.
Want to see what overwintered dahlia tubers (and gladiolus corms) overwintered correctly looks like? Watch this video:
Can’t I just keep my dahlias in the Ground over Winter?
Unless you live in zone 8 or higher, you can not keep dahlia tubers in the ground over winter. If you do, they will rot and become compost. In fact, you must dig them out before the first hard frost, and it’s better if you can get them out shortly after the first hard frost.
Dahlias need to be stored in the darkness in a heated garage or cool (but not freezing!) basement. You’ll also need a container to store them in, such as a flat cardboard box or large plastic container.
Label Dahlias before the First Frost
Once there is a risk of frost in the forecast, label the plants by tying survey tape or some other label around the very bottom of the stem. You need to do this step before the first frost so you can see every dahlia and know what it is. Unless you have a large flower farm where every section is already labelled, you will not remember what dahlia is which once frost hits and they are all shrivelled up.
Take note of what height the dahlias are if you have a bunch that are of varying heights.
Remove dahlias shortly after a hard frost
Once a hard frost has hit, trim off the stems, leaving 10 cm/4 inches or a bit less. Dig out the dahlia with a shovel, brush off the dirt, and let it dry for a day or two. Let them dry in a spot that gets no direct sun or frost.
While the typical wisdom says to remove dahlias after the first hard frost (around -5 degrees Celsius), the weather in Saskatchewan and other cold climates is not typical and you should use your best judgement.
There are years that we’ve had surprise -30 temperatures in October. Maybe you have a lot of dahlias to remove and are short on time. If you can’t get your dahlias out shortly after that first hard frost, it’s better to start removing them shortly after the first light frost, than risk time getting away from you and the weather getting too cold.
How cold is too cold? There’s no simple answer, but consider this–Dahlias can survive a frost or two if the ground is still warm. I’d be worried if my dahlias were exposed to multiple nights of -4 to -10 degrees Celsius outdoors. And finally, a frozen dahlia tuber is a dead dahlia tuber.
Again, use your best judgement and err on the side of caution.
How to Store Dahlias So They Bloom next Year
There are a couple of different methods for storing dahlias so they survive the winter in your basement. No method is better than another, pick one that you feel is most convenient for you.
- Wrap in Plastic: Place clean and labelled dahlias in a plastic grocery bag or black garbage bag. Do not tie the bag shut–you still want there to be some airflow. Place the bag in a cardboard box, making sure that the dahlias stay dark.
- Place the dahlias in a container with moist (but not wet!) peat moss, growing mix, or vermiculite–or a combination of all three. It doesn’t matter which one you choose. Cover loosely with a black plastic bag to keep it dark.
- Are your dahlias in containers? You can just trim the stem, cover the dahlia loosely so it is dark and there is still airflow, and store the whole container in the basement. You could also store it in an attached garage as long as the temperature doesn’t go below freezing.
Don’t set and forget–Check your dahlia tubers once a month.
Caring for dahlias over the winter is not a set it and forget it proposition. Each month, you must uncover your dahlias to check and see if they have rotted or are starting to dry out. Ideally, your dahlia tubers should remain plump through out the winter.
The key to preserving dahlia tubers over Winter: Darkness and Slight Moisture
Dahlias are a bit of a diva when it comes to winter storage. They need utter darkness, and conditions that are moist, but not watery. The best way to achieve this is to mist them with water before storing them, and then to mist them again every month or two if they look like they’re starting to dry out.
Don’t set your boxes of dahlias directly on a cement floor. The cement will steal the moisture and dry them out.
If your dahlia tubers dry out over the winter, you can try to revive them by soaking them for an hour or two in a bowl of water, or giving them a generous misting. I’ve heard varying results on the internet about whether this works or not, but it’s worth a try.
You can also just try and plant the dried out tuber directly into the ground in Spring. You might get lucky and it will grow.
Get a head start on Your Dahlia Flowers in Spring
The following instructions are best suited to my zone 3 climate in Saskatchewan, Canada, but can easily be adapted for zones 2-5. I’m assuming a last frost date of approximately May 21st.
When you live in a place with a short growing season (90-100 days) you need to get a head start on growing your dahlias. Dahlias typically bloom from mid-to-late August and through September. If you get your tubers in late and there’s an early frost, you might not even get any blooms. (Ask me how I know!!)
I bring my dahlias out from storage in mid to late April, mist them with water if they need it, and put them under a grow light or on a window ledge. A south facing window is best, but an East or West facing window will do. Most tubers start to get green shoots in about a week or so.
If you have tubers that don’t get any green bits, don’t worry. You can still plant them and they will still grow once you put them outside. I would also try planting any tuber that look dried up on the off chance that they’ll survive. Don’t plant any tubers that have gone moldy.
Once all risk of frost has past, take your tubers out to the garden and plant them. You’ll be rewarded with beautiful dahlias at the end of the season.
Planting and overwintering dahlias is a bit of extra work, but the gorgeous blooms are well worth the effort. Will you grow dahlias this year?
Kristen is a former farm kid turned urban gardener who owns the popular gardening website, Shifting Roots. She is obsessed with growing flowers and pushing the limits of what can be grown in her zone 3b garden. She also loves to grow tomatoes, but oddly enough, dislikes eating them raw.