Dahlias are arguably one of the most beautiful cut flowers to grow, whether you’re a beginner cut flower farmer or an experienced farmer florist. That being said, they can be one of the trickiest cut flowers 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, which will destroy your precious dahlias! And, let’s be real, a Cafe au Lait dahlia tuber is not cheap. 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. And, if you want to see what dahlia tubers (and gladiolus corms) look like when they are properly overwintered, check out 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 light 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 which dahlia is which once the frost hits and they are all shrivelled up.
Take note of what height the dahlias are, as well, if you have a large number 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°C/23°F C), the weather in Saskatchewan and other cold climates is not always typical. You should always use your best judgement. There are even 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: Dahlia tubers can survive a frost or two if the ground is still warm. I’d be worried if my dahlia tubers were exposed to multiple nights of -4 to -10 degrees Celsius outdoors. And finally, a frozen dahlia tuber is a dead dahlia tuber. 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, as 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 barely moist (but not wet!) peat moss, growing mix, 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 throughout 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.
[Update: I recently had a set of almost dried-out tubers that I put under grow lights and have been misting daily. A month later, they are looking better, and hopefully will start producing shoots soon!]
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 through September. If you get your tubers in late and there’s an early frost, you might not even get any blooms.
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 also 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 looks dried up on the off chance that they’ll survive. Don’t plant any tubers that have gone mouldy.
Once all risk of frost has passed, 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. I hope these tips help you in your own cut flower garden! What do you think, will you grow dahlias this year?
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