Last night the temperature dipped to the mid-30s, so I’m glad I had the dahlias safely tucked away for the winter.
Dahlias are a delightful flower that is ubiquitous around here (USDA grow zone 7-8ish) but unheard-of back in Indiana and Illinois, where I learned how to garden (USDA grow zone 5.) This is both because frozen ground in the winter will kill them and because they won’t bloom and thrive in intense summer heat. Here in the PNW, though, the major danger to them is rotting in the wet winter soil.
Last year I joined a local dahlia society to learn how to grow and care for them. A lot of those people overwinter their tubers and generally have good results by doing so; you need to make sure they’re in a well-draining place, either in amended soil or a raised bed, and you’re mostly good to go. The occasional cold winter or long, cold, wet spring (hello 2017…) will kill some, but for the work saved, to the people who grow hundreds, it’s worth the risk.
I only have a few and I have tender feelings about some of my varieties, though, so this year I dug mine.
The first step to putting them away in the winter is to cut back the foliage to 4-6″ of stem. Yes, it’s sad, I know. You can wait until frost has destroyed the foliage if you want to. Compost the foliage and think of how much good it’ll do the plants next year.
At this point, decide whether you’ll want to divide your clumps of tubers or not. Like potatoes, a dahlia tuber will create many new dahlia tubers during the growing season. Each tuber is capable of producing a new plant, though leaving clumps alone for a few years will create a big mass that grows a bushlike plant rather than a single, tall one. It’s up to you.
If you’re going to divide, you should leave the tuber in the ground for a week or so after cutting the foliage, to let the tubers grow “eyes.” The eyes will turn into flowering stalks in the spring, and without at least one still attached, a tuber won’t grow. So if you’re dividing you need to be able to see them, and to see them the dahlias need another week in the ground to grow them.
If you aren’t dividing, the way I’m not, then you can instantly dig your tubers up. Use a fork and start 10-12″ away from the stalk to avoid spearing tubers. Gently prise the soil up, then pull the mass of tubers free (they look so much like the mandrake roots from Harry Potter, it makes me laugh) and spray them clean with the hose. Now lay them out somewhere peaceful and not too sunny to dry. They can dry for up to a week.
A freshly-dug mass of tubers. Do you have your earmuffs on?
Once they’re a little dry, you need to move them into conditions that provide three things:
2. Enough warmth that they won’t freeze
3. Something to prevent them from drying out completely
Monty Don of Gardener’s World packs his around with spent compost and stores them in a shed. In my dahlia society we put them in trash bags full of cedar shavings (the kind used in kennels), then into styrofoam coolers, which people stored in basements, crawlspaces, garages, and sheds.
Mine are spending the winter in the potting shed.
There is a possibility of fungal activity rotting your tubers over the winter. There are two things you can do to minimize this risk:
1. Trim off all excess material, like the tiny trailing roots and any damaged tubers. The damaged ones you see above simply split when they were pulled because it had rained recently and they were so full of moisture. It’s okay; there are other, whole tubers on the bunch; but you don’t want the damaged ones to stay because they can only rot. Cut them off.
2. Add a fungicide to the cedar shavings. I put a couple tablespoons of powdered sulfur fungicide in each of my bags and shook it around before I put the tubers in.
Having taken these precautions, you’ve done just about all that can be done to prevent fungal rot. If you really love your dahlias, check them once or twice over the winter, and if you see powdery black mildew cut off the affected parts and store in fresh substrate. When fungus does take hold it tends to affect all the tubers in the same bag, so consider spreading your tubers out into multiple bags, just to hedge your bets.
And that’s it. Tubers are tucked away for the winter. Congratulations!