Q: What is the proper way to dig up and keep Dahlia tubers for the next season? I love your show!
---Leah in Boalsburg, PA ("in the middle of the state")
A: Well thank you, Leah! Now, dahlias are just one of a fairly large group of 'summer-blooming bulbs' that need to be dug up, brought inside and protected for the winter in most areas of the country. And every one of these plants is a little different. Practically the only thing they have in common is that none of them are bulbs—they grow from roots, corms, rhizomes and other underground structures.
Now, with dahlias and tuberous begonias, I personally cheat (because cheaters always win). I grow them in hanging baskets and bring them inside to hang in sunny windows before it gets really cold outside. If you've got the space, they make great houseplants.
But if they're in the ground, you have to dig them up and store them—unless you live in a zone where that specific plant can survive winter under a heavy mulch; or you don't mind buying new ones every Spring, which is what a lot of people do.
Now, a fellow garden writer I consulted, Marie Iannotti warns that there is no definitive way to store summer blooming bulbs over the winter; every gardener has their own little style. The most important guidelines, she notes, are to keep the stored roots in an area that won't freeze; don't seal them up in airtight containers (or they'll rot); and check them a few times during the season for 'the bad one'. Same as apples and potatoes; if you have eight dahlia tubers in a box and one starts to get all soft and smooshy, frequent checking will allow you to get it out of there before it infects the others.
But first we have to dig these puppies up. With dahlias, Marie says to allow frost to kill the tops of the plants, but to dig the roots up before the ground freezes hard. Then pack the roots in cardboard boxes filled with slightly damp peat moss and store the boxes in a place that ideally stays between 40 and 50 degrees. Dahlia tubers don't like to dry out completely, so check them every month or so and mist the peat moss lightly if it seems dry. But don't overdo it! We're not watering the plants; just keeping the humidity up a bit.
The care for those big beautiful tuberous begonias is pretty much the same, but less moist. Let frost kill the tops and dig the roots up before the soil freezes hard, but then let them 'cure' in a dry airy spot and then pack them in a cardboard box filled with peat moss, but keep these roots more on the dry side.
And there's lots of other summer blooming bulbs. Canna lilies are one of the hardiest; they can often survive being left in the ground under a heavy mulch in protected areas in USDA Zones 7 and up. But if you don't want to risk it, they're also the easiest to overwinter—just wrap them up in newspaper and store them in boxes somewhere between that magical 40 and 50 degrees F.
However, as is often the case in gardening, the devil is in the details. For instance, it's really important to get the roots out of the ground without damaging them. The best way to do this is to use a garden fork or pitchfork to slowly work the soil a good foot away from the base of the plant. Take your time, keep forking down lower and ideally, you'll be able to 'lift' all of the root system out of the ground without injury. Don't rush this step; any little nicks or cuts could mean a rotten root in February.
Some people then wash their dug up roots off, but I don't—I think it increases the chance of rotting. I prefer to just let them sit out in a cool dry spot for a few days, gently brush away any dirt, then carefully trim off the tops—any old greenery would turn into a mold-inducing mush in storage. Then I let them sit for a few more days to seal and heal and examine them again. If there's a few bad looking sections, prune them off and let the wounds heal for a few more days. But if large parts of a bulb are soft or rotten, toss it.
The hardest part is often finding the ideal storage area for the boxes these bulbs will spend the winter in. You need an area where temps stay below 50 but don't drop below 40. You have to use cardboard or something else that can 'breathe' (no Tupperware!). And most of us also have to protect the boxes against mice. Hungry mice love to try and get at stored bulbs. So I always put a few traps around and check them frequently. Note to fellow men: Mouse control and the adjacent dead animal disposal is one of the top spousal attributes we can achieve!
And finally, remember that this is the art of gardening, not science. There are a lot of different ways to do this, and you may have to experiment for a few years to get your perfect technique down. And even then, you're going to have the occasional plant that won't make it. Just be proud of any successes—and happy that in gardening you can easily replace your failures.