Since dahlias are native to Mexico, it’s not surprising that they can’t survive our winters here. What makes them so popular with northern gardeners, however, is the fact that they grow from tubers, which we can dig up and store inside during the winter. This makes them “perennial” in some respects, because we can grow the same tubers year after year. But they’re annuals in that we have to replant the tubers outside every spring.
At the end of autumn, we thrifty and labor-loving gardeners will rescue dahlia tubers from the ground before it freezes. The timing for this task doesn’t have to be exact, but it’s best to follow a few rules. First, wait until the first “killing frost.” This means that the formerly lush-and-green tops of the plant will turn brown and look dead. The flowers will die, too. Once this happens, cut the dead plant material back almost to the ground, leaving a few inches of stem to remind you where the tuber is located underground. Then, wait a week or so before digging.
Use a potato fork to dig, inserting the tongs straight into the ground about eight to twelve inches away from the base of the remaining dahlia stems. Lift gently; you want to unearth the entire clump of tubers without breaking them away from the central connecting tissue, known as the crown. If you find yourself breaking the tubers, try loosening the soil on all four sides before lifting upward. Pierced or broken tubers are more susceptible to disease and rot during storage.
Once I’ve unearthed the weird, dangly tuber clumps, I usually spray them off with the hose or dunk them repeatedly in buckets of water until the tubers are fully cleaned. This helps eliminate pests or fungi that might be living in the soil. Then, the tubers need to dry off in a dark and humid place that also has good airflow, like a basement or garage. I dry my dahlia tubers in plastic crates, but cardboard boxes will also work. When they’re ready for storage, the tubers should be dry to the touch; this shouldn’t take longer than a day or so. You want to completely dry the outside of the tubers without also drying out the insides as well -- hence the humidity. Don’t use a fan or dehumidifier to dry your dahlias!
Since dormant dahlia tubers need to stay in this admittedly confusing state of dry-but-not-too-dry, you should store them in peat moss, vermiculite, or wood shavings. These media help conserve moisture within the tubers while also fending off fungus and rot. I usually fill gallon Ziploc bags with peat moss, dahlia tubers, and plastic labels for keeping track of the cultivars. I try to keep the individual tubers from touching to protect them from each other in case one starts to decay. I cover the tubers entirely but leave the bags partially unsealed to allow a little air into the mix.
The ideal dormancy storage temperature for dahlias is between 40 and 55 degrees Fahrenheit, but that can be tricky to find within a fully heated home. My advice is to find a space that is as close to this temperature range as possible: a root cellar, basement, or attic will probably be your best bet for storing dahlia tubers. For the past few years, I have been storing my dahlias in the uninsulated part of our upstairs closet, which seems to work just fine.
Laura Schwarz gardens and writes in Minneapolis.
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