In the Northwest Territories, dahlias, cannas and gladiolus all come out of the ground every fall. Wait for a frost that blackens the foliage, then lift them within about a week or two. In a typical Yellowknife-area year, that means late September into early October. Dry them briefly, then store them cool, dark and barely damp, and look at them once a month. Soil here freezes far too hard for any of the three to overwinter outdoors.
Do you have to dig them up in Northwest Territories?
Yellowknife sits in Canadian hardiness zone 1b, according to Natural Resources Canada’s plant hardiness site (1991-2020 normals), and at that zone there is no gray area: lift all three every fall. The ground freezes deep and stays frozen for months, and none of these plants can take that. Mulch slows the freezing but does not stop it.
A caution about that zone number. It belongs to the Canadian system, which is built on seven climate variables, while the USDA system relies on a single one (winter minimum temperature). Natural Resources Canada states plainly that there are no simple conversions between them, and that Canadian zone 4a alone overlaps nine USDA zones, from 3a to 7a. So 1b should not be read as a USDA zone or lined up against American maps. It describes Yellowknife, the reference city, and not the whole territory.
What the cold actually does to the tubers
Environment and Climate Change Canada’s 1981-2010 normals for Yellowknife put the mean daily minimum of the coldest month at -29.5 °C (-21.1 °F, for readers comparing with US winters). That is the average overnight low, not a record. Plenty of nights run colder. Dahlia tubers, canna rhizomes and gladiolus corms are all storage organs from plants that evolved in places without hard freezes. Their tissue is mostly water, and once ice forms inside it, the cells rupture. The tuber thaws in spring as a gray, mushy lump.
Colder districts of the territory only firm up the verdict. Farther north, the first fall frost arrives earlier, so the lifting window opens sooner there than at Yellowknife. Nothing in the climate data for the territory points to a pocket where these plants could be left in place.
Hardy bulbs are a different story
Tulips, daffodils and similar spring bloomers need that deep cold, and many of them spend winter in the ground. Which ones will cope with a northern winter is covered in Which Spring Bulbs Survive Your Winter, and the wider category is laid out under Spring Bulbs. Dahlias, cannas and gladiolus sit on the opposite side of that line. They are summer bulbs, treated here as tender plants you keep in the basement.
Digging them up is not much work. A dahlia clump or a canna rhizome comes out in a few minutes with a spade and a bit of leverage. The effort is mostly in the timing, which is where gardeners most often slip.
When to lift them
The signal is a frost that blackens the foliage. A light touch of frost that nips a few leaf edges does not count. When the leaves of dahlias and cannas turn dark and collapse, the plant has shut down for the year, and lifting within about a week or two of that frost gives the tubers time to finish maturing before you handle them.
Reading the calendar
Environment and Climate Change Canada’s 1981-2010 normals for the YELLOWKNIFE A station put the first fall frost on average at September 18. Treat that as the midpoint of a window, not a safe date. In one year out of two the frost lands later, and in the other half it lands earlier. Counting a week or two from that average, the usual lifting window runs from late September into early October, shifting later in a mild fall and earlier in a cold one.
Those normals also cover 1981-2010, an older stretch than the 1991-2020 period behind the zone figure, and they come from a single station. Communities farther north or on higher ground run weeks behind Yellowknife in the other direction: their frost comes sooner, so their lifting dates move earlier. The First and Last Frost Dates in Northwest Territories page gives the frost timing to build your own schedule around.
One practical move helps in a year when frost arrives ahead of the average. Throwing an old sheet or frost cloth over the plants on a clear, cold evening can carry them through a single early frost. If the cold then settles in for good, you simply start the lifting clock a bit later than planned.
Why earlier is worse
Gardeners are tempted to lift in late August or early September, mostly out of nervousness about the weather. It backfires. While the foliage is still green, it is feeding the tuber, and the skin is still thickening. A tuber dug at that stage has thin skin and soft, underdeveloped flesh. It looks fine on the day you lift it, then it shrivels or rots in storage by midwinter, because there was nothing to protect it from either moisture loss or fungus.
The frost kills the top growth, the plant pulls its remaining reserves down into the roots, and the skin toughens. That short pause between blackened leaves and the spade is what makes the difference in March.
Each plant has its own routine at lifting time:
- Dahlias: cut the stalks back to a short stub, wait a few days, then loosen the soil well away from the stem and lift the clump by prying from several sides so the tuber necks do not snap.
- Cannas: cut the foliage back, then lift the whole rhizome clump with some soil still attached, before the ground freezes hard.
- Gladiolus: lift the corms once the leaves yellow or frost has hit them, and cut the stems off just above the corm.
Do not wait for the ground to freeze solid. A spade will not go in, and tubers that sit in frozen soil for even a few nights are the ones that turn to mush later.
Storing them over winter
The routine is the same in outline for all three: cure, clean lightly, store cool, dark and slightly moist so they neither rot nor shrivel. The details differ by plant, and the differences matter. A canna rhizome that is stored as dry as a gladiolus corm will wither. A gladiolus corm stored as damp as a dahlia tuber will go moldy.
Curing and cleaning
Curing means letting the surface dry in a sheltered, airy, frost-free place out of direct sun, usually for a few days. Brush off loose soil by hand instead of washing, and skip scrubbing. Washing is not forbidden, but wet tubers must dry completely before they go into storage, and a light brushing avoids that step altogether. Trim away dead roots and obviously damaged pieces, and leave the rest intact.
Gladiolus corms are the most fussy about drying. They need a longer cure than the other two, because stored damp they invite fungus. Once dry, snap off the old shriveled corm at the base and set aside the tiny cormels if you want to grow them on.
How the three differ
| Plant | What you store | Packing | Moisture level |
|---|---|---|---|
| Dahlia | Tuber clump, divided or whole | Boxes or crates with a barely damp, loose material such as wood shavings or vermiculite | Slightly moist; too dry and the tubers shrivel |
| Canna | Rhizome clump, some soil clinging | Crates or bags with lightly damp material, loosely closed | Slightly moist; they dislike drying out completely |
| Gladiolus | Cleaned, fully dried corms | Mesh bags, paper bags or open trays, in a single layer if possible | Dry and well ventilated; moisture invites rot |
Wherever you put them, the spot must stay above freezing and out of light. With the coldest month in Yellowknife averaging a nightly low of -29.5 °C, an unheated garage or porch can freeze solid in a cold snap, so an insulated basement, a heated crawl space or an interior closet is the safer choice. Too warm is a problem as well, since a heated room dries tubers out faster than a cool one.
The monthly check
Storage is not a set-and-forget job. Check the bins about once a month from the day you pack them until spring. A quick look through each container takes a few minutes. Soft, dark or smelly pieces get cut away with a clean knife and the rest is left to dry. If the tubers feel rubbery or look wrinkled, dampen the packing material a touch, a light misting at most. If you see condensation or mold on the container, open it up and let it air.
That regular inspection saves more stored plants than any special material or trick. A single rotting dahlia tuber can spread trouble to its neighbors within weeks, and noticing it early is the difference between losing one piece and losing a crate.
What goes wrong
Most failures fall into four patterns, and each has a visible sign and a straightforward prevention.
Rot shows up as soft, dark or slimy patches, sometimes with a sour smell or gray-white mold. It comes from storing tubers too wet, packing them tightly without airflow, or putting away pieces that were never fully dried. Cure properly, use only barely damp material, and cut away any affected tissue at the first monthly check.
Shriveling looks like wrinkled, rubbery or hollow tubers that have lost their firmness. Storage that is too dry or too warm causes it, and so does lifting early, since immature tubers have thin skin. Slightly damp packing and a cooler spot fix most cases, and a light misting rescues a tuber that has only started to wrinkle.
Lifting too early produces the same two symptoms in a more stubborn form. The tubers look healthy in October and fall apart by January. The only prevention is patience: wait for the frost that blackens the leaves, then dig within about a week or two.
Leaving them in the ground is the fourth mistake, and it tends to come from advice written for milder places, where a heavy mulch can carry tubers through a winter that barely freezes the soil. In Canadian zone 1b, with a coldest-month average low of -29.5 °C, that approach does not hold. Check where a tip came from before trusting it with a plant you have grown all summer.
The same weekend you are lifting tender plants is a good time to put hardy ones in the soil. Details for that side of the fall job are in Planting Spring Bulbs in Northwest Territories.
Common questions
Can I leave dahlias in the ground with a thick layer of mulch?
Not in the Northwest Territories. With Yellowknife’s Canadian zone 1b and a mean coldest-month minimum of -29.5 °C, the ground freezes deeper than any mulch layer can prevent. Tubers left in place usually come up in spring as soft, discolored remains. Lift them each fall and store them indoors.
Should I dig them up before the first frost to be safe?
Waiting is better. The frost that blackens the foliage signals that the plant has finished feeding its tubers. Lifting while the leaves are still green gives you immature tubers that tend to rot or shrivel in storage. If frost is forecast earlier than you expect, cover the plants for the night and keep the growing season going a little longer. Then lift within about a week or two after the damaging frost.
Do I need to wash the tubers before storing them?
A light brushing is enough. Shake or brush off loose soil, trim dead roots, and let the surface dry in a sheltered, frost-free place. If you do wash them, make sure they dry completely before packing. Gladiolus corms in particular should go into storage fully dry.
When do I plant them again, and does the date vary across the territory?
Replant after the last spring frost, and expect that date to move later the farther north you garden. The same logic applies in fall, when the frost arrives earlier in colder districts. The Yellowknife figures here come from one station, so use them as a starting point rather than a promise for your own yard. More local timing is collected in the Northwest Territories guides.