In North Dakota, dig all three. Dahlia tubers, canna rhizomes and gladiolus corms will not survive a winter in ground where the coldest month at Fargo averages an overnight low of 0.2°F. Lift them after a frost blackens the foliage, which at the Fargo airport station typically arrives between September 25 and October 5. Then cure them, and store them cool, dark and slightly moist. The timing and the storage are what decide whether they bloom again next summer.
Do you have to dig them up in North Dakota?
Yes, everywhere in the state. The USDA Plant Hardiness Zone Map 2023 puts North Dakota in zones 3a to 4b. That system rests on a single variable, the average annual extreme minimum temperature, and in both ends of that range the ground freezes deep and stays frozen for months. Wherever a garden falls inside 3a-4b, the verdict is the same: dig all three every fall, after the first frost blackens the tops and before the soil freezes hard.
That range covers the whole state, so it’s a poor basis for a dated plan. Fargo is the better yardstick here. NOAA’s 1991-2020 climate normals show the mean daily minimum of Fargo’s coldest month at 0.2°F. That’s an average, not a record, so plenty of nights run colder. Mulch can blunt a cold snap, but it can’t hold off a North Dakota January.
Why mulch is not enough
Gardeners in milder parts of the country sometimes leave dahlias and cannas in the ground under a thick blanket of leaves or straw. It works there because the soil never freezes deeply. These three plants store water in fleshy tissue: dahlia tubers are swollen with it, cannas pack it into thick rhizomes, and gladiolus corms are dense little reservoirs. When that tissue freezes, the cells burst. Nothing looks wrong until spring thaw, when you dig and find mush that smells sour. A heavy mulch over a canna clump in Bismarck or Grand Forks can also hold wet, cold conditions around the rhizome, which does more harm than good.
Hardy spring bulbs are a different matter. Tulips and daffodils need the winter cold and stay put. If you’re sorting out which bulbs can overwinter in your ground and which can’t, Which Spring Bulbs Survive Your Winter lays out the dividing line.
Where your yard sits matters for timing, not for the verdict
The verdict doesn’t change across the state, but the calendar does. The colder parts of North Dakota, meaning farther north and at higher elevation, see their first fall frost earlier. So the lifting window opens earlier there than it does in Fargo. Figures from an airport weather station also tend to run warm compared with suburban and rural gardens, which often see the first fall frost one to two weeks earlier. Treat the Fargo dates in this article as a reference point, then check your own county’s frost history and watch your own plants. Your dahlias will tell you more than any chart.
When to lift them
The signal is the foliage, not the calendar. When a frost turns dahlia and canna leaves black and limp, the plant has been told to shut down, and the energy that was going into leaves and flowers settles into the storage organs. Lift them within about a week or two after that frost blackens the foliage. At the Fargo reference station, NOAA’s 1991-2020 normals put the first 32°F freeze in a band from September 25 to October 5. That’s the 30-70% probability band, meaning a freeze in that stretch is a coin flip or better in a typical year. Count your week or two from whenever the blackening actually happens in your garden.
Why waiting for frost helps
Tubers and rhizomes dug while the plant is still green and growing haven’t finished maturing. Their skins are thin and their tissue is still full of free moisture. Cured and stored, they lose water unevenly, nick easily, and invite rot. A tuber that sits in the soil for a short stretch after frost has time to firm up and thicken its skin, and it stores far better. Lifting a dahlia in late summer just to get ahead of the weather is the classic way to open a storage box in February and find a tray of soft, gray tubers.
Why not wait longer, then? Because the soil keeps cooling, and a deep, hard freeze that reaches the crown can ruin the tuber you were trying to ripen. A cold, wet fall can also start rot while the clump is still in the ground. In a year when the first freeze comes early and hard, shorten the wait. In a year when the first frost is late and mild, you get a few extra days, but don’t count on them.
Three plants, three cues
Dahlias and cannas follow the frost rule closely. Cut the blackened stalks down to a short stub so they’re easy to handle, then loosen the soil around the clump with a garden fork before lifting. Pry from a distance, not right at the stem, because dahlia necks snap easily and a broken neck means a tuber that won’t sprout.
Gladiolus are a little different. The corms can come up once the leaves begin to yellow, even if a frost hasn’t blackened them yet, and they shouldn’t sit in cold, wet soil after the tops die back. If frost arrives first, lift them soon afterward. Because the whole planting window for hardy spring bulbs falls around the same weeks, it’s handy to plan both jobs together. Planting Spring Bulbs in North Dakota covers the other half of the fall workload, and First and Last Frost Dates in North Dakota helps you pin down the frost window for your own part of the state.
Storing them over winter
Storage comes down to three steps: cure, clean lightly, and store cool, dark and slightly moist. The goal is to keep the tissue from rotting and from shriveling. Those two failures pull in opposite directions, which is why moisture is the thing to get right.
Cure first, clean lightly
After lifting, shake off the loose soil, but skip the hose. A heavy wash leaves water in the crevices, and that’s where rot starts. Let the clumps dry in an airy, shaded spot out of the rain and away from freezing temperatures, such as a garage on a mild day or a covered porch. Curing lets the skin firm up and any small cuts seal. Dahlias and cannas need only a short drying period, until the surface is dry to the touch. Gladiolus corms need longer, since their papery husks and the old shriveled corm beneath each new one have to dry fully before storage. Once they’re dry, snap off the old corm and the tiny cormels, and save the cormels only if you want to grow them on.
How the three differ
| Plant | What you store | Packing | Moisture |
|---|---|---|---|
| Dahlia | Tuber clumps, usually divided in spring | Boxes with wood shavings, vermiculite or peat moss, loosely packed | Slightly moist, so tubers stay plump |
| Canna | Whole rhizome clumps with a little soil left on | Peat moss, coir or newspaper in a box or crate | Barely damp; they dislike drying out completely but rot if wet |
| Gladiolus | Cleaned, dry corms | Mesh bags, paper bags or shallow trays in a single layer | Dry, with good airflow |
The pattern is that dahlias and cannas, with their fleshy storage organs, need a little humidity around them or they wither. Gladiolus corms are the opposite. They’re dense and papery, they shrivel less readily, and they rot fast if packed in damp material.
Where to put them, and the monthly check
The spot should be cool, dark and steady. A basement corner, an insulated cellar or a closet against an exterior wall can all work. An unheated garage or shed often can’t, which is worth remembering given that Fargo’s coldest-month overnight average is 0.2°F. A shed that never sees heat will freeze everything you stored, and a spot beside the furnace will dry the tubers out. The aim is cold enough to keep them dormant and never cold enough to freeze.
Whatever you store, check the boxes once a month through winter. Pull out a few pieces, press them, and look for softness, mold or wrinkling. A tuber that has started to rot can usually be saved if you cut away the bad spot, let the cut dry, and repack it in fresh material. One that has wrinkled badly needs a light mist of water on the packing, not the tuber itself. Catching either problem in December saves you from discovering it in April, when the damage has already spread to the tubers around it.
What goes wrong
Most failures with these plants fall into four patterns, and each one leaves a recognizable mark.
Rot, shriveling and early lifting
Rot shows up as soft, dark patches that give under your thumb, sometimes with gray mold or a sour smell. It usually traces back to packing material that was too wet, tubers stored before they had dried, or poor airflow in a sealed container. The fix is prevention: cure properly, use damp-to-the-touch (not wet) packing, and open the boxes monthly. Pull out and discard any piece that has gone fully to mush so it doesn’t spread.
Shriveling is the mirror image. The skin wrinkles like a raisin, and the tuber feels light and hollow. This comes from storage that’s too dry or too warm. A light misting of the packing material, or moving the box to a cooler place, usually stops it if you catch it early. Gladiolus corms tolerate drier conditions than the other two, so the wrinkling that’s alarming in a dahlia tuber is often harmless in a corm.
Lifting too early gives you a particular kind of rot: thin-skinned, immature tubers that never cure properly. If the frost hasn’t come yet, resist the urge to dig just because the calendar says October.
Leaving them in the ground one zone too far north
Some gardeners hear that a neighbor’s cannas survived under mulch and decide to gamble. In a mild stretch of country that bet can pay off. In zones 3a to 4b of the USDA system, it almost never does. The result is a clump that doesn’t send up shoots in June, and a dig in July turns up nothing but soft remains. If you want to experiment, do it with a spare division, and store the rest the safe way.
Common questions
Can gladiolus stay in the ground in North Dakota?
No. Gladiolus corms are tender and won’t survive a winter in frozen soil anywhere in the state. Lift them when the foliage yellows or soon after the first frost, cure them, and store them dry. Keep in mind that spring-planted hardy bulbs are a separate group; see Spring Bulbs for those.
What if the ground freezes before I get to them?
Lift them anyway, as long as you can get a fork in. If the top few inches have frozen hard, wait for a thawing afternoon rather than chipping them out, because frozen tubers damage easily. Check each one for soft spots before storing, and discard anything that feels mushy.
Should I wash the tubers before storing them?
Brush off loose soil and leave it at that. A light rinse is fine only if you let the clumps dry completely afterward, but most gardeners are better off skipping water altogether. Wet crevices are where storage rot begins.
When do I put them back out in spring?
After the danger of frost has passed, since none of the three tolerates a freeze. The last spring frost comes later in the colder northern part of the state than in Fargo, so your own county’s history is a better guide than a statewide date. More local detail is gathered in the North Dakota guides.