Dig dahlias, cannas and gladiolus in Montana once a frost has blackened the foliage, and get them out within a week or two. In Billings, NOAA’s 1991-2020 normals put the first 32°F freeze between October 1 and October 14, so lifting usually lands in mid to late October. Farther north and at higher elevations, frost comes sooner and the shovel should follow it. When a forecast promises a hard freeze, trust the plants and the thermometer over the calendar.
Do you have to dig them up in Montana?
Yes. Plan to dig these three up every fall anywhere in Montana, because no part of the state has the mild ground that lets tubers, rhizomes and corms overwinter under mulch. The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) places Montana in zones 3a to 6a. Every one of those zones is a place where winter freezes the soil solidly.
What the numbers say
The USDA system rests on a single variable: the average annual extreme minimum temperature. It tells you the coldest night a plant should expect in a typical year, which is the number that decides whether a tender plant lives or dies outdoors. For a sense of the everyday cold, NOAA’s NCEI Climate Normals for 1991-2020 give Billings a mean daily minimum of 17.9°F in its coldest month. That is the average low, not a record. Half the winter nights are colder than that in the mildest large valley in the state, and the soil freezes to a depth well past where a dahlia tuber sits.
Dahlias, cannas and gladiolus all come from warm climates. Their storage organs are packed with water and starch, and once ice crystals form inside them, the tissue collapses. A tuber that has frozen looks fine in the ground until spring. Then it turns to mush at the first warm spell, or it simply never sprouts.
Where the line falls inside the state
The line between “stay put” and “come up” does not run through Montana. It sits well south of the state, in ground that stays open through winter. Zone 6a country and zone 3a country in Montana get the same verdict, and what changes is the timing. Billings is one of the milder valleys, so its dates run later than almost anywhere else in the state. Colder locations, meaning farther north or higher up, get their first fall frost earlier, and lifting moves earlier with it. Mountain valleys can see frost in any month, so a gardener there should watch the overnight forecast from late summer on.
Mulch does not change this. A thick layer of straw or leaves can hold a few degrees against a short cold snap, but it cannot stop a soil profile from freezing through across a Montana winter. Chinook winds can also thaw the surface mid-winter, then a hard freeze returns. That freeze-thaw cycling leaves tubers sitting in cold, wet soil, which is the classic setup for rot. Hardy bulbs such as tulips and daffodils handle this fine (see Spring Bulbs), and the Which Spring Bulbs Survive Your Winter guide sorts out which ones stay in the ground. The three plants here belong on the other side of that sorting.
When to lift them
The signal to watch for is a frost hard enough to blacken the foliage. Dahlia leaves go dark and limp, canna leaves collapse into brown mush, and gladiolus leaves yellow and flop. That one event is your cue, and lifting one to two weeks after the first killing frost gives the underground parts time to finish maturing.
Turning the frost date into a window
For Billings, NOAA’s NCEI 1991-2020 Climate Normals list the first 32°F freeze in the 30-70% probability band between October 1 and October 14. Add the week or two of waiting, and the lifting window runs from roughly the second week of October to the end of October. That is a working range rather than a promise, because a given fall might bring frost as early as late September or as late as the last days of October. The plants respond to the actual frost, not to the normals.
Colder Montana locations shift the whole schedule earlier. Someone gardening at a higher elevation, or farther north than Billings, should expect the first frost to come earlier, and plan to lift earlier as well. For the frost dates in other parts of the state, see First and Last Frost Dates in Montana. If the forecast calls for a deep freeze a night or two after a light one, don’t wait out the full stretch. A soil freeze can reach tubers, so lift promptly and skip the extra days.
Why lifting earlier backfires
It is tempting to dig in September on the theory that earlier is safer. Tubers lifted while the plant is still actively growing tend to be immature, with thin skins and soft tissue. They lose moisture quickly, and they are easy targets for fungal rot in storage. The foliage feeds the roots until frost cuts it off, and the plant pulls stored energy downward as it dies back. Digging before that step is finished takes the crop out of the oven half-baked.
The frost also acts as a trigger. It signals the plant to shut down and harden, and the week or two afterward is when the outer skin firms up. Tubers with firm skins cure well and hold moisture through months in a box. Tubers dug early and left with soft, thin skins are the ones that come out in January as wrinkled or slimy lumps. Gladiolus follow a slightly different pattern, since their corms are often lifted when the foliage yellows, but in Montana the first frost usually does that work for you anyway.
When you do dig, work a few inches out from the stalk and loosen the soil before lifting, so a shovel doesn’t slice through the clump. Cut dahlia and canna stalks down to a short stub. For gladiolus, trim the foliage after you lift, not before.
Storing them over winter
The formula is short: cure, clean lightly, store cool and dark and slightly moist. The goal is to keep the tubers and corms from rotting on one side and from shriveling on the other. Every step in between exists to hit that narrow middle. Set a reminder to check them once a month, because that is the step that saves the most of them.
Cure first, then clean lightly
Curing means letting the surface dry in a sheltered, airy spot out of direct sun and away from freezing, such as a garage, porch or basement corner. Keep going until the skins feel dry and papery to the touch. Then brush off loose soil by hand. Skip a heavy washing, or if you do rinse dahlias to spot problems, dry them completely before packing. Wet tissue against wet tissue is where rot begins. Cut away any soft or damaged spots, and dust cut surfaces if you like, but don’t strip the skin.
Dahlias, cannas and gladiolus side by side
The three do not want exactly the same treatment. Dahlia tubers dehydrate easily and want a lightly damp packing material. Canna rhizomes are tougher and also like slight moisture. Gladiolus corms want to be dry, and they suffer when kept damp.
| Plant | What you store | Curing | Packing | Moisture |
|---|---|---|---|---|
| Dahlia | Clumps of tubers with a fragile neck | Air-dry until the skins feel dry, out of sun | Boxes or crates layered with barely damp peat, sawdust or vermiculite | Slightly moist; the biggest risk is shriveling |
| Canna | Fleshy rhizome clumps | Air-dry briefly, trim stalks to a stub | Loose in boxes with lightly damp packing material | Slightly moist; check for both rot and drying |
| Gladiolus | Corms, plus small cormels | Longest cure of the three, until husks are papery | Mesh bags or open trays in a single layer | Dry; damp storage brings mold |
Dahlias reward you for keeping the neck of each tuber intact, since that is where the growth eye lives. A tuber snapped off from the crown often won’t sprout. Gladiolus corms are the opposite in temperament. Once cured, you can snap off the old shriveled corm at the base of the new one and discard it, and you can keep the tiny cormels if you want to grow more. Label everything as you box it, because winter memory does not hold up against a dozen similar-looking bundles.
Where to keep the boxes
Find a spot that stays cool and dark all winter but never freezes, such as an unheated basement, an insulated crawlspace or a closet on an outside wall. Avoid anywhere near a furnace, and avoid an attached garage that drops below freezing on a Montana January night. A basement beats a garage in most Montana houses for exactly that reason. Once a month, open every box. Look for soft or moldy spots, firm up any shriveled tubers with a light misting of the packing material, and pull out anything rotting before it spreads. That monthly ten minutes is worth more than any special packing trick.
What goes wrong
Most storage failures come from a short list of causes, and each leaves a recognizable sign. Catching the sign early is usually enough to save the rest of the box.
Rot and shriveling
Rot shows up as soft, dark, sometimes slimy patches, often with a sour smell or a fuzz of gray or white mold. It comes from packing too wet, storing before the skins dried, or leaving the boxes somewhere with no airflow. Remove the affected tuber, cut back to firm, clean tissue if only part is damaged, and let the cut face dry before repacking. Shriveling is the mirror image: tubers turn wrinkled and light, like an old apple. That means the air is too dry or the packing material has dried out. A light mist on the packing material, not on the tubers themselves, usually reverses it. If the tuber has gone hard and hollow, it won’t come back.
Timing and placement mistakes
Lifting too early gives thin-skinned tubers that fail in storage, which is why waiting for the frost to blacken the foliage matters so much. Lifting far too late is a risk too, since a hard freeze that reaches the soil can damage the crown. In Billings the sweet spot follows the October 1 to October 14 first-frost range, and colder spots in Montana move earlier.
The other classic mistake is leaving them in the ground because a neighbor’s dahlias supposedly survived one year. A mild winter or a deep snow cover can protect tubers now and then, but Montana’s zones 3a to 6a make it a gamble. A gardener who tries one clump under heavy mulch as an experiment, while lifting the rest, loses little. Anyone who leaves the whole collection out risks losing years of divisions in a single freeze-thaw winter. For the spring side of the calendar, Planting Spring Bulbs in Montana covers the bulbs that are meant to stay put, while the ones in your storage boxes wait until the danger of frost has passed.
Common questions
Can I leave dahlias in the ground in Montana if I mulch heavily?
It is a gamble that usually loses. Montana falls in USDA zones 3a to 6a, and Billings, one of the milder valleys, still averages a daily low of 17.9°F in its coldest month, according to NOAA’s 1991-2020 normals. Mulch softens a short cold snap but does not keep frozen soil off the tubers all winter. If you want to test it, try a single spare clump and dig up the rest.
Should I dig them before the first frost to be safe?
No. Lifting before frost catches the plants while they are still feeding their storage organs, and the skins stay thin and soft. Those tubers lose moisture fast and rot easily. The exception is a forecast for a deep, hard freeze after a light one, in which case lift promptly. Billings normally sees its first 32°F freeze between October 1 and October 14, so most lifting happens shortly after that.
Do gladiolus need the same storage as dahlias?
Not quite. Gladiolus corms store dry, in mesh bags or open trays, after a longer cure until the husks turn papery. Dahlias and cannas prefer slightly damp packing material so they don’t shrivel. All three want a cool, dark spot that never freezes, and all three benefit from a check once a month through the winter.
What if my garden is higher or farther north than Billings?
Move everything earlier. Colder parts of Montana get their first fall frost sooner, so the lifting window arrives ahead of the Billings dates, and mountain valleys can frost in any month. Watch overnight forecasts from late summer, and lift a week or two after the frost that blackens the foliage, wherever that falls on your calendar. The Montana guides collect the frost and planting information for other parts of the state.