In Colorado, dahlias, cannas and gladiolus come up every fall. Denver’s coldest month averages a daily low of 18.4°F (NOAA NCEI 1991-2020 normals), too cold for any of the three to overwinter reliably in the ground. The trigger is the first frost that blackens the foliage, which in Denver normally arrives between September 30 and October 12. Lift the plants one to two weeks after that, cure them, and store them cool, dark and barely damp. Get the timing wrong in either direction and the tubers rot.
Do you have to dig them up in Colorado?
Yes. The rule is simple: where the ground does not freeze deeply, these plants can stay in the ground under mulch, and where it does, they must come up. Colorado sits firmly in the second group, so plan to dig all three every fall. The state’s own climate numbers show why.
What the climate numbers say
The USDA Plant Hardiness Zone Map 2023 places Colorado between zones 3b and 7b, one of the widest spreads of any state. The USDA system rests on a single variable, the average annual extreme minimum temperature. That makes it a measure of the coldest night in a typical year, not of how long the cold lasts. For duration, look at the NOAA NCEI 1991-2020 normals for Denver, where the mean daily minimum of the coldest month is 18.4°F. That is not one freak night. It is the ordinary overnight low, repeated through the middle of winter.
Soil under air like that does not stay thawed. The water inside a dahlia tuber, canna rhizome or gladiolus corm turns to ice, the cells burst, and the plant you dig in spring is a bag of mush that smells like a wet basement. All three originate in warm climates, and none has a way to protect itself from a hard freeze.
Denver’s figures anchor the dates in this article, and Denver sits at 5,280 feet on the plains. Mountain towns run far shorter seasons and can frost in July. The western slope differs again. Wherever you garden, the pattern holds: the colder and higher the site, the earlier the first fall frost, so the lifting calendar moves earlier there.
Why the mulch gamble is a poor one
Somewhere in that 3b to 7b range are pockets milder than Denver, and gardeners in them sometimes report a mulched dahlia clump coming back in spring. Treat those reports as luck rather than a method. One winter without a long cold stretch says nothing about the next one, and a thick mulch blanket over wet soil can rot tubers even when it keeps the frost out. Digging takes an afternoon. Replacing a collection of named dahlias takes a lot longer, and costs more.
Hardy plants are a different matter. Tulips, daffodils and their relatives are built for cold, and they belong to the family of Spring Bulbs that stay in the ground all winter. If you want to know which of those will take your particular winter, Which Spring Bulbs Survive Your Winter sorts it out. Dahlias, cannas and gladiolus are the summer-flowering exceptions that need the lift-and-store routine.
When to lift them
The signal is a frost that blackens the foliage. Dahlia leaves turn dark and limp, canna leaves collapse into gray-green ropes, and gladiolus foliage browns and flops. In Denver the first fall frost normally arrives between September 30 and October 12, according to NOAA NCEI 1991-2020 Climate Normals (32°F freeze probabilities, 30-70% band, station USW00003017). So the lifting window opens one to two weeks after the first frost blackens the foliage. Where you land inside that Denver range depends on the year, and the calendar matters less than what the plants look like.
Reading the frost and the calendar
A light frost that nips the tips of a few leaves is not the signal. You are waiting for a freeze that kills the top growth outright. Fall in Colorado can swing from a first cold snap to a run of warm, sunny afternoons, and plants that survived the first scare keep working until a harder freeze finishes them. For a closer look at frost timing where you live, First and Last Frost Dates in Colorado is the place to check.
Elevation and latitude shift the schedule in one direction only. Colder sites frost earlier, so they lift earlier on the calendar, and warmer, lower sites lift later. A gardener in a mountain town may be digging tubers while a Front Range neighbor is still cutting bouquets.
Gladiolus can run slightly ahead of the others. Their leaves often yellow and brown on their own as the season ends, and once the foliage has clearly declined, the corms can come up whether or not frost has arrived. Dahlias and cannas are more reliable about waiting for the freeze.
Why lifting early gives tubers that rot
After frost kills the top growth, the underground parts are not done. In that short stretch they finish maturing: skins toughen, the crown of a dahlia clump firms up, and the plant pulls the last of its energy back into storage tissue. Dig while the stems are still green and you get thin-skinned, water-heavy tubers that bruise easily and dry unevenly. The cut stem end stays open, and that open wound is where rot usually begins, often weeks later in a dark box where nobody is watching.
The back end of the window matters too. Wait much longer than a couple of weeks and a hard freeze can reach the tubers themselves, especially in a cold, wet fall. Pick a dry day so the soil crumbles off rather than clinging in clumps.
For the mechanics, cut dahlia stems back to a few inches above the ground, and do the same with canna stalks. Push a garden fork into the soil a generous distance from the clump, because the tubers spread farther than most people expect, and lever the whole mass up gently. Stab the fork too close and you skewer the tubers you came to save. Gladiolus corms are smaller and easier to loosen, and they can be lifted with a trowel.
Storing them over winter
The goal in storage is to keep them from rotting and from shriveling at the same time. That takes three steps: cure, clean lightly, then store cool, dark and slightly moist. Those conditions suit dahlias and cannas, while gladiolus corms want the moist part left out.
Curing and cleaning
Once lifted, let the clumps sit in a sheltered, airy spot out of direct sun for a few days so the skins dry and surface moisture evaporates. A garage, covered porch or shed works if it stays above freezing. Then knock off loose soil with your hands or a soft brush. Skip the hose. Washing tubers adds moisture right when you want them dry at the surface, and scrubbing can tear skin. Check for damage while you work: cut away anything soft or mushy, and let small cuts dry before packing.
How each one wants to be stored
The three plants store differently, and treating them the same is a common reason a box goes wrong.
| Plant | Storage organ | Curing | Packing and moisture |
|---|---|---|---|
| Dahlia | Tuberous roots, joined at the crown | A few days in shade, until skins feel dry | Box or crate with barely damp peat moss, wood shavings or vermiculite; slightly moist |
| Canna | Thick rhizomes | Dry the surface, keep a bit of stem stub | Similar packing, kept barely damp so they never dry out fully |
| Gladiolus | Corms, with small cormels attached | Longer, in a dry, airy spot until husks turn papery | Mesh bag or shallow tray, kept dry and well ventilated |
Gladiolus is the odd one out. Its corms behave more like a dry onion than a tuber, so damp packing invites mold. Snap off the old shriveled corm at the base and keep the fresh plump one on top, and save the tiny cormels only if you want to grow more plants from them.
Colorado’s dry winter air changes the picture a little. Shriveling is a bigger risk here than in humid states, and a heated basement can pull moisture out of packing material faster than you expect. Freezing is the opposite risk: an unheated attached garage can follow the outdoor temperature down when the coldest month brings nightly lows averaging 18.4°F in Denver (NOAA NCEI 1991-2020). An interior closet or a cool, unheated basement room is a safer bet.
Whatever the spot, look through every box once a month. Squeeze a few tubers. If they feel like a firm potato, all is well. If they feel spongy, pull them out and cut away the bad part. If they have wrinkled like a dried apricot, lightly mist the packing material. That monthly check is the difference between a box that comes through winter and a box that looks fine until April.
What goes wrong
Most losses trace back to one of four mistakes, and each leaves recognizable signs before the plant is beyond saving.
Problems in storage
Rot shows up as soft, dark, sometimes weeping patches, often with gray or white fuzz and a sour smell. It starts at cut ends or bruises and spreads through damp packing. Prevent it with a proper cure, gentle handling, dry-surfaced tubers and packing that is barely damp rather than wet. Cut out any soft spot early and give the piece more air.
Shriveling is the mirror problem. Tubers turn wrinkled, light and leathery, and eventually they cannot sprout. It is common in dry Colorado basements. A light mist on the packing material during the monthly check, or slightly tighter packing in a lidded box, usually brings them back before they cross the line.
Problems in timing
Lifting too early looks like tubers that felt fine in October and turned soft by January. The skins never toughened. The fix is patience: wait for a frost that blackens the foliage, then let a week or two pass. Also cut the stem to a short stub rather than a hollow straw. Open stem ends can pull water in.
The costliest mistake is leaving them in the ground after a warmer-climate tip. Advice from a neighbor at a lower elevation, or from a gardener in a milder state, does not travel north or uphill. The tubers that come up in spring are either mushy or gone, and there is no fixing it then. Leaving them in the ground is the mistake that costs the most plants, so if you want to test a mulched clump, do it with one you can afford to lose and lift the rest.
Common questions
Can I leave dahlias in the ground in Denver if I pile on mulch?
Denver’s coldest-month mean daily minimum is 18.4°F (NOAA NCEI 1991-2020), and mulch slows freezing rather than preventing it over a whole season. Some clumps survive a mild winter, but wet soil under mulch can rot them anyway. If you want to experiment, lift most of your clumps and mulch a single one as a test.
Does the timing change in mountain towns or on the western slope?
Yes. The September 30 to October 12 first-frost range describes Denver, at 5,280 feet on the plains. Mountain towns run far shorter seasons and can frost in July, and the western slope differs again. Colder places get their first freeze earlier, so lifting moves earlier there too. The Colorado guides cover more of the state’s regional differences.
Do gladiolus need a frost before I dig them?
Not always. Dahlias and cannas wait for the blackening frost, but gladiolus foliage often yellows and browns on its own late in the season. Once the leaves have clearly declined, the corms can come up. If frost arrives first, use the same one-to-two-week wait as for the others.
What goes in the bed once the tubers are out?
Fall is the planting season for hardy spring bulbs, so an emptied bed can take tulips or daffodils right away. Planting Spring Bulbs in Colorado covers how to time that. Before you pack anything away, label each dahlia tuber with a tag or marker: bare tubers of different colors look identical in April.