In Iowa, dahlias, cannas and gladiolus come out of the ground every fall. The signal is a frost that blackens the foliage, and the job follows within one to two weeks. At Des Moines, the first fall freeze normally arrives October 14 to October 25. None of the three survives an Iowa winter reliably outdoors, so lift them, cure them, and hold them cool and dark until spring. Timing, storage and the usual failures are covered below.
Do you have to dig them up in Iowa?
Yes. Iowa spans USDA zones 4b to 6a on the USDA Plant Hardiness Zone Map 2023, and the whole range falls on the dig-them-up side. That map is built on a single variable, the average annual extreme minimum temperature, which is why it answers a survival question rather than a scheduling one. Where the ground never freezes deeply, dahlia tubers, canna rhizomes and gladiolus corms can sit under mulch all winter. Iowa is not that kind of place, and the decision to write down is to lift all three every year, in every part of the state.
What the Des Moines numbers say
NOAA’s 1991-2020 Climate Normals for Des Moines put the mean daily minimum of the coldest month at 13.8°F. That is an average night in midwinter, not the record low, so plenty of nights run colder. Tubers and corms are storage organs full of water and sugar, and they are built for warm soil. Once the soil around them freezes, ice crystals rupture the cells, and what you dig in spring is mush. A 13.8°F typical low leaves no room for argument.
Where the line falls, and why mulch doesn’t move it
The line between “lift” and “leave” does not run through Iowa. It sits well south of the state line, in territory where winter minimums stay comfortably above freezing, and every Iowa county is on the cold side of it. Zone 6a in the southern part of the state is milder than zone 4b up north, but a milder Iowa winter is still a freezing one. The practical differences inside the state affect timing, not the verdict. Northern Iowa is colder, so its first fall frost comes earlier and its lifting window opens earlier. Southern Iowa runs later. Des Moines is the reference point used throughout this article, and your own yard may sit a little on either side of it.
Mulch is where many gardeners get talked into a gamble. A thick layer of shredded leaves or straw slows the freeze, and a sheltered bed against a south wall may carry a dahlia through an unusually mild winter. That is a bet, not a plan, and the losing years cost you the whole clump. Mulch earns its keep on hardy bulbs like tulips and daffodils, and Spring Bulbs covers how those differ from the tender ones here. If you are unsure where a particular bulb sits, Which Spring Bulbs Survive Your Winter sorts the question by hardiness.
When to lift them
Wait for a frost that blackens the foliage, then lift within about a week or two. That’s the whole rule. At Des Moines, the first fall frost normally arrives between October 14 and October 25, according to NOAA NCEI 1991-2020 Climate Normals, which measure the 30 to 70 percent probability band of a 32°F freeze. So lift one to two weeks after the first killing frost, which at Des Moines means roughly from the second half of October into early November.
Why the frost matters
Frost kills the tops, but the tubers stay snug in the soil, which holds heat far longer than the air does. That pause is useful. The plant pulls its last energy down into the roots, and the skins toughen. Dahlia tubers dug straight after the frost are still soft, with thin skins that bruise and lose water quickly. Giving them that extra week or two lets them finish maturing while they’re still in the ground, protected from everything except a hard freeze that goes deep.
Lifting early, say in September because the plants look ragged, produces the opposite result. Immature tubers carry more water and thinner skins, and they shrivel or rot in storage. Cutting dahlia stems also opens a hollow channel for moisture and pathogens, and a tuber that is still actively growing heals that wound poorly. Patience pays here more than anywhere else in the fall garden.
Adjusting for your part of Iowa
Calendar dates are a guide, and the plants are the signal. Iowa State Extension county figures (1950-2021) run about 10 days earlier in fall than the Des Moines airport station, so the airport dates are the optimistic end. In many counties, especially farther north, the frost and the lifting window arrive earlier than the Des Moines numbers suggest. Colder parts of the state get their first freeze sooner, which means fall tasks move earlier, not later.
Watch the forecast as closely as the calendar. If a hard freeze is coming before any light frost has blackened the leaves, lift at that point, since a deep freeze that reaches the crowns costs you the plant. For a county-by-county look at timing, see First and Last Frost Dates in Iowa.
Gladiolus follow a slightly different signal. Their foliage yellows and browns on its own schedule, and the corms can come up once the tops have died back, which sometimes happens around the same time as the first frost. Cannas behave like dahlias: frost blackens the leaves, you cut them back, and the rhizomes come up soon after. Work on a dry day, loosen the soil well away from the clump with a garden fork, and lift rather than pull.
Storing them over winter
The storage recipe is the same in outline for all three: cure, clean lightly, then keep them cool, dark and slightly moist so they neither rot nor shrivel. The word “slightly” does the work. Too wet and they mold, too dry and they wither into leather. Where the three differ is in how much moisture each one tolerates.
Curing and cleaning
Curing means letting the surfaces dry and the skin firm up before packing. Set the lifted clumps somewhere sheltered, airy and out of direct sun, such as a garage or porch. Brush off loose soil rather than scrubbing. A light rinse is fine for dahlias if you let them dry completely afterward, but washing is optional, and leaving a little soil on is often safer. Trim the stems close to the crown once they’ve dried, and remove any obviously damaged pieces.
How the three differ
| Plant | What you store | Curing | Packing | Main risk |
|---|---|---|---|---|
| Dahlias | Tuber clumps | Air-dry until the skin is firm | Lightly damp, loose material (peat, vermiculite or wood shavings) in a ventilated box | Rot if packed wet, shriveling if left bare |
| Cannas | Fleshy rhizomes, kept in large chunks | Brief air-drying, not bone dry | Barely damp medium in a box or crate | Drying out, which they tolerate poorly |
| Gladiolus | Corms, with old spent corms removed | Longer, in a warm airy spot, until the husk is papery | Dry, in mesh bags or paper, never in damp medium | Mold from any trapped moisture |
Dahlias sit in the middle, with cannas wanting more moisture and gladiolus wanting less. Cannas are the ones most likely to wither if the air in storage is dry, so keep their medium just barely damp. Gladiolus corms behave more like onions and want air moving around them. Put all of it somewhere that stays cool and dark and does not drop below freezing, such as an insulated basement or the interior corner of an attached garage.
The step that saves the most plants is the dullest one. Open every container once a month from lifting until planting time in spring. Remove anything soft or moldy before it spreads, and mist lightly if the tubers have started to wrinkle. Five minutes a month beats discovering a box of mush in April.
What goes wrong
Rot is the most common failure. It shows up as soft, dark, mushy spots, sometimes with gray or white mold and a sour smell. Trim back to clean, firm tissue with a sharp blade, let the cut surface dry, and repack in drier material with more airflow. Left alone, one rotting tuber infects its neighbors, which is why the monthly check matters.
Shriveling looks the opposite: wrinkled, rubbery or hollow tubers that feel light. The storage spot is too dry or too warm. Mist the packing material lightly, or move the box somewhere cooler. A tuber that has gone completely papery and hard will not recover, but one that is only wrinkled often plumps back up.
Lifting too early produces a hybrid of both problems. Immature tubers with thin skins dehydrate and rot at the same time, and the clumps look fine in November and fail by January. The fix is simply to wait for the blackened foliage and then give it another week or two.
The last mistake is geographic. Plant labels and neighbors’ stories often come from warmer states, where these plants really do overwinter. Iowa runs from USDA zone 4b to 6a, and Des Moines averages a 13.8°F minimum in its coldest month, so a clump left out “because it worked in Missouri” is a clump at risk. Lift every year, even in the mildest corner of the state. The same fall you are lifting tender bulbs is also the time to put hardy ones in the ground, and Planting Spring Bulbs in Iowa covers that side of the job.
Common questions
Can I leave dahlias in the ground in Iowa if I mulch them heavily?
You can try, but the odds are poor. Iowa winters freeze the soil, and mulch slows that freeze without preventing it. A very mild winter or a protected spot might let a clump through, which is why you occasionally hear of a dahlia that “came back.” Lifting is the reliable method.
Should I dig before or after the first frost?
After, for dahlias and cannas. Wait for frost to blacken the foliage, then lift within about a week or two. Gladiolus can come up once their tops have yellowed and died back, which may happen at about the same time. The exception to waiting is a forecast hard freeze before any light frost has arrived, in which case lift right away.
Do I have to wash the tubers before storing them?
No. Brush off loose soil and let everything dry completely. A gentle rinse is fine if you can dry the tubers thoroughly afterward, but trapped moisture is the bigger threat, and a little leftover soil does no harm. Gladiolus corms are best kept dry and unwashed.
Do tulips and daffodils need the same lifting?
No. They are hardy, so they stay in the ground all winter and get planted in fall, not dug. See Planting Spring Bulbs in Iowa for timing. Other seasonal tasks across the state are collected in the Iowa guides, and the same frost-watching habit that tells you when to lift tender bulbs applies to most of them.