In South Dakota, dahlias, cannas and gladiolus come out of the ground every fall, about one to two weeks after the first frost blackens the foliage. For Sioux Falls, NOAA’s 1991-2020 Climate Normals put the first 32°F freeze between September 24 and October 4. Gardens farther north, at higher elevation or out in the country usually get that frost sooner, so check your own county before you pick a weekend with a shovel.
Do you have to dig them up in South Dakota?
Yes, everywhere in the state. Unlike a warm-winter territory, South Dakota has no corner where the ground stays soft and unfrozen, so there is no part of it where these three can be left under mulch. Plan to lift all three every fall, whether you garden in the southeast or near the Black Hills.
The cold behind the verdict
The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) places South Dakota in zones 3b to 5b. That system rests on a single variable, the average annual extreme minimum temperature, and the territory-wide range simply tells you that parts of the state run noticeably colder than others. Where you fall within it depends on latitude, elevation and how exposed your lot is.
For a concrete anchor, take Sioux Falls, the reference station. NOAA’s 1991-2020 normals give a mean daily minimum of 8.5°F in the coldest month. That number matters more than any zone label here, because it describes an ordinary January night, not a freak cold snap. Soil under a lawn or garden bed freezes to a meaningful depth on a night like that, and it stays frozen for weeks.
Now consider what is sitting in that soil. Dahlias form tubers, cannas form thick rhizomes, and gladiolus grow from corms. All three plants trace back to warm or tropical regions, and their underground parts are packed with water and have no defense against ice crystals. Frozen tissue looks fine in the ground. The damage shows up at thaw, when the tuber turns to gray mush.
Why mulch does not rescue them
Heavy mulch is a common suggestion in online garden advice, and it does work in places where winter lows only occasionally dip below freezing. Mulch does not add warmth. It slows how fast the ground cools and warms. Against a stretch of nights near 8.5°F, a layer of straw only delays the freeze. Gardeners in South Dakota sometimes report a canna surviving one mild winter in a sheltered spot against a south wall. That is luck, and it should not become the plan.
The hardy spring-flowering bulbs are a different story: tulips, daffodils and crocus actually need that winter chill. If you are sorting your beds into what stays and what comes up, Which Spring Bulbs Survive Your Winter covers the plants that can be left alone. For these three summer bloomers, the decision is already made.
When to lift them
The signal is a frost that blackens the foliage. Lift within about a week or two after that, and the tubers, rhizomes and corms get time to finish maturing underground before they come out. In South Dakota, plan to dig one to two weeks after the first frost that blackens the foliage.
Reading the calendar and the plant
Start with the reference-city frost date. NOAA’s 1991-2020 normals for the Sioux Falls airport station put the first fall freeze between September 24 and October 4 (that is the 30 to 70 percent probability band for a 32°F reading). Your lifting window sits one to two weeks after whatever date your garden actually gets its first killing frost, which means the second half of that range and a bit beyond in a typical year.
Suburban and rural gardens usually see that first frost earlier than the airport does, often by one to two weeks, and more at elevation. Gardens in the colder northern and higher parts of the state move the whole schedule earlier. A gardener near the Black Hills may be cutting back blackened dahlias while someone in Sioux Falls still has green leaves. The First and Last Frost Dates in South Dakota page has county-level detail for pinning down your own date.
Let the plant tell you when the frost has done its work. Dahlia and canna foliage turns dark and limp overnight. Gladiolus is a little different: the leaves yellow and brown on their own as the season winds down, and light frost does not bother the corms nearly as much as it bothers dahlia tubers. Digging when the leaves have browned is the usual approach.
Why earlier digging backfires
Gardeners get anxious as nights cool and pull everything before the first frost. That is the mistake that costs the most tubers. During the weeks after the foliage dies back, the plant is still moving stored energy down into the underground parts and the skin is thickening. A tuber lifted while the plant is still green has thin skin and high moisture, and those are exactly the conditions that invite rot once it is boxed up in the basement. By February it has turned soft, usually starting at the neck.
Waiting too long carries its own risk. If you leave dahlias through a hard freeze that penetrates the soil, or through a wet, cold stretch, tubers can be damaged before you ever see them. A decent compromise when a frost forecast arrives while the plants are still green is to cover them overnight with sheets or row cover and keep the plants growing a little longer. Once the frost has blackened the tops, cut the stalks down to a short stub, loosen the soil with a fork well outside the clump, and lift gently so the necks do not snap.
Storing them over winter
All three need the same basic treatment: let them cure, clean them lightly, then keep them cool, dark and slightly moist so they neither rot nor shrivel. Where they part ways is how much moisture each one wants and how it is packed. Check the stored tubers, rhizomes and corms once a month from the day they go in until the soil warms after your last spring frost.
Curing and cleaning
Curing is a short drying period in a sheltered, airy spot out of direct sun, such as a garage or porch. It lets cut surfaces heal and surface moisture evaporate before storage. Shake off the loose soil and leave the rest. Washing is fine for dahlias if you let them dry thoroughly afterward, but scrubbing them clean invites trouble, and so does stripping every bit of dirt off a canna rhizome.
Gladiolus corms cure the longest, since the old corm at the bottom and the husks need to dry down until they feel papery. Once dry, snap off the shriveled old corm and the small cormels can be kept separately if you want to grow them on.
How the three differ
| Plant | What you store | Moisture | Packing |
|---|---|---|---|
| Dahlia | Tuber clump with the crown intact | Slightly moist, never wet | Boxes with barely damp peat moss, wood shavings or vermiculite |
| Canna | Thick rhizome clump with a little soil left on | Moister than the others | Lightly damp medium in a box or bin, kept from drying out |
| Gladiolus | Cleaned corms, old corm removed | Dry | Mesh or paper bags, or open trays, with plenty of airflow |
Location matters as much as packing. An unheated garage in South Dakota can sit well below freezing for stretches, given that a normal January night at Sioux Falls averages 8.5°F (NOAA, 1991-2020). A basement corner, an insulated crawlspace or the inside of a cool closet wall usually does better. The target is cool and above freezing, with no light.
The monthly check is what saves them. Open each box, look for soft spots or mold, and press a few tubers. If they feel rubbery or wrinkled, mist the packing material lightly. If anything smells sour or feels slimy, cut the bad part away or toss the piece before it spreads. Ten minutes once a month is a far better investment than the cost of replacing a favorite dahlia.
What goes wrong
Four problems account for most lost tubers in South Dakota, and each leaves a different clue. Dig every fall, even if a neighbor’s canna came through one mild winter.
- Rot. Tubers turn soft and mushy, often beginning at the neck, and sometimes grow gray or white mold. It comes from storing too wet, too warm or with no air movement, and from lifting before the tops matured. Prevent it with a thorough cure, barely damp packing and the monthly check, trimming off any damaged tissue early.
- Shrivelling. Tubers become wrinkled, leathery and light. The storage area is too dry or too warm, which is common near a furnace. Lightly misting the packing material, or moving the box somewhere cooler, usually brings them back if you catch it early.
- Lifting too early. Thin-skinned, undeveloped tubers rot or dry out regardless of how carefully you store them. The fix comes in the previous fall: wait for the frost to blacken the foliage, then give it one to two weeks.
- Leaving them in the ground one zone too far north. Guides written for warmer states say cannas and dahlias can overwinter in the ground. South Dakota is zones 3b to 5b on the USDA map, and those guides were not written for it. The tuber that survived was usually in a protected pocket during an easy winter, and next year’s cold will be less kind.
Fall is also the time to get hardy bulbs in, since you will be out with a trowel anyway. The Spring Bulbs section and the guide to Planting Spring Bulbs in South Dakota cover that side of the job, and more regional advice sits in the South Dakota guides.
Common questions
Can I leave dahlias in the ground under mulch in South Dakota?
It is a gamble that usually loses. The USDA places South Dakota in zones 3b to 5b, and the Sioux Falls reference station averages 8.5°F for the daily minimum in the coldest month. Mulch slows the freeze but does not stop it, and frozen tubers rot at thaw.
Do I have to wait for frost before digging gladiolus?
Not exactly. Gladiolus foliage yellows and browns on its own as the season ends, and corms are lifted once the leaves have died back. A light frost does less harm to them than to dahlias, but do not leave them in the ground through a hard freeze.
What if a hard freeze hits before I get to the garden?
Cut the blackened stalks down and lift within a week or two, as planned. If the soil has not frozen solid, the tubers underneath are generally fine. If the ground has frozen hard around them, thaw it gradually and check each one for soft, dark flesh before storing.
Is an unheated garage good enough for storage?
Often not. A normal January night in Sioux Falls reaches an average low of 8.5°F, so an unheated garage can drop below freezing. A basement corner or other cool space that stays above freezing is safer for all three.