Most Kentucky gardeners need to dig up dahlias, cannas and gladiolus each fall. The signal is the first frost that blackens the leaves. At Louisville that frost normally arrives between November 2 and November 12, and the tubers, rhizomes and corms come out within about a week or two afterward. Lift them any sooner and they rot in storage. Wait much longer and wet, cold soil starts to do the same damage in the ground.
Do you have to dig them up in Kentucky?
Yes, as a rule. Kentucky falls in USDA hardiness zones 6a to 7a (USDA Plant Hardiness Zone Map 2023, built on the average annual extreme minimum temperature), and the ground freezes through that whole range. NOAA’s 1991-2020 Climate Normals for Louisville show the coldest month with an average daily minimum of 27.8°F. That is an average of the nightly lows, so plenty of individual nights run colder, and a mean below 32°F means the soil is not just chilly but frozen for stretches of the winter. Dahlia tubers, canna rhizomes and common garden gladiolus corms are tropical or subtropical in origin. They have no defense against ice in the soil, and the dependable plan across zones 6a to 7a is to lift all three after the first killing frost.
Where the line falls
Staying in the ground under mulch only works where the soil does not freeze deeply, and that line does not run through Kentucky. Nothing in the state’s zone range reaches a climate where winters stay mild enough to count on. The warmer end of the range, zone 7a, has the gentlest winters, and gardeners in those pockets sometimes leave cannas under a thick blanket of shredded leaves or bark and see them return. That is a gamble, and it tends to fail in just the winters that matter: a wet stretch followed by a hard freeze turns a rhizome to mush. Farther north in the state and at higher elevations the winters run colder, so the gamble gets worse, not better.
Soil moisture matters as much as the thermometer. Heavy clay, which covers much of central Kentucky, holds water around a buried tuber all winter. Cold alone seldom does the killing, because freezing and thawing in soggy ground rots the tuber from the outside in. A raised bed with sharp drainage and a deep mulch layer improves the odds, but even then you are betting against the weather.
There is a middle path for the reluctant digger. Lift one plant of each kind, such as a favorite dahlia, and leave the rest mulched. If the mulched ones survive, you have learned something about that particular bed. If they do not, the lifted plant is your insurance. Anyone who has priced out replacing a collection of large-flowered dahlias will understand why digging beats guessing. For a broader look at which bulbs handle the cold and which do not, see Which Spring Bulbs Survive Your Winter.
When to lift them
The cue is on the plant, not the calendar. A frost that turns dahlia and canna foliage black and limp tells the plant to stop growing and send its last energy down into the roots. NOAA’s 1991-2020 Climate Normals put the first 32°F freeze at Louisville between November 2 and November 12 (the middle of the probability band, so some years come earlier and some later). Lift within about a week or two after that frost, which for most of Kentucky means roughly the second half of November in an average year. For a closer read on your own county, the guide to First and Last Frost Dates in Kentucky breaks down how the timing shifts across the state.
Why not earlier?
Digging while the foliage is still green looks like a safe, early start. It backfires. Until frost shuts the plant down, the leaves are still feeding the underground parts, and the skin on tubers and rhizomes is thin and wet. Cutting the stalks at that point leaves an open wound on a tuber full of water. In storage, that combination invites rot, and dahlias are the worst for it because the hollow stem stub acts like a funnel for moisture and fungi. Tubers lifted early also tend to shrivel, since immature skin loses water faster than skin that has had time to set.
The short wait after frost does real work. The tops die back, the skins firm up and the tissue matures. Gladiolus follow a slightly different clock. Their leaves yellow and brown on their own as the season winds down, often before the first frost, and the corms are ready once the foliage has turned. If a late-October night threatens the blooms, you can let the corms ride until the leaves brown rather than force the issue.
Why not much later?
Waiting too long has its own cost. A first frost only kills what is above ground, because soil holds its warmth longer than air. But a long stretch of cold rain after that frost soaks the bed, and tubers sitting in cold, saturated soil start to rot even if the ground never freezes. Pick a dry day, loosen the soil with a garden fork a good distance from the stalks so you do not spear the clump, and lift gently. Rural gardens should lean toward the early side of the window. The National Weather Service in Louisville cautions that the city’s official station does not match the surrounding rural parts of Jefferson County, and a tighter window is the safer expectation outside town. The same goes for any spot farther north or higher up, where frost shows up sooner and lifting moves earlier with it.
Storing them over winter
The routine has three beats: cure, clean lightly, store cool and dark. Curing means letting the freshly dug clumps dry in a sheltered, airy spot out of direct sun and out of the rain, such as a garage or covered porch, until the skin feels dry to the touch. Brush off loose soil but skip the pressure washer. A light rinse on dahlias is fine as long as they dry completely afterward, but scrubbing strips protective skin and leaves open scrapes where rot begins. The target for storage is a space that stays cool but never freezes, like an unheated basement, a crawl space or an insulated garage, and that stays dark and has some air movement.
How the three differ
Each of the three has its own tolerance for moisture, and mixing them up in one box is how collections get lost.
| Plant | What you store | Packing | Main risk |
|---|---|---|---|
| Dahlia | Fragile tuber clumps with a stub of stem | Boxes or crates with barely damp peat moss, vermiculite or wood shavings | Rot at the neck, or shrivelling if packed too dry |
| Canna | Thick, fleshy rhizomes | Slightly damp packing in a box, some soil left clinging | Drying out into hard, wrinkled lumps |
| Gladiolus | Flat, round corms | Dry, in mesh bags or paper in a single airy layer | Mold if kept damp; thrips riding over in the husks |
Gladiolus want the driest treatment of the three. After curing, snap off the shriveled old corm from the bottom, pull away the tiny cormels (or keep a few to grow on, which takes years to produce blooms) and store the plump new corms in a mesh onion bag or paper sack. Dahlias and cannas, in contrast, are fleshy and lose moisture fast in dry air, so a slightly damp packing material keeps them plump without soaking them. The word is slightly: the medium should feel barely cool and moist in your hand, never wet enough to squeeze out a drop.
Before winter settles in, label each box with the variety or color. A dahlia labeled only “red” will not help you in May. Check every box once a month through the winter. Open each one, look and feel. Pull out and discard any tuber that is soft, slimy or smells sour, and if the others look wrinkled, mist the packing lightly. If you see mold spreading, open the container to air it and move the stock to a drier spot. This habit takes ten minutes a month and saves more tubers than any other step.
What goes wrong
Rot is the biggest loss, and it shows up as soft, dark, sometimes foul-smelling patches on a tuber or rhizome, often starting at the neck on dahlias. It comes from lifting before the skins set, packing the stock in material that is too wet, or storing it somewhere with no air movement. Cut away small spots with a clean knife, let the cut surface dry, and the rest of the tuber is usually fine. Rot that has spread through the heart of the tuber is a lost cause, so remove it before it contaminates its neighbors.
Shriveling is the opposite failure. Tubers turn wrinkled and light, like a raisin, because the storage spot is too warm or the packing is bone dry. A mild case recovers with a little added moisture in the packing, but a fully mummified tuber will not come back. Dry heated basements and spots near a furnace are common culprits, and a cooler corner solves the problem.
Lifting too early, as covered above, produces both problems at once: thin-skinned tubers that rot at the cut and dry out at the surface. Wait for the blackened foliage and the week or two after it, and the tubers go into storage ready to hold.
The other trap is the opposite mistake, leaving everything in the ground a zone too far north for the plant’s comfort. The plant looks fine all winter under its mulch and then simply never comes up, or comes up as a sickly, thin shoot in June. Nothing at the surface warns you, and by the time you know, a season has passed. Keep in mind that the zone map describes the extreme minimum, which sets what is possible, but a wet winter kills tubers in conditions the map calls mild. Anything that cannot be replaced cheaply deserves a spot in the basement.
Pests ride along too. Thrips overwinter on gladiolus corms and can wreck the next season’s flowers, so inspect the husks and discard anything with silvery, scarred or gummy patches. Once spring arrives, the same corms and tubers go back into warm soil, and the timing for that comes later in the year, as outlined in Planting Spring Bulbs in Kentucky for the hardy bulbs that go in during fall.
Common questions
Can I leave dahlias in the ground in Kentucky?
It is a gamble. Kentucky sits in USDA zones 6a to 7a (USDA Plant Hardiness Zone Map 2023), and at Louisville the coldest month averages a nightly low of 27.8°F (NOAA NCEI Climate Normals 1991-2020). That is cold enough to freeze the soil, and wet clay finishes off tubers that the cold alone might spare. Gardeners in the mildest spots sometimes get away with heavy mulch and good drainage. Anyone with prized varieties is better off lifting them.
Do I dig up the plants before or after frost?
After. Wait for a frost that blackens the foliage, then lift within about a week or two. At Louisville the first 32°F freeze normally comes between November 2 and November 12 (NOAA NCEI 1991-2020 Climate Normals). Gladiolus are the exception, since their leaves often yellow and brown before frost, and the corms can come out once the foliage has turned.
What should I do if a hard freeze comes before I get to them?
Cut the blackened stalks and lift as soon as the soil is workable and not soggy. Frost that only kills the top does not harm what is underground, but a deep freeze that reaches the tubers does, and thawed tubers feel soft and watery and will not store. If they still feel firm, cure and pack them as usual and check them often in the first few weeks.
Which bulbs can stay in the ground all winter?
Hardy spring bulbs such as tulips and daffodils belong in the ground and need the winter chill. For the full picture of what survives Kentucky winters, see our Spring Bulbs section, and browse the Kentucky guides for the rest of the seasonal calendar. Tender types like dahlias, cannas and common garden gladiolus are the ones to lift, and a few extra minutes in November decide whether they bloom the following summer.