Wait for the first frost that blackens the leaves, then lift within a week or two. In Burlington, NOAA’s 1991-2020 Climate Normals put the first 32°F freeze between October 3 and October 13, so the digging window opens just after that frost there and earlier in the Green Mountains and the Northeast Kingdom. Dahlia tubers, canna rhizomes and gladiolus corms all follow the same signal. Dig too early and they rot in storage. Wait too long and a hard freeze can reach them.
Do you have to dig them up in Vermont?
Yes. Vermont falls in USDA zones 3b to 5b on the USDA Plant Hardiness Zone Map 2023, and no part of that range has winters mild enough to leave these three in the ground. Plan to lift dahlias, cannas and gladiolus every fall, in every part of Vermont. The “stay in the ground under mulch” option belongs to places where the soil never freezes deeply, and that line runs well south of the state.
The zone map is built on a single variable: the average annual extreme minimum temperature. It says nothing about wet soil or snow cover, but it does tell you how brutal the coldest nights get. NOAA’s 1991-2020 Climate Normals for Burlington show a mean daily minimum of 12.9°F in the coldest month. That is Burlington, on the shore of Lake Champlain and the mildest corner of the state. Higher ground and the northern counties run colder.
Why mulch does not rescue them
Dahlias come from the highlands of Mexico, cannas from the tropics and subtropics of the Americas, and gladiolus (the common garden kinds) from southern Africa and the Mediterranean. None of them evolved to sit in frozen ground. When a tuber’s water-filled cells freeze and thaw, the tissue collapses. In spring you dig down and find a gray, mushy lump where a plump tuber used to be.
A thick blanket of straw or shredded leaves slows the freezing. It can’t hold the soil above freezing through a Vermont winter. Mulch also holds moisture against a dormant tuber, which is a recipe for rot on top of cold damage.
The one-mild-winter trap
Every year someone reports a dahlia that came back after being forgotten. That tells you about one winter, usually one with deep early snow acting as insulation. It doesn’t tell you the next winter will behave. A gardener who has spent two seasons growing a prized dahlia is gambling more than a few dollars’ worth of tubers on a lucky snowfall.
Hardy bulbs play by different rules. Daffodils and tulips want the cold and stay put, and Spring Bulbs covers that world. If you’re unsure where a plant falls, Which Spring Bulbs Survive Your Winter explains how hardiness works for bulbs that do overwinter outdoors.
When to lift them
Let the plant tell you. The signal is a frost hard enough to blacken the foliage: leaves go limp, then dark and slimy-looking. That frost tells the plant to stop sending energy into its underground parts, and the tubers, rhizomes and corms finish ripening. Lift a week or two after the first frost that blackens the foliage.
The window, from Burlington outward
For Burlington, NOAA’s 1991-2020 Climate Normals give a first fall freeze (32°F, in the 30 to 70 percent probability band) between October 3 and October 13. So the lifting window there opens a week or two after that frost, which puts most of the digging in the second half of October, depending on the year. Burlington is the mildest corner of the state, though. In the Green Mountains and the Northeast Kingdom the first frost arrives earlier, and so does the lifting. A garden at elevation may be digging while Burlington’s dahlias are still in bloom.
Use the calendar to prepare and the weather to decide. A light frost that only nips the tips of the leaves doesn’t count. If it’s followed by a warm stretch, the plants may keep growing, and you wait for the frost that finishes them. For a closer look at how your own town compares, see First and Last Frost Dates in Vermont.
Why lifting early backfires
An impatient gardener sees a dahlia full of buds in late September and worries about the weather. Digging then gives you tubers that haven’t finished maturing. Their skins are thin, they’re full of water, and the crown, where next year’s growth buds form, isn’t fully developed. Stored in that state, they rot or shrivel before spring. Canna rhizomes and gladiolus corms behave the same way: green foliage means the plant is still feeding what’s underground.
The lifting window has an upper end, too. Waiting beyond a week or two risks a hard freeze that reaches the soil and damages the tubers. A frost that only blackens the leaves is fine, because the ground still holds heat. A sustained deep freeze is not.
Pick a dry day so soil falls away easily. Cut the stalks back to short stubs, work a fork in well away from the stem to avoid spearing tubers, and lever the clump up rather than yanking it by the stalk. Fall is also the time to plant hardy bulbs, so schedule the lifting before you start on Planting Spring Bulbs in Vermont.
Storing them over winter
The goal is simple: stay cool, dark and just slightly moist, so nothing rots and nothing shrivels. Rot comes from too much moisture and too little air. Shriveling comes from drying out. Every step from digging to packing is about steering between the two.
Cure first, clean lightly
Once lifted, let the clumps dry somewhere sheltered and airy, out of direct sun and away from freezing, until the skins feel dry to the touch. Brush off loose soil. Don’t scrub or wash them, because broken skin invites rot and a wet tuber is hard to dry back. Remove any obviously mushy pieces, and label as you go. A tuber with no label becomes a mystery in April.
How the three differ
Dahlias, cannas and gladiolus all need the cool, dark storage, but they don’t want identical conditions.
| Plant | What you store | Curing | Packing | Moisture |
|---|---|---|---|---|
| Dahlias | Tuber clumps | Dry until skins feel dry, stems drained | Ventilated box with barely damp peat moss, vermiculite or wood shavings | Slightly moist; they shrivel quickly if dry |
| Cannas | Rhizomes | Short air-dry, soil left on | Box or bag with lightly damp packing material | Slightly moist; they shrivel if dry and rot if wet |
| Gladiolus | Corms | Longer, warm and airy | Mesh bag or open tray, no packing needed | Dry; dampness invites rot and mold |
Gladiolus is the odd one out. Each corm forms a new one on top of the old, which withers. Once cured, snap off the old shriveled corm and the tiny cormlets (those can be saved or discarded), and keep the plump new corm dry.
Keep all three somewhere cold that never freezes. Burlington’s coldest-month minimum of 12.9°F, per NOAA, is a good reminder of what an unheated shed or garage can do on a bad night, and a basement corner or insulated space away from outside walls works better. A single freezing night can ruin a whole box.
Check the boxes once a month through winter. Pick a date on the calendar and stick to it. Remove any tuber that is soft or moldy before it spreads, and if the others are shriveling, mist the packing lightly. Five minutes a month is the step that saves most of them.
What goes wrong
Rot and shriveling
Rot shows up as soft, dark, sometimes foul-smelling spots, often with white or gray mold. It usually means the packing was too wet, the box had no air, or the tubers went in before curing. Pull out the affected pieces, let the rest dry, and repack in something drier. Shriveling is the opposite: wrinkled, leathery, lightweight tubers that look like raisins. It means the storage was too dry or too warm. A light misting of the packing, or a cooler spot, usually fixes it if you catch it in time.
Timing and zone mistakes
Lifting too early looks like rot and shriveling that no amount of adjustment cures, because the tubers were never ready. Wait for the blackened foliage, then lift within a week or two of that frost. At the other end, tubers left in the ground are the classic mistake for gardeners who moved from somewhere milder. A neighbor’s survivor or advice from a warmer state doesn’t change the zone you live in. In Vermont, with zones 3b to 5b on the USDA Plant Hardiness Zone Map 2023, leaving them out is a bet that rarely pays, and the farther north and higher up you garden, the worse the odds get.
Common questions
Can I wait until after a hard frost to dig?
Yes, and it’s the normal approach. The first frost that blackens the foliage starts the clock, and a week or two later is the target. The risk is waiting through several hard freezes in a row, which can penetrate the soil and damage tubers. If a deep freeze is forecast right after the first frost, dig sooner rather than later.
Can gladiolus stay in the ground in Vermont?
Not reliably. Gladiolus corms are tender, and the USDA Plant Hardiness Zone Map 2023 puts Vermont in zones 3b to 5b, with Burlington’s coldest-month minimum at 12.9°F per NOAA. Lift them, cure them dry, and store them in a mesh bag. Check the Vermont guides for other fall garden tasks that follow the same logic.
Does Burlington’s milder climate mean I can skip lifting?
No. Burlington is the mildest corner of the state because of Lake Champlain, but its coldest month still averages a daily minimum of 12.9°F. That is far too cold for tender tubers in unprotected soil. The lake gives you a slightly later first frost, between October 3 and October 13 by NOAA’s 1991-2020 normals, not a different winter.
When do I plant them back out?
After the last spring frost has passed, which comes later in colder, higher parts of the state. Dahlias and cannas can be started indoors in pots a few weeks earlier if you want a head start. Gladiolus corms go straight in the ground once the soil is workable and frost danger is over. A tuber that has sprouted in storage is not a problem, just a sign it’s ready.