In Idaho, dahlias, cannas and gladiolus come out of the ground in the week or two after the first frost blackens their leaves. In Boise, NOAA’s 1991-2020 normals put that frost between October 7 and October 21. Higher and colder ground gets it sooner, and the digging calendar moves up with it. Leave the roots in the soil and you are betting on a winter mild enough to spare them. Idaho rarely obliges.
Do you have to dig them up in Idaho?
Yes. The USDA Plant Hardiness Zone Map 2023 places Idaho anywhere from zone 3b to 7b. That rating rests on a single variable, the average annual extreme minimum temperature, so it says little about how long the cold lasts or how deep the ground freezes. For Boise, the mildest reference point we have, NOAA’s 1991-2020 climate normals show the coldest month averaging a daily minimum of 25.4°F. That is well under freezing, and it is an average night, not a record low. Plan to lift dahlias, cannas and gladiolus every fall.
The verdict does not depend on finding your exact zone number. The real test is whether the soil freezes down to where the tubers sit. Boise is a low valley, and most of Idaho is higher and colder, with a shorter growing season. If the valley floor fails the test, the mountain valleys, the high plains and the northern panhandle fail it harder. Gardeners in a sheltered pocket may occasionally get away with it, but no figure supports betting on that.
Why a thick mulch does not change the answer
Mulch slows the soil from cooling. It does not stop it. A deep pile of leaves or straw might delay a freeze by days, but a winter whose coldest month averages nights of 25.4°F, even in the valley, will eventually push frost past it. Dahlia tubers, canna rhizomes and gladiolus corms are all fat storage organs, mostly water. Once that water freezes inside them, the tissue collapses. In spring they come up as mush, or they never come up at all.
The confusion is understandable, because the neighbor’s tulips and daffodils sit out the same winter without any help. Those are hardy bulbs, and they actually need the cold to set flowers. Which bulbs belong in which category is laid out in Which Spring Bulbs Survive Your Winter, and the wider family is covered under Spring Bulbs. Dahlias, cannas and glads are the exceptions that come from warmer climates and never adapted to a hard freeze.
The one caveat
A container plant is a different story from one in the ground, and a pot sitting on an exposed patio can freeze faster than open soil, so it does not count as protected. Whatever the setup, the decision is the same: lift them, and do it at the right time, which is the next question.
When to lift them
Wait for a frost that blackens the foliage. That blackened, limp top growth is the signal that the plant has shut down for the season and that the tubers underneath are finishing their growth. In Boise, the first 32°F freeze of fall typically arrives between October 7 and October 21. That range comes from NOAA’s 1991-2020 Climate Normals for the Boise station, and it marks the band where the odds of a freeze run between 30 and 70 percent. Treat it as the season’s turning point, not a promise.
Lift within a week or two after the frost that blackens the foliage. In Boise that puts the work roughly in the stretch that follows the October 7 to October 21 frost range. Gardeners farther north or at higher elevation should expect the frost, and so the digging, to come earlier than that range. A garden a couple of thousand feet above the valley floor can blacken well before Boise does. Look at the plants, not the calendar. The local dates are laid out in First and Last Frost Dates in Idaho.
Why lifting early backfires
A green plant is still feeding its underground parts. Dig in the middle of that process and the tubers are undersized, thin-skinned and full of water. They look fine on the day, then turn soft in the box a couple of months later. Dahlias make it worse because their stems are hollow, so moisture can travel from a freshly cut stalk straight into the crown. Waiting for the blackening frost, and then a few more days, lets the skin toughen and the tissue firm up. That toughened skin is what carries them through months of storage.
Waiting too long has its own price. The frost that kills the top growth is usually a light one. A hard freeze that follows can reach the tubers themselves, especially in the higher and colder parts of the state. So the window has an edge on both sides: not before the foliage blackens, and not long after it.
How to lift without wrecking them
Cut the stalks down to a short stub, then work a garden fork into the soil well away from the clump, since dahlia tubers spread wider than most people expect. Pry up gently and lift the whole clump. Snapping a tuber off at the neck leaves a root that will not sprout, so patience matters here. Cannas come up as thick knotted rhizome masses and can take a bit of muscle. Gladiolus corms pull more easily, and they are the most forgiving of the three about timing, since the foliage often yellows and collapses on its own.
Storing them over winter
Storage comes down to three steps: cure, clean lightly, then hold them cool, dark and just moist enough. Let the clumps dry in a sheltered spot out of rain and direct sun, with good airflow, until the surface is dry to the touch. Then brush off loose soil rather than scrubbing or hosing them, because water left in crevices invites rot. Curing seals the cut ends and skin, which is what keeps the tubers from rotting on one side and drying out on the other.
The storage spot matters as much as the prep. It should stay cool, stay dark and stay above freezing. An unheated basement or a cellar room usually works. An uninsulated garage can drop below freezing during a cold snap in Idaho, and that will kill everything in the box. Warm, heated rooms are the opposite problem, because they dry tubers out and push early sprouts.
The three plants want slightly different things
Cure and cool storage are common to all three. The moisture is where they part ways.
| Plant | Storage organ | How to pack | Moisture |
|---|---|---|---|
| Dahlias | Tuber clumps | Ventilated box or crate with lightly damp peat, wood shavings or vermiculite around them | Slightly moist; too dry and they shrivel |
| Cannas | Rhizomes | Similar packing, or wrapped loosely in paper with a little damp medium | Slightly moist; they dry out and wrinkle easily |
| Gladiolus | Corms | Mesh bag, paper bag or open tray, stored dry, with old shriveled corm and husks removed | Dry; damp corms mold |
Gladiolus are the odd one out. Corms are drier and denser than tubers, and they keep best in open air with no packing material at all. Dahlias and cannas hold more water in their tissue and need a little dampness around them to stay plump. If you store all three in the same damp bin, the glads mold while the dahlias do fine.
The monthly check that saves them
Check the stored roots once a month. Open the box, pick up a few pieces and squeeze gently. A firm tuber is fine. A soft or slimy one is rotting and needs to come out before it spreads to its neighbors. A wrinkled one is drying out, so mist the packing lightly and close the lid. Glads should feel hard and papery, with no mold. Five minutes a month catches problems that would otherwise wipe out the whole box by March.
Hardy spring bulbs run on the opposite schedule, going into the ground in fall and staying there. That job is covered in Planting Spring Bulbs in Idaho.
What goes wrong
Rot is the most common loss. The tuber feels soft or mushy, may smell sour, and often shows gray or white mold. Causes include lifting before the foliage blackened, packing in material that is too wet, poor airflow and storing anywhere warm. Prevention comes from the front end: wait for the frost, cure properly and keep packing only barely damp. When one piece goes bad, cut it out or discard it right away. Dahlias with a small soft spot can sometimes be saved by trimming back to clean tissue and letting the cut dry before repacking.
Shrivelling looks different. The tuber wrinkles, feels light and hollow, and eventually will not sprout. The cause is storage that is too dry or too warm, and the fix is a light mist on the packing material and a cooler spot. Catching it early is easy, and catching it late is not, which is why the monthly check matters.
Lifting too early is a quieter failure. It produces thin-skinned, undersized tubers that look fine for a few weeks and then rot. Nothing in the box tells you what went wrong, so the lesson usually arrives in spring. Wait for the blackening frost and lift in the week or two after it.
The costliest mistake is leaving them in the ground one zone too far north, which in Idaho means almost anywhere. Frozen tubers look normal in fall and turn to mush by spring, and nothing comes up in June. Boise’s coldest month averages nights of 25.4°F, and most of the state runs colder than the valley. The other fall and winter jobs for the state sit in the Idaho guides.
Common questions
Can dahlias stay in the ground in Boise if I mulch heavily?
Not reliably. NOAA’s 1991-2020 normals show a coldest-month mean daily minimum of 25.4°F at the Boise station, which is well below freezing, and Boise is one of the milder places in the state. Mulch slows the freeze but does not prevent it, and a frozen tuber does not recover.
Do I have to wait for frost before I dig?
Yes, ideally. A frost that blackens the foliage tells you the plant has finished, and lifting within a week or two after it gives the tubers time to mature. If a hard freeze is forecast well before the foliage blackens, it is safer to cut the stalks and lift than to let the ground freeze around them.
Do all three go into storage the same way?
No. Dahlias and cannas want cool, dark storage with a little dampness around them so they neither rot nor shrivel. Gladiolus corms store dry, with air moving around them. All three need curing first and a check once a month.
Do the dates change in colder parts of Idaho?
Yes. The first fall frost comes earlier the farther north or higher you go, so lifting moves earlier too. The October 7 to October 21 range comes from Boise, a low valley, and much of the state is higher and colder. Watch your own foliage: when a frost blackens it, the clock starts.