Dig them up. In New Brunswick, dahlia tubers, canna rhizomes and gladiolus corms do not reliably survive winter in the ground. The signal to lift is the first frost that blackens the foliage, followed by a wait of one to two weeks. Moncton’s average first fall frost is October 2, which puts an average-year lifting window at roughly October 9 to 16. A late frost pushes that later, and colder districts run earlier.
Do you have to dig them up in New Brunswick?
Yes, and the numbers leave little room for argument. Natural Resources Canada’s plant hardiness site places Moncton in zone 5a, and that is the Canadian system, not a USDA zone. The two systems do not convert. The Canadian one blends seven climate variables, the USDA one uses a single winter-low figure, and NRCan itself says there are no simple conversions between them. Whatever zone label you carry in your head from a catalog, lift all three every fall once frost has blackened the tops.
What the cold does underground
Dahlia tubers, canna rhizomes and gladiolus corms are fat storage organs, packed with water and starch. When the soil around them freezes, that water turns to ice and ruptures the cells. The tissue thaws in spring as a gray, mushy mess. Mulch slows the cooling, but it cannot rescue a plant in a place where the ground freezes deeply and stays frozen for months.
Environment and Climate Change Canada’s 1981-2010 normals for the Moncton station show a mean daily minimum of -13.9 °C (6.9 °F) in the coldest month. That figure is an average of nightly lows, so real cold snaps run well below it. A nightly average that far under freezing, sustained through a whole month, is the kind of winter where frost reaches well past the top layer of soil.
Where the line falls
Staying in the ground under mulch works where winters barely freeze the soil, which describes milder regions far from here. No such line shows up in New Brunswick’s data. Moncton’s figures are the reference, and the province is not a single climate: northern and upland districts run weeks behind Moncton in spring and ahead of it in fall. That makes the verdict firmer there, not looser. Nothing in these figures supports leaving tender tubers out anywhere in the province.
A few small, truly hardy gladiolus species exist, but the large-flowered garden types sold each spring are tender. The same goes for cannas, which are tropical in origin. These plants behave very differently from the tulips and daffodils in the Spring Bulbs category, which actually want a cold winter. If you are unsure which of your plantings belong to which camp, Which Spring Bulbs Survive Your Winter sorts them out.
When to lift them
The cue is a frost that turns dahlia, canna and gladiolus leaves black and limp. Lift one to two weeks after that frost. In an average Moncton year, with the first fall frost around October 2, the window works out to roughly October 9 to 16. Treat those dates as the middle of a range, not a deadline, because the real trigger is the frost, not the calendar.
Why the frost date is a midpoint
Environment and Climate Change Canada gives October 2 as the mean first fall frost for the Moncton station (normals for 1981-2010). A mean date means that in one year out of two the frost comes later. Some autumns, the first blackening frost arrives well into October, and your lifting window slides with it. Other years, it comes earlier, and you are digging while the maples are still turning.
Distance from Moncton matters too. Northern and upland gardens get their first frost earlier, so the lifting window moves earlier with it. A garden on a hilltop can be lifting while a garden in a sheltered valley has not yet seen a frost. For a closer read on your own area, First and Last Frost Dates in New Brunswick lays out the local timing.
Why the wait matters
The frost tells the plant to stop growing and shift everything underground. During the week or two after, the tuber skins thicken, the eyes (the growth buds near the stem) firm up, and starch settles in for the winter. Tubers lifted at that point are mature, and mature tubers keep.
Dig too early, while the foliage is still green, and the tubers are still filling. Their skins are thin, their flesh is waterlogged, and the crown is soft. They look fine on lifting day, then turn to rot in storage by midwinter. Dahlia stems are hollow, and cut green stems can also channel water straight into the crown, which makes things worse.
Practical details help. Cut the stems down to a short stub after the frost and lift on a dry day, working the fork well out from the plant so you do not spear the clump. Dahlia necks snap easily, and a broken neck means a tuber with no future. Cannas come up as heavy mats of rhizomes that need real leverage. Gladiolus corms are easier, but let the foliage yellow or blacken before you cut it, because the leaves are still feeding the new corm.
If a hard freeze looks imminent before any frost has blackened the tops, cover the plants overnight with a sheet or lift early. A slightly immature tuber beats a frozen one. And while you are out there with the fork, October is also when the hardy bulbs go in, so Planting Spring Bulbs in New Brunswick is worth a look for that job.
Storing them over winter
The routine has three steps: cure, clean lightly, then store cool, dark and slightly moist so the tubers neither rot nor shrivel. The details differ for each plant, and getting them wrong is the usual reason a stash fails by March.
Curing and cleaning
After lifting, let everything dry for a few days in a sheltered, airy place out of direct sun and well away from frost, such as a garage or porch. Curing lets the cut surfaces seal and the skins toughen. Then brush off loose soil with your hands or a soft brush. Skip scrubbing, because damaged skin is an open door for rot. Dahlias are best left undivided until spring, when the eyes are easier to see. Trim cannas to manageable clumps only if they will not fit your storage container.
How the three differ
| Plant | Storage part | Curing | Storage medium | Main risk |
|---|---|---|---|---|
| Dahlia | Tuber clump | A few days, airy, out of sun | Barely damp peat, vermiculite or wood shavings in a box | Rot at the neck, or shriveling |
| Canna | Rhizome | Short, just to dry the surface | Lightly damp medium, a bit moister than dahlias | Drying out completely |
| Gladiolus | Corm | Longer and drier, until fully dry | Dry, in a breathable mesh bag or open tray | Mold and thrips if kept damp |
Gladiolus is the odd one out. After curing, snap off the old shriveled corm at the base, discard the tiny cormels or save them for growing on, and store the new corms in a breathable bag or open tray, not in damp medium. Dahlias and cannas, by contrast, are fleshy and lose moisture quickly, so a barely damp packing material keeps them plump. Cannas take this furthest, because a dry canna rhizome can simply desiccate over winter.
Whatever the plant, aim for a place that is cool, dark and reliably above freezing, such as an unheated but frost-proof basement or a cold cellar. Too warm and the tubers sprout early or dry out. Too cold and they freeze, which is exactly what you lifted them to avoid. Use shallow boxes with ventilation, not sealed plastic, and label each variety.
The step that saves the most plants is simple: open every box once a month from November through March. Pull out any tuber that is soft, moldy or leaking. Mist very lightly if the rest are wrinkling. A single rotting dahlia can spoil its neighbors, and you catch it early only if you look.
What goes wrong
Four problems account for most failures, and each shows itself differently.
- Rot. Soft, discolored patches, a sour smell, sometimes fuzzy mold. It comes from too much moisture, poor airflow, or tubers that were damaged or immature when stored. Cut away the bad tissue to firm flesh, let the cut surface dry, and move the tuber away from the others. Prevention is a barely damp medium, ventilation, and the monthly check.
- Shriveling. Wrinkled, light, leathery tubers, common in dahlias and cannas. The storage place was too dry or too warm. A very light misting of the medium, or a cooler spot, usually stops it if you catch it early.
- Lifting too early. Thin-skinned, heavy, soft tubers that look fine in October and collapse by midwinter. The fix is patience: wait for the blackening frost, then one to two weeks more, which in an average Moncton year means roughly October 9 to 16.
- Leaving them in the ground. Nothing sprouts in spring, and the digging fork turns up gray mush. The tubers froze.
The last mistake often starts with a catalog. American sellers quote USDA zones, and a plant labeled hardy to a certain USDA zone is not a promise about Canadian zone 5a. NRCan is explicit that the two systems cannot be matched number for number. Do not leave dahlias, cannas or gladiolus in the ground in New Brunswick, even under a heavy mulch, on the strength of a zone number from another system. The New Brunswick guides are a better starting point for timing that fits your climate.
Common questions
Can I store dahlias in pots instead of boxes?
Yes. Lift the clump, let it cure, then set it in a pot or crate of barely damp medium and keep it in a cool, dark, frost-free spot. The monthly check still applies, and the medium should stay only faintly damp.
What if the frost has not come by late October?
Environment and Climate Change Canada’s October 2 is a mean for Moncton, so in half of years the frost lands later. Keep waiting for the blackening, but if a hard freeze is forecast before it happens, cover the plants overnight or cut and lift them. A slightly immature tuber is a better bet than a frozen one.
Do cannas and gladiolus follow the same schedule as dahlias?
The trigger is the same: frost on the foliage, then lifting a week or two later. Cannas then want lightly damp storage, gladiolus want dry storage, and dahlias sit in between with a barely damp medium.
Does the Moncton frost date apply to my garden?
Only roughly. The figure comes from the Moncton station and covers 1981-2010. Northern and upland parts of the province run weeks behind Moncton in spring and ahead in fall, so their lifting window moves earlier. Watch your own garden’s first frost and count from there.