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Canna rhizomes are killed by freezing. Where winter ground does not freeze, the clump can stay where it is: cut the canes down after the first frost blackens the foliage, cover the crown with 4 to 6 in (10 to 15 cm) of coarse mulch, and pull the mulch back as growth begins in spring. In those conditions the clump simply gets bigger every year and only needs dividing every third or fourth season.
Where the ground freezes at all, the rhizomes have to come up, and they have to come up before the freeze reaches them rather than after. The first light frost that blackens the leaves does not harm the rhizomes, which are protected by several inches of soil, and it is actually a useful signal. What kills them is the first hard freeze that penetrates to planting depth.
Containers are simpler. A potted canna can be moved whole into a frost free shed or garage, cut down, and left dry in its pot until spring, watering perhaps once mid winter if the compost becomes dust dry. It takes more space than bare rhizomes in a crate but it involves no lifting, no curing and no soil to clean off.
Do not lift while the foliage is still green and growing. The plant is still moving starch down into the rhizome, and a rhizome lifted early is lighter, softer and stores far worse than one lifted after the first frost.
Lifting after the first frost
Wait for the first frost that turns the leaves black and limp, then act within a week or so. Cut the canes down to a stub of 4 to 6 in (10 to 15 cm), which gives you a handle to lift by and makes the clump easy to see. Take all the cut top growth away from the bed rather than leaving it, since leaf roller overwinters in old stems.
Lift with a fork, not a spade. Push the fork in 12 in (30 cm) out from the stubs, all the way round, and lever the whole clump up in one piece. Canna rhizomes spread far wider than the visible base and a spade slices straight through the outer, youngest, best pieces. Cuts and bruises are exactly where storage rot starts.
Shake the loose soil off and let it fall. Do not wash the rhizomes. A thin film of dry soil is harmless and protective, while washing soaks the surface and forces you to dry them thoroughly before packing, which is one more chance for things to go wrong. Only wash if the soil is a heavy wet clay that will not shake off at all, and then dry them well afterwards.
- Cut the canes to a short stub After the first blackening frost, cut every cane to 4 to 6 in (10 to 15 cm) above soil level. Remove all the cut foliage and stems from the garden and dispose of them rather than composting.
- Fork the clump out whole Work a fork in 12 in (30 cm) from the stubs, all the way around, and lift the whole clump intact. Avoid cutting into the rhizome mass, because every wound is a starting point for storage rot.
- Shake off soil and inspect Knock away loose soil by hand. Discard soft, hollow or foul smelling sections and any clump showing the yellow vein streaking of virus. Keep only firm heavy rhizomes with visible growing points.
- Cure in a dry, frost free, airy place Lay the clumps in a single layer out of direct sun at around 60F (15C) for three to seven days, until the cut stubs are dry and corky to the touch and the surface soil is dusty.
- Pack loose in a breathable container Put the cured clumps in crates or open boxes, packed loosely in barely damp wood shavings, coarse peat or dry sand so that no two rhizomes touch. Do not use sealed plastic bags, which trap moisture and guarantee rot.
The storage conditions that decide the outcome
The target is cool, dark, frost free and slightly humid: 45 to 55F (7 to 13C) is ideal. Below 40F (4C) risks cold damage, and above 60F (15C) the rhizomes start into premature growth in the dark and emerge in spring pale, drawn and exhausted. An unheated cellar, an insulated garage or a frost free shed all work. A heated living space is too warm and too dry.
The packing material has one job, which is to hold the humidity steady without wetting the rhizome. Barely damp wood shavings, coarse peat, vermiculite or dry sand are all suitable. The test for the material is that it feels cool but leaves no water on your hand when squeezed. Wetter than that and rot spreads through the crate; bone dry and the rhizomes shrivel by late winter.
Keep the clumps whole rather than dividing them in autumn. A whole clump has less cut surface, stores better and loses less moisture. Division is a spring job, done when you can see which growing points have survived and are swelling.
| Storage factor | Target | What goes wrong outside it |
|---|---|---|
| Temperature | 45 to 55F (7 to 13C) | Below 40F (4C) cold damage, above 60F (15C) premature sprouting |
| Light | Dark | Light with warmth starts weak pale shoots |
| Packing medium | Barely damp shavings, peat or sand | Wet medium rots the crate, dry medium shrivels the rhizome |
| Container | Open crate or ventilated box | Sealed plastic traps condensation and causes rot |
| Curing before packing | 3 to 7 days until stubs are corky | Packing damp seals in moisture at the cut surfaces |
| Inspection | Once a month | A single rotting rhizome spreads through a whole crate unnoticed |
Checking through winter and waking them in spring
Look through the crates once a month. Take out anything that has gone soft, dark or smells sour, because one rotting rhizome will infect its neighbors in a closed box. If several are affected, unpack the whole crate, discard the losses and repack in fresh material. If the rhizomes are wrinkling and lightening in weight, mist the packing material very lightly, never the rhizomes themselves.
Towards the end of winter the growing points begin to swell and show pink. That is the cue to bring the clumps out, divide them into pieces with three to five growing points each, and either pot them up under cover at 65 to 70F (18 to 21C) or hold them until the soil outside reaches 60F (15C). Let cut surfaces from dividing dry for a day before planting.
Expect to lose some every year, and treat the losses as information rather than failure. Consistent softness and rot means the store is too damp or too warm; consistent shriveling means it is too dry or too airy.
Records that make next winter easier
Write down the date of the frost that triggered lifting, the date you actually lifted, and the number of days you cured for. Paired with how well the batch came through, these three numbers converge after two or three seasons on a routine that works in your climate rather than in a generic one.
Record where each clump came from in the garden and how large it was, then compare with its size the following autumn. That comparison is the clearest measure of whether a position suits canna, and it tells you when a clump has grown enough to need splitting.
Finally, keep a simple tally each spring of how many rhizomes came out sound, how many rotted and how many shriveled. The direction of the losses points straight at the correction needed: rot means drier or cooler storage, shriveling means slightly more humid packing.