I routinely overwinter canna tubers in large pots/tubs, covered with peat/soil mix in the unheated garage, which during the coldest weather might get down into the upper 20s.

If you use pots or bags containing a similar medium to insulate them, I'd expect them to handle temps around freezing handily.

I tried the peat moss covering of cannas in storage in my basement last year. When spring came about 60% of all my cannas had shriveled. Must have been too dry or the peat moss sucked all the moisture from them. So I don't intend to do the peat covering routine again,

Let’s be perfectly clear; if cannas are exposed to below
freezing temperatures they will be damaged. The severity of damage is simply a
product of the time duration the rhizome is subjected. Cell walls contain a
high concentration of water. When water freezes the cell wall expands and
ruptures. The most exposed areas obviously freeze first and the damage
progresses as time passes. The colder it is the more rapid and severe the
damage. Protect from freezing is the safest solution.

Freezing is not the only obstacle for winter storage but it
is the primary concern; dehydration and fungal infection can be an issue.
Unfortunately there are some cultivars (varieties) that simply do not winter
store because of poor or inadequate hybridizing selection.

Just for the record, most experienced gardeners would recommend
using moist peat moss as a storage medium to prevent dehydration if applicable.

Kent

Glad you could chime in Kent! I'd like to ask you a few questions based on your knowledge of cannas.

Yes, I thought about wetting the peat moss before I used it, I then got worried about promoting fungal growth because of that moisture being there constantly. I do sprinkle sulphur on them before storage to help with fungal resistance.

From what you said, freezing point of water, 32 F, is the bad zone. That implies Cannas do not know how to be hardy at all, as in, unlike plants adapted to cold weather, they can't thicken the fluid within them such that they only freeze at really low temps. This would make sense considering they are tropical plants.

1) Does that mean that cannas that survive in the ground do not really undergo freezing at all or not for long anyway, long enough to kill? That just seems hard for me to imagine. Over here we pretty much had a zone 7 winter last year, all cannas people lazily left in the ground made it and were in bloom by May! Yet there were many, many nights of temps in the 10 F to 0 F degree range, I even saw -5 F once but not sure how long it persisted. Hard to imagine that with those temps the top 6 inches or so of the ground didn't freeze down into the 20s.

2) I've read plant tags that state cannas as hardy to 10 F. What do you say to that? Is that air temperature maybe?

Thanks Kent, appreciate all that info.

One more question: my canna clumps that were dug out are all full of dirt - this is clay heaven where I garden. Any real pros/cons to removing the dirt for storage? Or is it just a space saving and aesthetic thing?

I am not sure I would consider dealing with cannas
at or after harvest for an “aesthetic” reason. I am reasonably sure that every
grower has to deal with this problem in a manner that best accommodates their
special needs and limitations.

With the above in mind, my first choice would be to
leave some of the soil attached and place the clumps in an old fashion underground
frost proof root cellar. This allows me to quickly dig and store the roots.
Little muss, fuss or expense for harvest or storage and roots can be divided in
the spring for planting: dig-store-forget until spring. Unfortunately not many
of us have that option, but it was done this way for many years in the past
quite successfully.

For those whose winters are milder, I would remove
the top growth and mulch heavily to protect from freezing. In the spring I
would lift, divide and replant.

The bottom line has less to do with which is best
and more to do with what works best under one’s particular circumstance. That
decision may require a few trials and errors.

For those with heavy clay soils or low organic soils
consider amending your soil with suitable organic supplements.

Kent

I planted a canna in a pot after buying it at Walmart.

& forgot to take it out side, it is still growing in the pot that is about 7" round pot.