Cannas can be overwintered in the ground âÂÂmost yearsâ in Maryland, but not without certain considerations. Location of the planting, relative to exposure to full sun and possible protection from the north winter winds would be beneficial. Your soil in the bed should be amended or replaced to provide reasonable drainage and aeration. Obviously a heavy mulch needs to be applied in the late fall over the cut off canna stalks to a depth to prevent freezing. Most of this mulch can be removed in the early spring to facilitate earlier growth. It would likely be advisable to lift and divide your canna clumps each spring to prevent overcrowding if the clumps have shown good growth. This would help to aerate and speed up soil warming.
If you wish to really speed up the soil temperature increase in the early spring consider covering the bed with clear plastic and remove it once the plants begin to emerge.
Regards,
Kent

I know I'm one zone warmer than you, in SE Virginia, so you'll have to take that into consideration. But this is my experience with cannas:

My first year with them I was given 4 from a neighbor. I put them in the ground where they promptly started multiplying. My neighbor told me that she lifted them in the winter and stored them in her unheated but attached garage. I don't have a garage or a basement, so I lifted mine and stashed them in my crawl space. Of course when I did so, I did not get every bit of the thick roots with them.

The next year I pulled the cannas out of the crawl space and they looked... dead. Crossing my fingers, I potted them up and waited. Only to have it confirmed that they were in fact, dead. So I guess the crawl space was not a good way to protect them for the winter.

However, from the bits of roots left in the ground, a dozen more cannas came up and have multiplied quite invasively ever since (and I obviously don't dig them up for the winter anymore). So much so, in fact, that this year I've decided it's time to purge myself of them as they are taking over. They had absolutely no mulch covering them, and the root system seems very shallow imo. And I've got the same clay muck that you do :( It would seem to me that nothing will kill them, including the winter. Even with as cold as it was this winter, I have 100% faith that my cannas are still there alive and kicking.

If I were you, I'd try a few in the ground (with some protection, since you're a zone colder than me) and, if you can, lift a few just in case.

This post was edited by sara82lee on Wed, Mar 19, 14 at 20:53