I was attempting to overwinter my canna in its container this year, as opposed to digging out the bulbs. I cut it down to soil level and put it in a corner of the basement. To my surprise, when I checked on it, it grew like crazy. It's dark, but it's probably not cool enough, and the container did have a lot of moisture from the fall rains.

What do I do? Do I just keep cutting it down to soil level, until it's time to bring it out in spring?

Is there a window? I would give it a little drink if the soil gets dry & keep ignoring it. When taking outside, I would expect all of that inside growth to burn, so I would cut it off then. New foliage should pop up fast in warm sunshine, plenty of water. If spider mites appear, cutting it off sooner would be good in that case.

There's no window? Why do you ask?

Just curious because there's so much foliage. My Canna storage was always dark and cool when I lived where storing for winter was needed, so it must just be the warmth like you said.

The foliage does look surprisingly green for not having any light.

I guesss the choice is-
-ignore and hope it has enough energy to make it to spring
- find light and then support the growth.

you can certainly put it indoors near a window and let it grow, watering sparingly if it's below 65F, normally if it's above 70F.
i grew Canna indica from seeds and i cut it down to 12" around nov 1 ev year and it resprouts again within a month given light and warmth and keeps going. It's in a 3rd year now and finally it slowed down, but still producing off-shoots. You can then cut it down and put outside when the warmer weather starts. If it's shaded at first it should adjust to light levels and new leaves will be able to take more light.

Alex, sorry I didn't see this post sooner. I'd be inclined to do what skylark suggests and bring the pot upstairs to a sunny window if you have one. All that nice foliage - it would be a waste to cut it all down and you also might not have a whole lot of rhizomes left if you just keep cutting off the foliage until the weather is warm. Healthy foliage makes new rhizomes.

Otherwise, you need to find somewhere cooler to keep that pot.