Ah, see there, sooby, you added another (not obvious to me) layer of good garden thinking! In my zone 7 we have plenty of time for huge cannas.

Update on the mystery canna. Yes, my neighbor's son said that was the flowers that grew in a bed close to his foundation. So, they are canna. He had moved into his house in the early summer, and they were already there. So, apparently the owners put them in the ground before they moved.
Now for the bad news. None of them survived. I put them in a paper bag in a pot and kept them in a garage until it got freezing, then I moved that pot to my basement. Last month I brought them out to inspect and they were dried out, hollow, and lightweight. Clearly dead. Oh well, I would have no place for them here anyway, but at least I tried. Now I want to find out if one's at my neighbor's son's house survived. I doubt it.

The bulbs posted look like what I get from the dark-leafed type of canna. I've grown them in a couple different pots for a couple years. I was blaming the pots on them having small bulbs; while I did keep most of them over winter, they looked that size, and they struggled to to get any size the year after.
Canna indica and some other cannas can make huge rhizomes that store well. As big as they were, the ones I left in ground here in zone 7b and even had leaves over are mostly rotted.