I haven't raised any cannas in my zone 5 garden for quite a few years. Yet, I will still occasionally have one come up in the garden. It has always been too late for them to mature and bloom. However, this year, I have one that may make it.
1. At first, I thought this was due to small pieces of rhizome left deep in the ground below frost line, but that would only explain the first year.
2. Are the seeds very hard and long-lived? Perhaps they have been in the ground waiting for idea conditions to germinate?