Hi Nick,

Canna
hardiness in our cold zones has been discussed in numerous threads on this
site; the most recent was in early January of this year.

Relative to
your particular location and assuming the planting location has no special
micro environment that prevents excessive freezing we can assume that any canna
growth would be the result of canna seed survival and germination.

Canna seed,
much like many of our annual flowering plants that have produced viable seed in
a previous growing season typically survives harsh cold weather seasons and
will germinate when conditions are suitable. Often, that may be a few years
later.

Just for the
record; your observation is quite common but not commonly recognized.

If you care
to investigate your own special situation, simply lift the growing plant and look
for a source of initial growth; rhizome remains or nothing (seed).

Kent