That is strange. I guess the saying is true one man's poison is a another man's.....

I am not in your zone so they are not hardy here. Given your weather conditions they probably multiply pretty fast but they are a bulb. I would imagine that alone would make them easy to pull out if it grew wider than you wanted like any other plant that would need to be divided when it is bigger than you wanted.

It does not have underground roots that will jump and become a thug.

what marquest said.

& besides, extra cannas, being so easy to grow & giving such a good return in flowers, are always welcomed by the Master Gardeners & other groups for municipal gardens, school gardens, Habitat for Humanity, etc.

Certain cannas are more aggressive than others. 'Tropicanna' aka 'Phaison' is notorious for aggressive growth, for instance.

In frost-free zones (I'm in the SF Bay Area) I have had to fight to keep some varieties from overrunning their neighbors. Pulling leaves does nothing; the rhizomes spread wherever they can. I shovel prune some of the clumps at least once or twice a year.

Here is a link that might be useful: 'Out of control cannas' discussion