Good chance you would kill out your Canna too. Best thing to do is old fashion weed pulling if you want to save your canna's.

Some of my cannas are planted in an area where we put down several thickness of newspaper with grass clippings on top for weed control.

Thus far the canna have not shown a problem with it. I have begun to take up the paper in some areas that no longer have a weed problem. I put back the grass clippings for mulch to hold moisture since our climate is on the dry side. The canna I have moved to that bed seem to like it since they are multiplying like crazy and bloom soon after transplant.

Peggy