I have grown Cannas for years, even in the most miserable and coldest climates, but I never knew they were edible. All those years of thinning out the tubers and either giving them away or throwing them away, when I could have had a sort of potatoes substitue. Go Figure, here is a latest post from Dave's Garden Newsletter" You Can Have Your Canna and Eat it, Too? --
You didn't know that? It's common knowledge in Ecuador and Peru, which are part of the Canna's natural habitat - heck, I can buy Achira flour at my local market; it's a staple crop. Some species are tastier than others - particularly, if you're going to eat them, Canna discolor (commonly known as the Achira Cannas) are probably the tastiest, followed by C. edulis. Apart from the rhizomes, the leaves are excellent wrappers for steaming foods (proper Tamales and Quimbolitos, which are steamed breads, are made in Canna leaves), and young seeds can be ground and added to flours for frybreads. Young shoots of C. edulis are a fairly tasty vegetable in stirfries as well.
Actually I had in times past wondered about it, but most of my edibles search dealt with southwestern native plants, as that has always been my main interest. Dump me out in the bush and I could survive there. The link I gave at least had some recipes for cooking the tubers, so that will be helpful.
