Earlier this year I worked at a retail plant nursery where I was able to get lots of free plants off the dump rack. On one occasion I lucked into a number of ornamental canna plants. They were being "dumped" because they carried a virus which caused visible streaking on the leaves (not to be confused with the natural desirable streaking of some cultivars). Viruses are very common in cannas. I figured I would take them and just grow them for a season to get experience growing cannas as I had not grown them before. I repotted them in larger pots and grew them on for the season. Now that the winter is upon us they are going dormant. I dumped out the first of the five potted plants and cleaned the tubers. It took a few minutes to remove all the roots which held a lot of potting soil, but once they were all off not too much soil was left to clean off the actual tubers. I have read very good things about the edibility of canna tubers both for varieties cultivated specifically for larger tubers as a crop and also for the tubers of the ornamental types. After slow cooking the tubers in a crock pot with a little water for many hours I tried a tuber. I can confirm that it is a great source of starch, and also a little fibrous as I had read they would be. However, they lacked the soft texture and sweetness that I had been looking forward to after reading their description in my Perennial Vegetables book by Eric Toenmeier. It may be that I need to store them for a time after harvest to allow the starches to partially convert to sugars. I imagine that would affect both flavor and texture. Based on my experience cooking them freshly harvested I would say that they do have good food value, and they did not have any off putting flavors. They were not particularly delicious, but I will not count this against them because this is my first try at preparing them. I imagine if I had never eaten potatoes in my life, and had no real idea how to prepare them I might not think very highly of them after giving them a poorly prepared taste test. Also, this was not a cultivar specifically selected for food production. On a side note: After I acquired the virus ridden ornamental cannas I lucked out into finding another canna for sale in the water garden section of the nursery (later in the season when I was no longer working there). The canna I found was the less ornamental plain Jane green leaved type, but it was labeled as "Achira" which is the term used for cannas grown specifically for their starchy tubers in some parts of the world. I bought one, and potted it up as well. It has been doing very well for me, and I have been keeping it in my greenhouse at my parents' home to avoid the spread of virus to it from my ornamental cannas. I will not try harvesting any tubers from the Achira this year as I want it to spread more first giving me a better harvest as well as tubers to grow on for future years. The less ornamental Achira is still a lovely plant so I enjoy having it even if it doesn't have the same eye catching effect of some of the ornamental cultivars. I have four more ornamental cannas in pots, and over the coming months I plan to un-earth them one at a time and try preparing them for food. I hope I can start getting enough of a feel for them to be able to start making them into something that is enjoyable to eat in addition to being a good source of carbohydrates. Unlike some people who are always trying to limit their calories I am usually trying to figure out what I can grow that will actually provide maximum calorie yields. I feel that if it came down to it we could easily starve if we had to rely on some of the "edible" foods that we can come by locally. All those leafy weeds that we can eat may be nutritious, but they're not usually too high in calories.
Note: Canna "tubers" are actually rhizomes, but "tuber" sounds better than "rhizome". Hence my improper use of the word.