I looked at what climate those other tubers beside potato grow and I see theoretically they should grow up to a hardiness zone of 8. I live in a Mediterenean climate and theoretically it should work. If olive trees grow.
The sweet potato is succesfully grown in California and southern US. Then there is the taro and yam. I wonder is there any problem that you would encounter, I read that for the cassava you have to get 8 warm months in a row at least, there could be a problem there.
Be content with sweet potatoes in Zone 8. Everything else is outside your climatic reach. Even with sweet potatoes, you want to grow the more northern varieties, like Korean Giant. In another post 3-4 days ago I listed a dozen tubers/roots that will grow in your area. No need to put up a heated greenhouse to make foods nutritionally similar to what grows spontaneously there.
I just found out something about cassava that makes me very sad, its oxalate content is enormous, 1260mg per 100g, beating even spinach, at it.
So that soaking in the river that Africans do is not just because of the cyanide.
In Los Angeles I have grown it all - growing things is my business. Cassava is a common name for Manihot esculenta, and it grows well in zones above 8, especially when there is humidity. That said, a sheltered microclimate in zone 8 would work - nothing wrong with trying to push a zone or two. It is a beautiful plant, and one of my favorite starches, Try it - get an organic yuca root from a latino market and put it in water for a few days - you'll have roots and leaves and the whole thing can be planted out within a month. Start your own yuca farm! YUM!!!!