Opsec, we're pretty sure it IS poisonous, and deadly so, raw. Cyanide. A highly volatile form that cooks out in a few minutes (don't cook it all day, and sniff the fumes. That's how fufu (starchy West African dish) makers get chronic low-level cyanide poisoning from it's relation Cassava).
The question is whether it contains any problem chemicals other than cyanide. I'm guessing they are pretty comparable to domesticated Cassava. The plant looks so much like its domesticated cousin it's hard to tell them apart at first glance. I'm pretty sure they're capable of crossing too; I think the genus is a small one of closely-related species.
Right now I still have a lot of options. I don't think this will be a significant food any time soon; the question is more one of opportunism. I don't think anything bad would happen to the hypothetical test animal; I doubt they contain anything their relations don't. It's also a matter of taking prudent precautions to avoid nasty surprises.
Chaya is another Euphorbaceae member with the same form of cyanide in the leaves, also rendered edible by cooking. I've finally found a source for it; I think it is more of a passalong plant in Texas and Florida than one that moves through commercial channels. In fact, that's how I found it; a guy wants to make a little money from his Chaya prunings from his shrubs in his back yard.
Chaya however is not as coldhardy as Manihot grahamii. It will survive light frosts but probably not severe ones. It will need greenhouse treatment over the winter. Point is not so much as a major food source, but to have it to sell to others in suitable climates--including having it for ourselves if we offshore ourselves. AND it's a food source, insofar as surprisingly few plants will provide greens for a family. Several websites said to the effect that 1 or 2 would feed a family, a hedge of them a whole neighborhood. One of the universities that tested it in Puerto Rico said highest value of greens for the space that they could find. So a few plants in the greenhouse for taking cuttings, could also provide some cheap groceries.
Chaya is not as rich in protein as Cassava leaves are, but the protein is better-balanced (Cassava leaves don't have enough sulfur-containing amino acids--eat them with cereals), and the overall nutritional profile is excellent. Much more nutritious than any temperate-climate green, including even healthy Brassica greens.
People who eat them say they taste "like spinach, but milder", and "need more cooking, because the leaves are tougher than spinach". One minute cooking will do it, but they suggest more like 5 to 10 to tenderize them. Otherwise, you can sneak them into anything that is good with spinach. Popular mixed in fritatas, or served steamed with garlic and butter.
Because of the cyanide, all of these species are fairly pest-free. I could plant the Manihot in my deer-infested property and not have to worry about it.
