Its about as poisonous as any other Alliaceous plant. Haven't you noticed the onion-like odor of the crushed leaves? Most animals will avoid it. It should be Ipheion rather than Tristagma but the genus is long overdue for an update.
Which actually doesn't mean a lot! Onions are edible for most people, but cause stomach problems for some, and are definitely poisonous for cattle. Chives are edible for people, but poisonous for horses. I'd not be surprised if at least some species are also poisonous for people. In many plant families, including this one, there is no predicting toxicity of one species from another, each combination of plant species and feeding species has to be taken individually.
