Milkweeds are one of my favorites, probably because of the comet effect of the flower structure. And their variations on that theme are fascinating. Green milkweed is one of those fascinations. This plant may not be one you've seen. Not because it grows in a bog, or because you have to get down on the ground to see it, but because it blends in. I added Colorado to the known distribution based on the image taken along a freeway close to the northeast corner. This species was used as medicine by Indians, mostly externally. Milkweeds are poison. But Blackfoot used the roots for food. Perhaps the root doesn't contain the noxious chemicals milkweeds develop to discourage grazing and protect pollinators.