Ask Mr. Smarty Plants

Polygonum ramosissimum (Bushy knotweed) is native to and invasive in just about all of North America. We could find no evidence of its being toxic. If you follow that plant link you will see that we have very little information on the plant and no pictures. From the University of Wisconsin Plants of Wisconsin, here is a page with a picture and a little more information. From Flora of North America, here is a more scholarly discussion of the plant. Another article from the University of Michigan.

From the Nova Scotia Museum, we found an article The Poison Plant Patch. If you scroll down that page, you will find a list of carcinogens and co-carcinogens. None of them was Bushy Knotweed.

Since we were not having much luck getting information on that species of the Polygonum species, we found a non-native Polyganum avicular, that would not be in our Native Plant Database. It had some more information on medical uses of species of this genus. We thought that might be why you were interested in the toxicity of this one.

We tried once more to determine carcinogenic qualities of the plant. We searched on the genus name, Polygonum, and found this article on Medical Attributes of Polygonum cuspidatum (Japanese knotweed), which may give you some more clues for your research. Otherwise, we found no indication that the North American native Polygonum ramosissimum (Bushy knotweed) had either toxic nor carcinogenic characteristics.