Polygonum Cuspidatum var. Japonicus is the species of plant most commonly referred to as Japanese Knotweed. This plant is an invasive species yet has been used traditionally in Chinese and Japanese medicine for its benefits on gastrointestinal health and circulatory health (among some other claims such as cancer prevention). Composition analysis of Japanese Knotweed reveals it to be a vessel for Resveratrol (as well as a few other compounds structurally similar to resveratrol and may act in the same way) and anthraquinone compounds that possess slight laxative effects; like Senna root but less potent.
Most of the effects of Japanese Knotweed can be traced back to either the stilbenes (resveratrol) or the anthraquinones (emodin) for the circulatory and gastrointestinal help, respectively.
Most research on Japanese Knotweed has been pertaining to either suppressing its invasive tendencies, or controlling it to become a large scale producer of Resveratrol for medical or supplemental usage. Limited trials have been conducted in humans, but it appears to have similar effects to resveratrol due to the resveratrol content.