Japanese knotweed is the most concentrated, available, and economical source of resveratrol. It is extracted from this plant that grows wild throughout the world. Giant knotweed is not subjected to herbicides as are grown crops. Giant knotweed provides resveratrol along with emodin, a natural molecule that may induce episodes of diarrhea. To eliminate diarrhea side effects, resveratrol should be an 80-plus percent extract from giant knotweed.