I thought resveratrol came from red wine or grape skins

Isn't Resveratrol made from grapes??

Japanese knotweed, an herb which you may see listed as Polygonum cuspidatum or Fallopia japonica on your supplement label, is a common source of resveratrol. In fact, every resveratrol product we tested in our review contained Japanese knotweed, although some contained additional sources. This may be because red wine extract, red grape skin extract and other sources of resveratrol tend to be more expensive and contain smaller amounts of resveratrol than Japanese knotweed.