One of the plants with the highest levels of resveratrol (and trans-resveratrol) is Japanese knotweed (called Polygoni cuspidate or its synonym Fallopia japonica).
What is also interesting is that this plant also includes a smaller level of Emodin and Chloroquine, which also have research papers showing that these have effects against the Coronavirus.
Japanese knotweed is found very commonly in the UK where it is considered an invasive weed. However, if you are harvesting it you must ensure that you cause no spread of the plant, as this is illegal within the UK. We recommend that no root material is taken, just the tops of the shoots.
Great care is needed to find untreated plants because this plant may have been treated by weed killer containing glyphosate. The leaves show signs of burning and the stems shrivel. It is advisable to contact your local Council to find out when they spray.