Ross Heaven has recently written two articles on shamanic herbal medicine which you can access through the links below.
How Plants Really Heal
https://drive.google.com/file/d/0Bye3nrdkFjwcR3hkRDBsaWR2a2c/view?pli=1
How to be a Plant Spirit Shaman
https://drive.google.com/file/d/0Bye3nrdkFjwccHpSVzktRnBZS3c/view?pli=1