https://www.fantasyflightgames.com/en/news/2017/5/30/force-of-nature/
I like the part where they say they can take trumpets... sure FFG, sure
I think this is a more precise explanation on the Overgrow mechanic though. Basically you add one overgrowth token to a piece of terrain for each Overgrow your army has during setup. The overgrowth does nothing on its own but it does fuel other abilities.
Edited by Willange