This quest can be received from Hakunin in Arroyo. He can be found by asking the tribespeople where to find him, but his is the furthest tent back. His garden is being ruined by spore plants, and he wants them cleared out. He asks the Chosen One for help. The garden he speaks of is the closest one to the back of his tent.
The spore plants do little damage with their ranged attacks, so either defeat them at a distance or hit them up close and move back at least three hexes in the same turn. Also, each plant has only a finite supply of spores to shoot at range, so it may be worth it to stand at or near max range, and just let the plant exhaust its supply of spores, before moving in for the kill.