The aromatic oils in plants contain the same sorts of chemicals like terpenes as in cannabis, and many are known to be pharmacologically active, including mental effects. Myrcene, for instance, is abundant in most cannabis and also found in verbena, lemon grass, hops, mango, West Indian bay tree, and cardamom.
Ever had black coffee with cardamom? It's very tasty, imo, and it has a strange calming effect on me, despite having a bunch of caffeine. Almost like the world just quieted, allowing me to hear everything more clearly. I've never known why, but maybe this is a reason.