Use the adjective botanical to describe something that has to do with plants. You might call your illustrated book of New England ferns and mosses a botanical guide.
Botanical research involves the study of plants, and a botanical scientist — or botanist — has focused her career on learning about plants. If there are botanical substances in your favorite soda, it means that in addition to sugar and chemicals, there are plant ingredients too. Botanical comes from botanic, which has its root in the Greek word botanikos, "of herbs."