The Enquiry into Plants (Greek: ,Περὶ φυτῶν ἱστορία) by Theophrastus is the earliest major work on botany in the classical tradition. Theophrastus seems to have specialised in applying the methods of the peripatetic school in this area, while his teacher focused on animals.
Loebulus. L079 - Theophrastus -- Enquiry into Plants II: Books 6-9. Treatise on Odours. Concerning Weather Signs. PDF of public domain Loeb edition in Greek and English. Also available via the Internet Archive.
ToposText: Enquiry into Plants, translated by Arthur Fenton Hort (1916). HTML text with linked Google Maps.
History of Philosophy without any gaps: The Next Generation: the Followers of Plato and Aristotle - podcast by philosopher Peter Adamson.
Edward Worth Library: Theophrastus of Eresus.
Simon Fraser University: Theophrastus Project.
Stanford Encyclopedia of Philosphy: Theophrastus, by Katerina Ierodiakonou
The Great Conversation: Further reading at Tom's Learning Notes
Aristotle: On the Soul (De Anima).
Aristotle: History of Animals.
Pliny: Natural History.
Ancient Greek resources: Learn to read Greek classics in the original.