h₃erbʰ-eh₂-
“herb, green plant”herb, herbal, herbivore
Extended form of *h₃erbʰ- giving Latin herba, English herb, herbal, herbivore.
Discussion
The PIE form *h₃erbʰ-eh₂- (herb, green growing plant) derives from the root *h₃erbʰ- (to change status, to pass from one state to another), with a nominal suffix. The connection between "changing" and "green plant" may pass through the concept of growth as transformation — the plant as something that changes, that emerges from seed to sprout to maturity.
Latin herba (grass, herb, green plant) is the direct continuation and the source of the English word family: herb (a plant valued for flavour, medicine, or fragrance — pronounced with or without the initial h, depending on dialect), herbal, herbalist, herbarium, herbivore (an animal that eats herbs/plants), and herbicide (a plant killer). The Latin word covered a broader range than modern English herb: herba meant any green plant, including grass, not just the aromatic kitchen plants that English speakers typically imagine.
The Germanic branch preserves the root in a different form. Old English eorþe (earth) and the related forms are sometimes connected to *h₃erbʰ- through the concept of the growing surface, though this etymology is debated — most scholars derive earth from *h₁er- (earth, ground) instead. More securely, the Greek érephos (a covering, a roof — perhaps originally thatch, woven green plant material) may continue the root.
The English word herb lost its initial /h/ in medieval French (Old French erbe) and was borrowed into English in that h-less form. The spelling was later re-Latinised to herb, but the pronunciation without /h/ persisted in many dialects — this is why American English says "erb" while British English says "herb." The discrepancy is not a matter of correctness but of which stage of the borrowing chain each dialect froze at.
The medical and culinary significance of herbs in ancient cultures ensured the word's survival and productivity. Herbal medicine predates written records, and the herbal (a book cataloguing plants and their uses) was one of the most important genres of medieval and early modern literature. The PIE root that named green growing things ultimately named an entire tradition of plant knowledge.