Why is the ninth month called September?
c.1300, herber, "herb garden," from Old French erbier "field, meadow; kitchen garden," from Latin herba "grass, herb" (see herb). Later "a grassy plot" (early 14c., a sense also in Old French), "a shaded nook" (mid-14c.). Probably not from Latin arbor "tree," though perhaps influenced by its spelling.
The change from er- to ar- before consonants in Middle English also reflects a pronunciation shift: cf. farm from ferme, harbor from Old English herebeorg.
arbor ar·bor (är'bər)
n. pl. ar·bo·res (är'bə-rēz')
A treelike anatomical structure.