Hydroxylates germacrene A acid to 6-alpha-hydroxy-germacrne A acid, a precursor of sesquiterpene lactones that spontaneously undergoes a lactonization which yields costunolide. Costunolide can then spontaneously conjugate to glutathione or cysteine. germacra-1(10),4,11(13)-trien-12-oate + O2 + reduced [NADPH--hemoprotein reductase] = (+)-costunolide + 2 H2O + oxidized [NADPH--hemoprotein reductase] Belongs to the cytochrome P450 family.