Mediates formation of germacrene C and germacrene D using farnesyl diphosphate as substrate. Can also catalyze formation of trace of germacrene B. (2E,6E)-farnesyl diphosphate = diphosphate + germacrene C (2E,6E)-farnesyl diphosphate = (-)-germacrene D + diphosphate Binds 3 Mg(2+) ions per subunit. kcat is 0.01 sec(-1) with farnesyl diphosphate as substrate. Predominantly expressed in root. Belongs to the terpene synthase family.