Catalyzes formation of valerena-4,7(11)-diene, one of the active ingredients responsible for the sedative effect extracted from Valeriana officinalis root. (2E,6E)-farnesyl diphosphate = diphosphate + valerena-4,7(11)-diene Binds 3 Mg(2+) ions per subunit. kcat is 0.013 sec(-1) with farnesyl diphosphate as substrate. Predominantly expressed in root. Belongs to the terpene synthase family.