The Collaborative International Dictionary of English v.0.48:

Synanthous \Syn*an"thous\ (s[i^]n*[a^]n"th[u^]s), a. [Pref. syn-
   + Gr. 'a`nqos flower.] (Bot.)
   Having flowers and leaves which appear at the same time; --
   said of certain plants.
   [1913 Webster]