The Collaborative International Dictionary of English v.0.48:

Amentaceous \Am`en*ta"ceous\, a. [LL. amentaceus.] (Bot.)
   (a) Resembling, or consisting of, an ament or aments; as, the
       chestnut has an amentaceous inflorescence.
   (b) Bearing aments; having flowers arranged in aments; as,
       amentaceous plants.
       [1913 Webster]

WordNet (r) 3.0 (2006):

amentaceous
    adj 1: (of plants) bearing or characterized by aments or catkins
           [syn: amentiferous, amentaceous]