The Collaborative International Dictionary of English v.0.48:

Iridaceous \Ir`i*da"ceous\, Irideous \I*rid"e*ous\, a. [From NL.
   Iris, Iridis, the Iris.] (Bot.)
   Pertaining to, or resembling, a large natural order of
   endogenous plants (Iridace[ae]), which includes the genera
   Iris, Ixia, Crocus, Gladiolus, and many others.
   [1913 Webster]

WordNet (r) 3.0 (2006):

iridaceous
    adj 1: of or pertaining to or characteristic of plants of the
           family Iridaceae