The Collaborative International Dictionary of English v.0.48:

collard greens \col"lard greens\ (k[o^]l"l[~e]rd z), n. pl.
   [Corrupted fr. colewort.]
   1. Same as collards[1]. [Colloq. Southern U. S.]

   Syn: collards.
        [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

collard greens
    n 1: kale that has smooth leaves [syn: collards, collard
         greens]