The Collaborative International Dictionary of English v.0.48:

Johnson grass \John"son grass`\ [Named after W. Johnson of
   Alabama, who planted it about 1840-1845.] (Bot.)
   A tall perennial grass (Sorghum Halepense), valuable in the
   Southern and Western States for pasture and hay. The
   rootstocks are large and juicy and are eagerly sought by
   swine. Called also Cuba grass, Means grass, Evergreen
   millet, and Arabian millet.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Johnson grass
    n 1: tall perennial grass that spreads by creeping rhizomes and
         is grown for fodder; naturalized in southern United States
         where it is a serious pest on cultivated land [syn:
         Johnson grass, Aleppo grass, means grass, evergreen
         millet, Sorghum halepense]