Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

Johnson grass
     n : 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: {Aleppa grass}, {means grass}, {evergreen millet},
         {Sorghum halepense}, {Sorghum halapense}]