The Collaborative International Dictionary of English v.0.48:

bottlegrass \bottlegrass\ n. (Bot.)
   a European foxtail naturalized in North America; it is often
   a troublesome weed.

   Syn: green bristlegrass, green foxtail, rough bristlegrass,
        bottle grass, Setaria viridis.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

bottle-grass
    n 1: European foxtail naturalized in North America; often a
         troublesome weed [syn: green bristlegrass, green
         foxtail, rough bristlegrass, bottle-grass, bottle
         grass, Setaria viridis]