The Collaborative International Dictionary of English v.0.48:

Cover crop \Cov"er crop\
   A catch crop planted, esp. in orchards. as a protection to
   the soil in winter, as well as for the benefit of the soil
   when plowed under in spring.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

cover crop
    n 1: crop planted to prevent soil erosion and provide green
         manure