The Collaborative International Dictionary of English v.0.48:

Catch crop \Catch crop\
   Any crop grown between the rows of another crop or
   intermediate between two crops in ordinary rotation in point
   of time. -- Catch"-crop`ping, n.

         Radishes . . . are often grown as a catch crop with
         other vegetables.                        --L. H.
                                                  Bailey.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

catch crop
    n 1: a crop that grows quickly (e.g. lettuce) and can be planted
         between two regular crops grown in successive seasons or
         between two rows of crops in the same season