Alasdair,
 
Some citations which may be of use:
 
  Westmacott, Richard.  "Pattern and Practice in Traditional
    African-American Gardens in Rural Georgia."  LANDSCAPE
    JOURNAL 10, No. 2 (Fall, 1991): 87-104.
 
  Westmacott, Richard.  AFRICAN-AMERICAN GARDENS AND YARDS IN
    THE RURAL SOUTH.  Knoxville, TN:  University of Tennessee
    Press, 1992.
 
Your intern might also want to look at slave narratives and
ex-slave interviews; the Virginia WPA material is unique in
that the interviewers were African-American.  For a start, see
WEEVILS IN THE WHEAT, which was first published in 1976 (there
should be a recent paperback edition available).
 
I know there are accounts of slaves having access to garden
plots, however there seems to have been a fair amount of
gathering of wild greens, persimmons, and so forth.
 
Good luck,
 
Marty Perdue
[log in to unmask]
(architectural historian with an interest in things vernacular
and one time employee of Doug Sanford and Bill Kelso at
Monticello)