Artists who work with plants - besides Pedro Ramirez - duh

Mark Dion
Phoebe Washburn
Hans Haacke 
Fritz Haeg
Mathilde Roussel
Edina Tokodi
Robert Cannon
Paula Hayes

just found this link too, i didnt do to shabby with my picks, there is some overlap.

http://beautifuldecay.com/2013/11/15/phoebe-washburn-artists-use-dirt-grass-trees-plants-artistic-medium/