• introductory text: http://en.wikipedia.org/wiki/Companion_planting
• http://en.wikipedia.org/wiki/List_of_companion_plants
• comprehensive article on companion planting: http://attra.ncat.org/attra-pub/complant.html
• companion planting table: http://www.gb0063551.pwp.blueyonder.co.uk/seeog/companion/
• companion planting table: http://eap.mcgill.ca/publications/EAP55.htm
• boek by Gertrude Franck http://www.ivydenegardens.co.uk/Companion%20Planting/companionintrodu.html

Related: plant_guilds