• introductory text: http://en.wikipedia.org/wiki/Companion_planting
• 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/

• introduction on gardening with plant guilds: http://members.optusnet.com.au/~rnbnielsen/hort%20course%20nscf/Course%20Resources/Guild%20theory.htm
• on working with plant guilds in permaculture: http://www.self-willed-land.org.uk/permaculture/plant_guilds.htm
• plant & funghi in food forests: http://www.permaearth.org/foodforestarticle.html
• Permaculture Guilds: A Primer: http://onestraw.blogspot.com/2007/03/permaculture-guilds-primer.html

A full permaculture guild consists of 7 layers (from Permaculture Guilds: A Primer):

• canopy layer (only for larger gardens, not on roofs, balconies, etc.)
• low tree layer
• shrub layer
• herbaceous layer
• rhizosphere
• soil surface (ground cover)
• vertical layer (climbers and vines)

Each guild needs 5 things:

• nitrogen
• nutrients (non mineral, mineral, soil)
• mulch
• pollination
• protection from external threats