Permaculture Principles - A great site explaining a lot about what permaculture is and the natural principles permaculture projects are guided by.
Permaculture Association - UK based organisation. Their website includes lots of information and advice about getting going on your own permaculture project and provides lots of opportunities for networking or finding out about permaculture enthusiasts.
Permaculture Videos - The most reliable source I've found to date is just to search for Permaculture videos on You Tube. There's lots of great inspiration on there with projects from a very good range of different climate zones around the world.
Plants for a Future - A very comprehensive, searchable database of useful plants, including over 7000 varieties, listed along with details of edible, medicinal and 'other' uses. Also includes details about propagation and cultivation.
Agroforestry Research Trust - UK based charity that researches agroforestry/forest gardening, sells good quality plants for that purpose and hosts the UK Forest Gardening Network.
Books
Below are some of the books that we've found particularly useful or just really interesting as we work through our permaculture project.
Plants for the Future, Ken Fern
A fantastic guide to useful plants. The book is arranged by growth type - trees, ground cover, climbing plants and so on.
Great practical guide to making forest gardens. Includes lots of useful information about preparing your site and maintaining it once it's ready. Also lots of useful and very interesting info about micro climates.
Very inspiring book about a Japanese chap who decides to reinvent his farming method to align more closely with nature.
Another very inspiring book about an Austrian farmer who takes organic farming to another level, producing a highly productive permaculture farm in the Austrian Alps. Particularly impressive is his micro climate manipulation, so much so that he is growing lemon trees in a usually cold, mountainous area.
A lovely book, full of great colour photos, all about making your vegetable patch beautiful. Lots of great ideas and inspiration here if this is the kind of garden you're after.
