I read in various places that the geometry and planting layout of a
permaculture garden should exploit "edge effects". The explanation
tends to be along the lines that where one planting meets another you
get an edge and at edges good things happen so therefore you should do
this.

What I haven't got yet is the detail of:
- what these good things at edges are
- very importantly why they happen and
- how significant the effects are.

I am not trolling for an argument - I really want to understand. Can
anybody point me to a resource that explains?

David