Bill Mollison - Source: permacultureprinciples.com
"Ascertain the general "hardiness" zone for plants and animals. This is based on temperature, with frost being the limiting factor. Make a survey of the plants that grow in the area, noting special circumstances surrounding plants that are "marginal“; what is the technique or microclimate that allows them to grow?"
~Bill Mollison
Permaculture: A Designers Manual - 1989
Page 136
