USDA zones
This is the dominant system used within the USA. It divides the USA and Canada into 11 zones of hardiness based on average lowest winter temperatures, each one having a range of 10°C. Each zone can also be subdivided into ‘a’ and ‘b’ areas to create 5°F intervals. As well as covering the USA and Canada, USDA zone maps also exist for Europe, Africa and China.
These are the USDA maps currently available on the internet:
