Hardiness Zones for Gardeners and Landscapers
A hardiness zone is a geographic area defined to encompass a certain range of climatic conditions relevant to plant growth and survival for farmers and gardeners.
The original and most widely-used planting zone system, developed by the United States Department of Agriculture (USDA) as a rough guide for landscaping and gardening, defines 13 zones by annual extreme minimum temperature. It has been adapted by and to other countries (such as Canada) in various forms to aid in planting and growing.
Unless otherwise specified, “hardiness zone” or simply “zone” usually refers to the USDA scale. For example, a plant may be described as “hardy to zone 10”: this means that the plant can withstand a minimum temperature of −1 °C (30.2 °F) to 3.9 °C (39.0 °F).
Other hardiness rating schemes have been developed as well, such as the UK Royal Horticultural Society and US Sunset Western Garden Book systems.
For those that constantly forget their zone, here is the best USDA Zone map I have found for answering that question “What planting zone am I in?”