The United States Department of Agriculture (USDA) has devised a system to provide gardeners with a basic guide to plant hardiness. The Hardiness Zone Map system divides the United States and Canada into numbered zones based on lowest average winter temperature. Even within a zone, different micro-climates exist, such as large urban areas which may be warmer or land situated at different elevations.
What Effects Plant Hardiness
The Hardiness Zone Map is a valuable tool but it is not foolproof. There are a variety of factors that can influence the hardiness of a given plant in a garden. These factors include: soil type, drainage, snow cover, exposure to wind, winter, rainfall (or lack of), as well as fluctuations in temperature.