This hardiness map was developed by the Agricultural Research Service of the U.S. Dept. of Agriculture. The Zones 3 to 10 are based on the average annual minimum temperature for each Zone. In the Zone information given for each plant in the database, the smallest number indicates the northern-most Zone it can be counted on to survive the winter, and the higher number, the most southerly area in which it will perform consistently. Many factors such as altitude, degrees of exposure to wind, proximity to bodies of water, snow cover, soil types and the like can create variations of as much as two Zones in winter hardiness, while cool nights, shade and amount of water received can extend the southern limits. If you are connected to the Internet, find your hardiness zone by entering your zip code. Review the AHS Heat Map.