The following maps show areas of winter hardiness for plants. Zones are based on a formula that takes into consideration several factors affecting the hardiness of a plant in a given location. The most important element in plant survival is minimum temperature during the winter. Other important considerations are the length of the frost-free period, summer rainfall, maximum temperatures, snow cover and wind.
The hardiness areas have been divided into zones with zero being the coldest and then progressively milder. Sometimes even though older plants are hardy, young plants may be tender and need protection the first winter.