The primary guide to determine plant hardiness is the USDA Hardiness Zone Map, which divides the United States into ten zones based on average minimum temperatures. Each zone is then subdivided into A and B sections. A plant is said to be hardy if it can tolerate the lowest average winter temperature that occurs in a zone. Hardiness is affected by duration and intensity of sunlight; amount and timing of rainfall; length and severity of summer drought; soil characteristics; slope; frost occurrence; humidity; and cultural practices.
For more information or questions contact your nearest forester for advice.