A hardiness zone is a geographic area defined as having a certain average annual minimum temperature, a factor relevant to the survival of many plants. In some systems other statistics are included in the calculations. The original and most widely used system, developed by the United States Department of Agriculture (USDA) as a rough guide for landscaping and gardening, defines 13 zones by long-term average annual extreme minimum temperatures. It has been adapted by and to other countries in various forms. A plant may be described as "hardy to zone 10": this means that the plant can withstand a minimum temperature of 30 to 40 °F.

Alfred Rehder was a German-American botanical taxonomist and dendrologist who worked at the Arnold Arboretum of Harvard University. He is generally regarded as the foremost dendrologist of his generation.