The hardiness zone is based on the Plant Hardiness Zone Map of the USDA.
Average minimum temperature is the main basis for these zones. May other factors beside winter temperature influence the hardiness and growth of plants. Rainfall, soil type, planting site, summer heat, age of plant, cultural conditions; all affect a plant's hardiness and must be considered when determing if this plant will grow in a certain zone. Under favorable conditions a plant listed in one zone may grow in a zone of a smaller number.