I refer to the USDA Hardiness Zone Map to locate the information I need to know before designing a garden. These zones refer to the coldest temperature that area experiences in any given year. A plant, tree, and/or shrub needs to be resilient to these low temperatures to be considered a perennial (a plant that returns year after year) for that area. There are 11 zones that divide up North America. Each zone is 10 degrees difference from zone before. This map was updated in 20