The Plant Hardiness Zones divide the United States and Canada into 11 areas based on a 10 degree Fahrenheit difference in the average annual minimum temperature. (The United States falls within Zones 2 through 10). For example, the lowest average temperature in Zone 2 is -50 to -40 degrees Fahrenheit, while the minimum average temperature in zone 10 is +30 to +40 degrees Fahrenheit. See more. Suggested hardiness zones have been indicated for all trees and perennials available online from the Foundation. If a range of zones, for example, zones 4-9, is indicated, the tree or perennial is known to be hardy in zones 4, 5, 6, 7, 8, and 9. Suitable hardiness means a plant can be expected to grow in the zone's temperature extremes, as determined by the lowest average annual temperature. Keep in mind that local variations such as moisture, soil, winds, and other conditions might affect the viability of individual plants. Please note that there can be quite a difference between the older "official" USDA hardiness zone map and this newer "unofficial" ArborDay Foundation hardiness zone map. This newer map has often moved zones up about "1", meaning a Zone 5 on the USDA map may now be a Zone 6 on this map. Please take this into account when researching and deciding on the varieties you may want. Often varieties in many places may still be described using the older USDA zones. Grandpa's Orchard will be adjusting some varieties accordingly to take this into account. We do not feel that the climate has changed quite this much yet in all areas of the country!