This is the new hardiness zone map. Listed are the average minimum yearly low temperatures, not the the coldest temperatures ever recorded. A plant not fully dormant can be damaged at much warmer temperatures. Other factors, including chilling requirements and heat units, are critical to plant performance within a climate zone. These are the zones listed for each variety throughout the catalog!
Note of Explanation from Raintree Nursery: Every 15 years, the USDA puts out a new Hardiness Zone Map. In 2005, a new map was due out, however it has not yet been published. The Arbor Day Foundation, a non-profit organization, used the same weather data that the USDA uses to make their maps and made a new Hardiness Zone Map. They also made another map that shows the differences in zones over the past 15 years. You will notice that much of the nation has become one zone warmer. While this is good in a limited sense because some people can now grow plants in areas that were previously too cold, it is alarming as further evidence that global warming is a real and significant phenomenon.