Why the New Map The "Plant Hardiness Zone Map" was published in 1960 and revised in 1965. Since 

 Was Created merl5 man y changes, new interests, and new responsibilities have emerged in North 

 American landscaping: 



Changes in Weather. We have been losing from our landscapes plants that 

 apparently survived the 1940's to the 1960's. Many of the hardiness zone classifi- 

 cations of plants are no longer considered valid. In North America, the ranges of 

 temperature and moisture for the past decade were wider than those recorded for the 

 1940's through the 1960's. 



Introduction of New Elite Forms. Our landscape industry has worked with plant 

 explorers and breeders to introduce many new forms of traditional plants that are 

 adapted to a wider range of environments than the older forms. 



Scope. The continental United States is contiguous with Canada to the north and 

 Mexico to the south. We share more than a common border — we share indigenous 

 plants and many introduced plants that can be grown successfully. We needed to 

 expand the scope to include the whole of North America. 



How To Use Zones 2-10 in the map have been subdivided into light- and dark-colored sections (a 

 the New Map ^d b) that represent 5°F (2.8°C) differences within the 10°F (5.6°C) zone. The light 

 color of each zone represents the colder section; the dark color, the warmer section. 

 Zone 1 1 represents any area where the average annual minimum temperature is 

 above 40°F (4.4°C). The map shows 2° latitude and longitude lines. Areas above an 

 arbitrary elevation are traditionally considered unsuitable for plant cropping and do 

 not bear appropriate zone designations. There are also island zones that, because of 

 elevation differences, are warmer or cooler than the surrounding areas and are given 

 a different zone designation. Note that many large urban areas carry a warmer zone 

 designation than the surrounding countryside. The map contains as much detail as 

 possible, considering the vast amount of data on which it is based and its size. 



In using the map to select a suitable environment for a landscape plant, today's 

 gardeners should keep in mind the following: 



Stress Factors. We became aware of additional stresses to plants during the 

 1970's. Acid rain, gaseous and particulate pollution, security lighting, and toxic 

 wastes, among many other stress factors, have significantly increased the potential 

 for unsatisfactory performance of landscape plants. We need to document the 

 tolerances of plants to these factors. 



New Plant Management Systems. New techniques of planting, transplanting, 

 watering, fertilizing, and providing pest control measures have done much to 

 increase the vigor of landscape plants. But used unwisely, these same measures can 

 reduce plant hardiness. 



