USDA Plant Hardiness Zone Map 



Henry M. Cathey 



his map supersedes U.S. Department of Agriculture Miscellaneous 

 Publication 814, "Plant Hardiness Zone Map," which was revised 



in 1965. It shows in detail the lowest temperatures that can be 



expected each year in the United States, Canada, and Mexico. These temperatures 

 are referred to as "average annual minimum temperatures" and are based on the 

 lowest temperatures recorded for each of the years 1974 to 1986 in the United 

 States and Canada and 1971 to 1984 in Mexico. The map shows 10 different zones, 

 each of which represents an area of winter hardiness for the plants of agriculture 

 and our natural landscape. It also introduces zone 1 1 to represent areas that have 

 average annual minimum temperatures above 40°F (4.4°C) and that are therefore 

 essentially frost free. 



Basic Plant All agriculturists, environmentalists, horticulturists, and home gardeners have one 

 Requirements all-abiding question about any plant they wish to introduce into their growing 

 spaces. Will it flourish? 



The ability to predict whether a newly collected species or cultivar can be success- 

 fully grown in a location is fundamental to the continued productivity of America's 

 agriculture and to the survival of our landscapes. The plants in our urban spaces, 

 farms, fields, and forests consist of native vegetation and exotic plants introduced 

 into our land from all over the world. All plants must be placed in an environment 

 that meets their basic requirements: 



Day length. Day length is usually the most critical factor in regulating vegetative 

 growth, flower initiation and development, and the induction of dormancy. Plants 

 survive only when the day length promotes their growth and prepares them for the 

 seasonal changes. 



Radiation. Most plants respond to radiation in the 270- to 3000-nm region. 

 Cloudy, rainy days coupled with the shade provided by nearby plants and structures 

 can significantly reduce the amount of radiation available. Plants survive only 

 where the amount is within a specified range. 



Temperature. Plants grow best within an optimum range of temperatures; and the 

 range may be wide for some species, narrow for others. Plants survive only where 

 temperatures allow them to metabolize. 



Frost. Plants differ in ability to survive frost, their responses varying from immedi- 

 ate death to sustained performance. The previous environmental and cultural 

 conditions of plants can often shift, but not permanently alter, their tolerance to 

 freezing. Plants survive only when they are adapted to subfreezing weather. 



Director, U.S. National Arboretum, Agricultural Research Service, U.S. 

 Department of Agriculture, Washington, DC 20002. Temperature data were 

 compiled and maps prepared under contract with the Meteorological 

 Evaluation Services Co., Inc., 165 Broadway, Amityville, New York 1 1701. 



