Heat. The thermal cutoff temperature varies widely from species to species. By 

 tradition we group plants into sun, partial sun, and shade types and plant them 

 according to their light and heat tolerances. 



Rainfall. Gardeners need to know how much water a landscape plant requires in 

 determining its usability in low maintenance landscapes. Rainfall gardening often 

 greatly limits which species can be used successfully. Gardeners also need to know 

 how much and how often to water plants in high maintenance landscapes. 



pH. The ability of plant roots to take up water and nutrients depends on the pH 

 (measure of acidity or alkalinity), presence of soluble and insoluble salts, and 

 aeration of the growing medium. The successful culture of all plant species requires 

 that they be grown in a medium within a definite pH range and with from 10 to 14 

 essential nutrients in appropriate balance. Although plants may tolerate some 

 extraneous elements and compounds, every plant species and cultivar has well- 

 prescribed limits. 



How the Map Every plant can adapt to a range of environments. Gardeners have learned through 

 Was Started experience where the great variety of landscape plants can be grown. Over the years 

 many schemes have been proposed to help gardeners locate those environments 

 when they introduce new species, forms, and cultivars. The pooling of many of 

 these schemes culminated in the development of the widely used "Plant Hardiness 

 Zone Map," under the supervision of Henry T. Skinner, the second director of the 

 U.S. National Arboretum. In cooperation with the American Horticultural Society, 

 he worked with horticultural scientists throughout the United States to incorporate 

 pertinent horticultural and meteorological information into the map. 



The elements of that map were — 



Zones. The contiguous United States and southern Canada were divided into 10 

 zones based on a 10°F (5.6°C) difference in average annual minimum temperature. 



Winter Hardiness. Survival of landscape plants over winter was selected as the 

 most critical criterion in their adaptation to the environment. 



Classification. The zone ratings were intended to indicate excellent adaptability of 

 the plants. Many plants may survive in warmer or colder zones. Usually, mere 

 survival does not represent satisfactory performance. 



Interactions With Other Environmental Factors. Many other factors may come 

 into play in determining satisfactory growth. Wind, soil type, soil moisture, humid- 

 ity, snow, and winter sunshine may greatly affect the adaptability of plants. 



Interactions With Cultural Factors. The way plants are placed in the landscape, 

 how they are planted, and their size and health can greatly influence satisfactory 

 adaptability. 



