According to the USDA, the plant hardiness index for Background
Information How the map was started Every plant can adapt to a
range of environments. Gardeners have learned through 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 horticulral scientists throughout the United States to
incorporate pertinent horticultural and meteorlogical
information into the map. Indicator Plant Examples The following
are names of representative persistent plants listed under the
coldest zones in which they normally succeed. Such plants may
serve as useful indicators of the cultural possib