16 MISC. PUBLICATION 303, U. 8. DEPT. OF AGRICULTURE 
DEVELOPMENT OF THE PLANT-GROWTH REGION MAP* 
The need for a map that designates zones or areas having fairly 
uniform growing conditions for plants was recognized when an at- 
tempt was made to discuss where in the United States different roses 
could be grown and also when recommendations for the planting of 
trees on city streets throughout the country were desired. : 
To show regions where the different types of roses could be grown 
with a minimum of attention a rose zone map was prepared and was 
published in the Atlas of American Agriculture (45) and in the Amer- 
ican Rose Annual (77). It raised a storm of protest by growers and 
occasioned much discussion among them because roses were being suc- 
cessfully grown far beyond the zones indicated for each group. This 
success was due to the fact that special attention had been given to 
their culture, particularly to winter protection. Eventually it was 
generally conceded that the map showed what it purported to show. 
A map was made as a basis for recommending street trees and was 
published in a Department Bulletin, Street Trees (417), and again 
in a Farmers’ Bulletin, Trees for Town and City Streets (478). This 
map divided the country into 13 zones. The zoning was based on a 
combination of field studies in different parts of the country and a re- 
view of the literature of tree planting. Before the tree lists to ac- 
company the map were completed it became evident that more zones 
would have been desirable but this limited number was what seemed, 
practicable at the time. 
The aim in preparing these maps, and the later one appearing in 
this publication (fig. 1), is to delimit areas with growing conditions 
sufficiently similar so that a. large proportion of the plants that will 
grow in any one part of each zone will grow reasonably well in all of 
it. This makes it possible to draw up lsts of plants that can be 
recommended for growing in each region. 
Factors influencing growing conditions are latitude and altitude, 
which have a direct bearing on temperature, frosts, and length of 
growing season as well as extremes of temperature; rainfall, totals 
and distribution through the year; humidity as influenced by rain- 
fall and winds; soil type and character; and amount and intensity of 
sunshine. Often these factors change very gradually so that there is 
no sharp natural line of demarcation. In such instances almost arbi- 
trary limits must be established in order that the regions shall not be 
too large to have reasonably uniform conditions throughout. Ex- 
amples of areas thus delineated are those included in regions 15 and 16 
and in regions 18, 19, and 20. In each of these two groups of regions 
the changes are so gradual from north to south that it is difficult to 
know where to draw the separating line or lines, but the total differ- 
ence between the northern parts and the southern appears to call for 
separation. In the absence of other marked factors the north and 
south limits of these regions have been determined by the character- 
istic farm crops of each. 
It is seldom that the range of natural distribution of any two plants 
is exactly the same; nor is the range of successful cultivation of any 
two plants more likely to coincide. Although the lines on the growth- 
region map give the approximate limit of distribution of plants it 
8 This section was written by Furman Lloyd Mulford, associate horticulturist, Bureau 
of Plant Industry. 
