Vol. XXXI Washington, D. C., September 1,1925 No. 5 
THE MINIMUM TEMPERATURE FOR GROWTH OF THE 
DATE PALM AND THE ABSENCE OF A RESTING 
PERIOD 1 
By Silas C. Mason 
Horticulturist, Office of Crop Physiology and Breeding Investigations, Bureau of 
Plant Industry, United States Department of Agriculture 
INTRODUCTION 
The culture of the date palm is a relatively new industry in the 
United States, though scattering trees grown for ornamental purposes 
or for domestic fruit supply have been known for many years in 
Southern California and Arizona and in the Gulf Coast regions. The 
first established date gardens in the United States where systematic 
observations could be made on the relation of the date palm to cli¬ 
matic environment were planted during the earl y y ears of the present 
century. The date-growing regions of the Old World are generally 
so remote from the centers of scientific research that definite published 
observations on the temperatures have been meager. The empirical 
knowledge of the natives as to the success or failure of their date 
varieties under different climatic conditions is often very accurate, 
but as yet little of this knowledge has been coined into a general 
circulatmg medium. In our own country the mistake is often made 
of assuming that where the date palm as a tree is successfully grown 
its commercial culture for fruit must be equally successful. 
Twenty years of observation of the date pahn in relation to tem¬ 
perature have established clearly the fact that Phoenix dactylifera as 
a species represented by the cultivated varieties has very definite 
temperature requirements which must be provided, often within very 
narrow limitations, if cultural success with a particular variety is to 
be secured. The zero point, or the temperature below which the 
f rowth of date palms ceases with prolonged exposure, seems to 
e the foundation upon which all study of its other temperature 
relations should be based. The relative temperature efficiency of 
two localities in which the date palm is able to grow will be measured 
most accurately, not by the summation of heat units above zero F. 
or zero C. but above the zero point of the date palm’s activity, 
which has heretofore been but imperfectly known. For this reason 
the writer’s studies of the zero point of the date palm, both under 
field and laboratory conditions, are offered here in advance of the 
studies of the more general relations of this species to climatic 
conditions. 
i Received for publication Sept. 26, 1924; issued October, 1925. 
Journal of Agricultural Research, Vol. XXXI, Na. 5 
Washington, D. C. Sept. 1,1925 
Key No. G-476 
63338—25t-1 
( 401 ) 
CEREAL INVESTIGATIONS. 
