sept. i, 1925 Minimum Temperature for Growth of Date Palm 
413 
Thus the greenhouse and cool-chamber tests confirm the widely 
separated field observations in placing the zero point for the date 
f >alm at 50° F. or slightly lower. The time of exposure was sufficient 
or the entire plant (leaves, stem, and roots, with the ball of earth) 
to have been reduced to the cool-chamber temperature, and we can 
conclude that the physiological activities which lead to growth were 
suspended at that point. The zero point for these palms , then , was 
the temperature attained by the growing center of the bud at which all 
growth ceased . We may assume that the same condition holds for large 
palms in the field. 
At the end of the cool-chamber experiment, after all three plants 
had been brought to a section of the greenhouse where they had full 
sunlight and a day temperature of 68° with night minima of 55° to 
57°, palm No. 2 began growth during the day, the others the follow¬ 
ing night, and made gains of from 3 mm. to 8 mm. on the different 
leaves during the next three days. 
These figures clearly show that the plants had sustained no injury 
by being brought to the zero point of growth. With a slight time 
allowance necessary to bring the roots in the mass of earth in the pot 
to the new temperature, they were ready to resume normal activity 
when exposed to the higher temperature. 
With the zero point for constant exposure known, the next step 
is the application of such knowledge to field conditions, subject to 
the varying ranges of daily temperature. 
Daily mean air temperatures of 50° F. seldom bring the date-palm 
growth to zero. At Indio a slight growth has been recorded under 
mean temperatures for weekly periods as low as 45°, but these were 
where the component maxima were in the 60’s and 70’s and frost 
minima were sustained for but brief periods. 
At Eureka, Calif., with but few frosty nights and minima rarely 
below 28° or 30° F., but with monthly means of from 46° to 56° F., 
giving a normal annual mean of only 51.3°, the growth of the date 
palm is entirely excluded. At San Francisco, nearly frostless, with 
monthly means from 49° to 62° F. and the normal annual mean only 
54.9°, the date palm makes a slow growth, but seldom, if ever, 
flowers. 
For a comparison of the relative temperature efficiency for the 
vegetative growth of the date palm of localities not subject to destruc¬ 
tive minimum temperatures, if the mean of 50° is taken as the base 
the monthly summations above that point during the growing season 
will afford a reliable scale of values. 
SUMMARY 
The date palm, Phoenix dactylifera , as represented by its horticul¬ 
tural varieties, has a wide range of temperature endurance, being 
able to survive without permanent injury temperatures in different 
localities from 4° to 125° F. 
The geographical range of the date palm as a plant is very much 
wider than the range of its successful fruit production, and the culti¬ 
vated varieties are often rather exacting in their temperature require¬ 
ments. 
Date trees, under favorable environment, continue growth through¬ 
out the year at a rate closely correlated with the current mean 
