72 BULLETIN" 479, U. S. DEPARTMENT OF AGRICULTT7EE. 
out than at one with a heavier, more retentive soil. It is also likely 
to be more serious with shade and moisture loving species, such as 
spruce and western red cedar, than with such drought-resistant 
species as western yellow pine. As the absorbing portions of the 
roots die at the same time or even just preceding the time of yellow- 
ing of the leaves, it is important that the trees never be allowed to 
reach this stage. Recovery is then doubtful. The remedy is obvious. 
Excessive crowding in seed or transplant beds should be avoided; 
the soil should be kept fresh at all times after the damping-off 
period has passed; exceptionally heavy watering should be prac- 
ticed during periods of particularly drying or hot weather; and 
shade should be supplied if necessary. 
A type of injury similar to summer drought injury but rather 
uncommon may be caused by an excess of water. This makes itself 
evident by the yellowing and final death of the plants. Recovery 
from this is claimed to be slower and even more doubtful than from 
sun scorch. 
WINTERKILLING. 1 
Winterkilling produces a condition in plants very similar to that 
of sun scorch and physiologically is due to the same cause; that is, 
the inability of the roots to furnish moisture to the tops as fast as 
it is lost by transpiration during periods of severe, trying weather. 
YV~interkilling differs from sun scorch, however, in that it occurs 
during the winter, and it is not necessarily due to insufficient water 
in the soil but rather to the fact that the water is frozen and thus 
not available to the plant. Periods of strong winds or of bright, 
warm days during the winter when the ground is frozen are most 
likely to cause winterkilling. Sometimes only a portion of the top 
of the plants is injured. This is especially likely to occur with any 
portion which protrudes above a snow covering. Mulching is the 
preventive for this type of injury. 
FROSTS. 
Losses from killing frosts are infrequent but usually extensive 
when they do take place. This is due to the relatively few species 
growing in any one nursery and the large number of plants which 
happen to be in the same class and in a like susceptible condition. If 
seed is obtained in localities which have a climate similar to that of 
the nurseries, if it is sown so as to give the trees the benefit of the full 
growing season, and if growth is not stimulated too late in summer 
through watering and cultivating, no further measures are required 
to prevent frost-killing under normal conditions. But additional 
^or full discussion, see Bulletin of the United States Department of Agriculture No. 44, 
" The Blight of Coniferous Nursery Stock," by Carl Hartley, of the Bureau of Plant 
Industry. 
