72 l.i'i I ETIH 4*79, I', s. DEPARTMENT OF AGRICULTURE. 



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 t} T pe 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. 

 Winterkilling 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 

 likety 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 

 glowing 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 



1 For 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 riant 

 Industry. 



