12 | BULLETIN 44, U. S. DEPARTMENT OF AGRICULTURE. 
The formation of ice crystals in the younger tissues at the close 
of the thaws may also be concerned in causing injury. Trees affected 
by winterkilling in the nursery look much like trees affected with sun 
scorch. Winterkilling can be distinguished from other types of 
blight by paying attention to attendant circumstances. The most 
damage may be looked for during the hardest winters or winters with 
little snow. It is likely to be worst where the beds are least pro- 
tected by windbreaks or a mulch and in the least resistant species. 
Winterkilling and sun scorch work differently in that winterkilling 
is worst in open stands, while with sun scorch the case is reversed. 
This may be explained not only on the ground that the closely sown 
trees protect each other from drying winds, but also, as pointed out 
by Forest Supervisor Elers Koch, act as a mulch, protecting the soil 
from deep freezing. 
The ordinary measures for preventing winterkilling are to pro- 
tect the nursery beds as thoroughly as possible by windbreaks and 
to mulch the beds with straw. Mulching must be done cautiously. 
Tt is likely, especially if heavy or close, to result in mulch injury and 
do more harm than good. Mulch injury is entirely different from 
winterkilling and will be considered later in this paper. 
Heaving is also distinct from ordinary winterkilling. The roots 
of nursery stock in heavy soil are sometimes lifted partly or entirely 
out of the ground by the action of alternate freezing and thawing. 
Both plants and surface soil are raised by the expansion of the soil 
in freezing. In thawing, the soil settles back gradually around the 
roots, which are left higher than they were previously. The process 
is entirely mechanical. Like winterkilling, heaving can be prevented 
by mulch. 
FROST INJURY. 
Frost injury differs from winterkilling in that it is due to the 
formation of ice crystals in unripened tissue, while winterkilling is 
probably due mainly to drying out, as above described. In addition 
to injury to unripened tissue above ground by early frosts, it is 
possible that early freezing of the soil injures by killing roots which 
have not yet stopped growth. According to Hartig,* trees with in- 
jured roots are likely to start growth the following spring before 
showing any effects and then turn brown rather suddenly. Very late 
spring seed sowing and encouraging the growth of stock toward 
the end of the season should be avoided, in order to get the tissues 
properly ripened up and able to endure freezing. Covering beds with 
a mulch before the first heavy freeze should prevent injury, though 
very early mulching must be avoided. 
1 Hartig, Robert. Text-book of the Diseases of Trees. Translated by William Somer- 
ville, revised and edited by H. M. Ward. London, 1894, p. 289. 
