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. 

 It 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, 1 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. 



' Hartig, Robert. Text-book of the Diseases of Trees. Translated by William Somer- 

 Tille, revised and edited by II. M. Ward. London, 1894, p. 289. 



