304 FUNDAMENTALS OF FRUIT PRODUCTION 



Table 41. — Maximum Depth of Frost Penetration at Lincoln, Neb. 



* Not recorded to maximum penetration. 



that soil does not freeze until it is cooled several degrees below 32°F.2s 

 Consequently since these figures were based on the assumption of freezing 

 at 32° the actual frost penetration was not so great as is indicated. 



Critical Temperatures for Tree Roots. — In the section on Water 

 Relations the extent and the depth of some fruit tree root systems are 

 indicated. The data there given indicate that in the majority of fruit 

 growing regions by far the greater part of the feeding roots is in the 

 surface foot of soil. 



The finer roots of beech, oak and ash, trees that are considered 

 at least fairly hardy, die at temperatures between 8.6° to 3.2°F.i" and the 

 roots of other hardy plants are reported killed at temperatures from 

 14° to 5°F.^^ Working with apple roots, under laboratory conditions, 

 Chandler found that "the killing temperature varies from — 3°C. in 

 summer to about -12°C. [26.6°F. to 10.4°F.] in late winter with rather 

 rapid freezing." He remarks further, "They are still very tender in 

 autumn when tissue above ground has begun to increase rapidly in hardi- 

 ness ... as the roots extend away from the crown they become more 

 and more tender and apparently this tenderness is greater on those roots 

 that extend downward into the soil." It may, then, be concluded that 

 the roots of most plants are more tender, at a given temperature, than the 

 parts above ground. Parentheticall,y, though Chandler's statement 

 as to increasing tenderness with increasing distance from the crown may 

 be accepted, it should be understood that root killing is frequently 

 observed at or near the crown and not elsewhere, probably because this 

 part is nearest the top soil and therefore exposed to colder temperatures, 

 as shown in Table 39. 



Carrick^^ found a marked difference in tenderness of roots at different seasons 

 in New York. "The material frozen in October and November," he states, 

 "shows a marked tenderness compared with roots tested in February and March. 

 The period of maximum resistance seems to end somewhat before the last of 

 March, tho the date would, of course, vary with the conditions affecting 

 after-ripening and possibly also with the variety . . . This range of hardiness 

 indicates a difference in resistance of between 3 and 4 Centigrade degrees. 



