CHAPTER XX 



PROTECTION AGAINST FROST 



The fruit grower should have, not only knowledge of the conditions 

 under which frost occurs, but information as to the exact danger points for 

 his various fruits and as to the value of different protective measures that 

 may be at his disposal. 



CRITICAL TEMPERATURES 



If heating is to be done it should be delayed until the temperature is 

 near the critical point to save expense and exhaustion of the fuel in the 

 heaters before morning. If it is known that the blossoms of one variety 

 or of one species are more tender than others protective effort may be 

 concentrated more or less on the tender plants. At times it has been 

 assumed arbitrarily that a certain temperature is fatal and that because 

 certain orchards had been exposed to that temperature they would bear 

 no crop. Accordingly the calyx spray was omitted, to save labor and 

 expense, only to have it appear later that a fair crop had survived the freeze 

 but had become thoroughly infested by codling moth, scab and other 

 pests. If, then, there is a certain temperature that is universally fatal 

 to the blossoms of all fruits or of one kind of fruit it should be known. 



A compilation of temperatures stated as dangerous to blossoms of 

 various fruits is reproduced here as Table 66. 



The considerable difference in the damaging points as stated by these 

 various writers is significant and it seems probable that the range of 

 killing temperatures is as great if not greater than indicated by the table; 

 West and Edlefsen state that there is sometimes a spread of 5°''. The 

 variations in temperatures between sheltered thermometers, exposed 

 thermometers and plant tissues make field observations of only limited 

 value. Variations in radiation conditions make the correction of ther- 

 mometer readings to plant temperatures uncertain. Furthermore, 

 different blossoms must be exposed to radiation in varying degrees because 

 of diversity in their positions in the cluster and on the branch. 



Assuming, however, that temperatures can be measured accurately, 

 as doubtless has been done in closely controlled work such as that of 

 Chandler and of West and Edlefsen, the final result is still a complex 

 involving several factors whose separate measurement is difficult. 

 Several blossoms, alike in development, will show differences in their 



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