PROTECTION AGAINST FROST 375 



Covering and Spraying. — The protection of plants from frost by- 

 covering tliem with paper or cloth is of course effected through saving 

 of the heat otherwise lost through radiation. The efficacy of this 

 method is well known though it is not practicable in the orchard. ^^^ An 

 experiment in California showed that with an outside minimum of 19° 

 the lowest temperature under a paper covering spread over an almond 

 tree was 24°, a saving equal to the raise in temperature secured in many 

 instances by orchard heating. 



The protective effects of water spray were investigated in Utah by 

 keeping a block of apricots under a continuous fine spray during a frost. ^°^ 

 Ice formed on the blossoms and it finally appeared that only the sprayed 

 trees were damaged. The injury was not a mere failure to set fruit; 

 there was actual killing. In view of the work of Harvey^'' it seems prob- 

 able that in this case the ice formation on the surfaces of the blossoms 

 inoculated the inside tissues with ice crystals and actually hastened their 

 freezing. 



Orchard Heating. — The most successful results so far achieved in pre- 

 venting low temperatures have been realized by the use of large numbers 

 of small heaters, warming the air itself. This practice has become a 

 settled part of orchard routine in some sections; in others it has been in 

 extensive use but is now almost obsolete. There can be no doubt of 

 its efficacy in some cases. Frequently, however, it has been considered 

 too expensive insurance. The heating capacity of a set of heaters is 

 limited and sometimes in a severe freeze the temperature sinks so low 

 they are unable to maintain a protecting temperature, or in some cases 

 a high cold wind renders them useless. On the other hand, a few degrees 

 of freezing rarely destroys a whole crop. The full value of these heaters 

 is, then, realized only with minima in a certain narrow range; with 

 minima outside, they are either unnecessary or useless. 



It is probable that failure to realize these limitations led to the instal- 

 lation, during the greatest vogue, of orchard heating equipment in many 

 places where its true usefulness is rarely available and that failure to 

 realize its limitations at the outset caused unjustifiable expectations of 

 its value. In either case the reaction was bound to cloud the instances 

 and circumstances in which it can be of real worth. 



Furthermore, orchard heating has been invoked at times when the 

 difficulty, supposedly frost, was in reality something entirely different. 

 At one time many cherry growers at The Dalles, in Oregon, installed 

 extensive heating equipment to induce a proper setting of fruit when 

 their orchards were of self sterile and inter-sterile varieties and what 

 they actually needed was provision for proper cross pollination. Orchard 

 heating cannot make weak trees set heavy crops. In view of the equal 

 influence of freezing on blossoms of weak and of strong trees, as cited 

 previously in this section, the increases sometimes reported in fruit set 



