344 



FUNDAMENTALS OF FRUIT PRODUCTION 



follow it. If the average date of blossoming and the average date of 

 the last frost for a given locality coincide there are possible four combi- 

 nations of events: (1) blossoming before the average and frost before 

 the average, a condition which may or may not be disastrous to fruit 

 at that point; (2) blossoming before the average and frost later than 

 the average, very likely to be a disastrous combination; (3) blossoming 

 after the average date and frost before the average date, a safe condition, 

 and (4) blossoming later than the average and frost after the average, 

 unsafe. In cases 1 and 4 the last frost may or may not precede the 

 blossoming, with chances balancing. Cases 2 and 3 balance each other. 



Fig. 33. — Average dates of last killing frost in spring. (After ReecP^^) 



It appears, therefore, that locations where the average blossoming date 

 and average last frost date coincide have an even chance of escaping 

 frost, a margin of safety that is rather small for growing of the fruit 

 in question. 



Determining Frost Risks in Different Sections a7id Localities. — Averages, 

 of course, do not indicate the range of the figures that they represent. 

 The range of last frost dates may be considerable at one point and limited 

 at another, with the averages identical. Table 55 shows variations in 

 the last frost dates on record for several stations with identical average 

 date for this event. Such averages have only a limited significance for 

 the fruit grower, unless the fruit he grows generally blossoms consider- 

 ably later than the average date of the last frost. 



The last column in Table 55 records standard deviations from the 

 average date of the last frost, Apr. 15 in each case. This standard 

 deviation means, taking Roseburg for example, that over a considerable 



