222 POMOLOGY 



192. Spring frosts. — Frosts in autumn are of some 

 economic importance to the pomologist, but those occurring 

 in the spring are usually much more disastrous to the fruit 

 crop, and are considered here more in detail. The destruc- 

 tion of blossoms and hence the prospective crop by 

 spring frosts either locally or over rather large areas is a 

 common occurrence and one of the most ruinous phases of 

 fruit-growing. 



The United States Weather Bureau distinguishes three 

 types of frost, based on the degree or severity of it, namely: 

 "light," "heavy," and "killing." The latter two are usu- 

 ally distinguished by the extent of injury to vegetation 

 rather than to the actual amount of deposit. The term 

 "killing frost" is described as one which is generally de- 

 structive to the staple products of the locality. Vegetation 

 may also be damaged by low temperature without an actual 

 deposit of frost, a condition due usually to cloudiness. The 

 probable dates of killing frosts for any locality are a valu- 

 able guide to the fruit-grower and gardener and some maps 

 have been prepared by the Weather Bureau showing the 

 dates of the last killing frosts in spring for the different re- 

 gions of the United States. The same sort of map is given 

 for the last killing frosts in the fall and for the average num- 

 ber of days without killing frosts.^ These maps are based 

 on a very large number of records from many regular and 

 cooperative stations of the Weather Bureau. There is 

 great irregularity in the dates of the last frosts in the spring 

 and the first ones in the fall for any given place, and usually 

 the arithmetical average of these dates is used in construct- 

 ing the maps but such a date entails a large amount of risk, 

 one year with another. On the other hand, if the latest 

 frost date recorded in the spring and the.earhest in the fall 



' Reed, William Gardner. Frosts and the growing season. U. S. 

 Dept. Agr. Off. Farm Manag. 1918. 



