24 MANUAL OF AMERICAN GRAPE-GROWING 



offset warm days and delay ripening. Certainly rains, fogs 

 and humid air delay maturity. The bottom heat of loose, 

 warm, dry gravelly or stony soils hastens maturity. Sunshine 

 secured by a sunny aspect or shelter hastens maturity. 



The seasonal sum of heat. 



Successful cultivation of the grape depends on a sufficient 

 amount of heat during the summer season. The theory is 

 that buds of the grape commence to start when the mean daily 

 temperature reaches a certain height, and that the sum of the 

 mean daily temperature must reach a certain amount before 

 grapes ripen. Manifestly, this sum must vary much with dif- 

 ferent varieties, low for the earliest sorts, high for the latest. 

 There have been many observations as to the temperatures 

 at which buds of the grape start growth, so that it is now 

 known that the temperature varies in accordance with lo- 

 cality and degree of maturity. Roughly speaking, grape 

 buds start at temperatures from 50° to 60° F. The seasonal 

 sum of heat for ripening is probably 1600 to 2400 units. A 

 variety ought not to be planted, therefore, in a region in 

 which the average seasonal sum of heat is not sufficiently 

 high. The seasonal sum of heat can be determined for a 

 locality from data published by the United States Weather 

 Bureau ; and by comparing with the sum of heat units in 

 localities where a variety is known to thrive, the grape- 

 grower can determine whether there is sufficient heat for 

 any particular variety. 



The grape seldom suffers from hot weather in a grape region. 

 The fruit is sometimes scalded in the full blaze of a hot sun, 

 but the ample foliage of the vine usually furnishes protection 

 against a burning sun. At maturing time, the heat of an un- 

 clouded sun, if the air circulates freely, insures a finely finished 

 product. Deep planting helps to offset the harmful influences 

 of warm climates. 



