Fruit trees are in danger of frost damage in cases where their average date of flowering stage is very close to the date of the last hoarfrost, because the fruit trees are the most sensitive to freezing in the flowering time.
From this point of view, the difference between the average date (A) of the last hoarfrost and the average date (B) of full bloom stage was adopted as an index of the risk with respect frost damage This index was applied to apple, peach and pear. The frequency of occurence of frost damage increases with decreasing the index value (B—A).
The results obtained in this manner are shown in Tables 1, 2 and 3. These tables show, in general, Suzuka and Hirano are the most dangerous places with respect to frost damage, because of early flowering and late date of the last hoarfrost. On the contrary, the frost damage scarcely occured at Sakata where the flowering date is late and the date of the last hoarfrost is early (Fig. 2).
Highly significant difference was recognized, furthermore, between full bloom stage of Jonathan and air temperature in April as shown in Tables 4, 5 and in Fig. 1.
Even in regions where there are no data on the flowering date of fruite trees, it is possible to know indirectly hazardous areas for planting fruite trees from the stand point of frost damage using a close relationship betwen air temperature in April and the date of the last hoarfrost. Figs. 3 and 4 show the results oftained by such a indrect method.
It is shown in Fig. 4 that Fukaura, Tomari, Oma, Mimaya, Wakinosawa and Tanabu are relatively safety places concerning frost damage, while Shichinohe, Tago and Ozawaguchi are dangerous places.