loo FRUIT TREES AND THEIR ENEMIES 



from spring frosts is probably greater than that caused 

 by all other pests together, and it is the one against 

 which they are most helpless. Spraying trees with 

 thick lime washes, a few weeks before the blossoming 

 season, retards, as a rule, the opening of the buds for 

 about a week, and treatment with caustic emulsions 

 seems to do the same ; but a week's delay may often 

 be of no avail, and, with the variable incidence of 

 frost in this country, may, on occasions, be actually 

 prejudicial. 



In considering other palliatives, it must be remem- 

 bered that there are two kinds of frosts affecting 

 trees : wind frosts, and radiation frosts. In the 

 former, a current of cold air, generally from the north 

 or east, causes the damage, and this damage is often 

 due, not so much to the actual lowness of the tempera- 

 ture, as to the drying effect of the wind, and to its 

 preventing the working of insects which fertilise the 

 blossoms. The only means of mitigating the effects 

 of such frosts is to provide ample shelter to the 

 plantations. 



A radiation frost is due to the earth losing its heat 

 by radiation at night, when the sky is cloudless, and the 

 air is clear and dry : the cooled earth then cools the 

 layer of air in contact with it, which, in its turn, cools 

 the blossoms of the trees, and thus destroys them. 

 The cooling of the air is accompanied by the precipi- 

 tation of the invisible moisture present in it, first into 

 the liquid form of dew, and then into the solid form 

 of hoar-frost, the minute particles of water, while still 

 suspended in the air, forming a mist. Radiation from 

 the earth is prevented by cloud, and diminished by 



