36 Farmers' Bulletin 1096. 



Although it is sometimes difficult to find time to keep records on 

 heating operations during the rush of firing, it should be done when- 

 ever possible. The temperature when firing is begun, time of ini- 

 tial firing and number of heaters fired, time of firing additional heat- 

 ers, the lowest temperature recorded during the night, can all be 

 jotted down from time to time as the work goes on. On the follow- 

 ing day an estimate can be made of the amount of oil consumed and 

 the extent of the damage to the fruit, if any. Eecords of this kind 

 will be found to be of great value in regulating later firing ; the more 

 information of this kind gathered, the more efficiently can the firing 

 be handled. Records of this kind will also help to determine whether 

 protection is profitable or not, a question which every grower should 

 solve for himself at the earliest possible time. 



If orchard heating is carried on in a careful, painstaking manner, 

 with ample equipment, there are probably few commercial fruit- 

 growing districts where the heaviest frost likely to be experienced 

 can not be successfully fought with orchard heaters. 



FROST AND MINIMUM TEMPERATURE FORECASTS 



General forecasts of frost for large areas are issued by the Weather 

 Bureau during the growing season, and in certain rather small dis- 

 tricts where protection against frost damage is practiced on a large 

 scale, forecasts of the minimum temperature to be expected from 

 night to night are issued. Farmers or fruitgrowers who have a 

 means of protecting their crops should arrange with the nearest 

 Weather Bureau station to obtain forecasts of the kind available in 

 their community. 



INJURIOUS TEMPERATURES 



So many factors must be taken into consideration in determining 

 whether a given temperature will cause damage that the matter is 

 one of great complexity. The length of time the low temperature 

 persists, the vigor and stage of advancement of the plant, the kind 

 of weather preceding the frost, and the rate of thawing all have 

 considerable influence on the amount of damage that will be done. 

 Other conditions being the same, a weak, undernourished plant will 

 show more injury than a strong healthy one after both have been 

 subjected to the same low temperature. 



Pure water has a higher freezing point than water carrying foreign 

 substances in solution. For example, the freezing point of a strong 

 solution of common salt may be 23 or lower, depending on the 

 concentration; the weaker the solution the higher the freezing 

 point. When the weather is warm and sunshine and moisture 

 plentiful, plants make a rapid growth and the sap is likely to be 



