120 PRINCIPLES OF PLANT CULTURE 



berries. The covering should not exceed an inch or two 

 in thickness, otherwise the plants may be smothered in 

 warm winter weather. For taller plants, such as the grape 

 and raspberry, the soil is usually the most convenient and 

 satisfactory covering, since a litter covering tends to 

 attract mice, that often injure woody stems. To assist 

 in bending down the stems, a little earth is usually re- 

 moved at the base on the side toward which they are to 

 be bent. Shrubs too large for bending down may be in- 

 closed in straw or similar material. 



203. Exposure. — A northerly exposure is generally 

 least trying to plants in winter, because it is least subject 

 to fluctuations in temperature. The influence of the sun 

 is here less perceptible and snow remains longer than 

 upon other exposures. The summit of a hill is usually 

 less trying than a valley, because the cold air tends to 

 seek the lower places, especially in still weather (209). 



Wind breaks, i.e., plantings of trees intended to break 

 the force of prevailing winds, act beneficially in lessening 

 damage from cold, in so far as they prevent snow from 

 drifting off the soil and mitigate the effects of drying winds 

 (189). They favor the formation of frost, however, by 

 checking the air currents (210). 



METHODS OF AVERTING INJURY FROM COLD IN THE 

 GROWING PERIOD 



204. The freezing point. — Plants are much more sus- 

 ceptible to injury from cold during their growth period 

 than during their dormant period (170). Comparatively 

 few plants, however, are injured by cold at any season 

 until the temperature falls below the freezing point of 

 water (32° F., 0° C.) or when so-called hoarfrost occurs. 



