44 THE TEMPERATURE FACTOR 



under which the crops must be grown to develop satisfactorily, 

 rather than merely designating their susceptibility to, or immunity 

 from, frost injury. 



While vegetables in general may be classified as cool season 

 and warm season crops, the different vegetables in each of these 

 classes do not thrive equally well under the same conditions of 

 temperature and moisture. Although all the cool season crops 

 will germinate and grow at a relatively low temperature, and 

 demand comparatively cool weather for their highest development, 

 some will endure and even demand lower temperature than others. 

 The converse also is true : Some will endure much higher tempera- 

 ture than others. 



Cool Season Crops. — From a cultural standpoint, the cool 

 season crops may be divided into three general groups: (1) 

 The first group is composed of short season crops which cannot 

 endure the heat of summer, but which can safely be planted in 

 the open ground sufficiently early to attain full development 

 before the normal season for hot weather. The following vege- 

 tables belong to this group: Garden cress, kohlrabi, leaf lettuce, 

 mustard, peas, radishes, spinach, turnips. Of these crops, cress, 

 lettuce, mustard, and spinach will stand slightly lower temperature 

 in the seedling stage than the other crops mentioned. However, 

 in practice they are usually all planted as soon as the ground can 

 be worked in the spring. Additional plantings of lettuce, radishes 

 and peas can be made somewhat later for the sake of securing a 

 succession, since radishes and lettuce grow quickly, and peas can 

 stand a sHghtly higher temperature than most of the other crops 

 of the group. Any later plantings of cress, mustard, spinach, or 

 early turnips are usually overtaken by such high temperature 

 that the products are of little value, so that it is usually unwise 

 to plant these crops in central latitudes any later than the very 

 opening of spring. 



Some of the crops in this group can also be grown in the autumn 

 if there is sufficient moisture available. Turnips, radishes, lettuce, 

 and spinach are the crops best adapted to this purpose. In the 

 extreme South, this entire group of plants may be grown in the 

 midst of winter. 



(2) The second group of cool season .crops consists of those 

 which cannot endure excessive heat, and which at the same time 

 have so long a period of growth that it is impossible for them to 

 complete their development before the normal season of hot 



