FORCING AND RETARDING PLANTS 161 



Cold frames. The cold frame differs from the hotbed in a 

 single feature: it lacks the pit of manure. It cannot therefore 

 be used for growing plants in very cold weather, though as 

 the air becomes warmer in spring it is often used to start 

 tomato, cabbage, and other plants for transplanting. Its great- 

 est usefulness is found in carrying half-hardy plants over the 

 winter and in prolonging the growing season of lettuce, 

 radishes, pansies, and the like in autumn. Cold frames are 

 banked up with manure to aid in keeping out the frost, and 

 in severe weather may be covered with mats. Both cold frames 

 and hotbeds do best if placed in sheltered situations, though 

 cold frames for carrying half-hardy or dormant plants through 

 the winter are often placed on the north side of a fence or 

 building. In the latter case the object is simply to protect 

 the plants, and no light is needed. Cold frames designed to 

 shelter plants for a few weeks in spring may be made with 

 oiled paper or cloth in place of the glazed sash. 



Forcing single hills. Single plants of such species as rhu- 

 barb, asparagus, and sea kale may be made to grow much 

 earlier than they would naturally, by placing a box or half 

 barrel over each hill and piling manure around it. The top is 

 sometimes covered with a light of glass, thus making a mini- 

 ature cold frame of it. Single boxes of this kind are now being 

 offered for sale and serve a variety of purposes. In autumn 

 manure is often piled over hills intended for forcing, to keep 

 the ground from freezing deeply. Asparagus, rhubarb, and 

 onions are often forced in the house by setting the "roots" 

 upright and close together in a box and placing the box in a 

 warm room or cellar. If kept moist, the young shoots will 

 soon appear. These plants may also be grown under the 

 greenhouse benches in the same way. Light is not necessary 

 for forcing plants of this nature, since the food stored in the 

 roots or other underground parts is drawn upon for making 

 shoots. If it is desired to force the same roots again, they 



