FORCING AND FORCING STRUCTURES 57 



Permanent cold frames. Cold frames are often more elaborate 

 and more permanent structures than those just described. Perma- 

 nent cold frames are sometimes spoken of as " pits." They usu- 

 ally consist of a structure, similar in design to the one shown in 

 figure 17, placed over an excavation varying in depth from fifteen 

 inches to four feet. Such excavations are walled up with brick, 

 stone, or concrete, and the superstructure is often made of the same 

 material. These permanent cold frames or pits are used for the 

 winter storage of semihardy plants or for carrying over winter let- 

 tuce, cabbage, and pansy plants, to be used for early spring plant- 

 ing. All such structures are covered with standard hotbed sash, 

 which is often supplemented by straw mats or shutters the same 

 size as the sash. 



Cold frames are built according to a variety of designs to suit 

 the climate and the purpose for which they are intended. The 

 convenience of permanent frames is greatly increased by the use 

 of supports and guide strips for the sash, but these are not abso- 

 lutely necessary. 



Hotbeds. Hotbeds are true forcing structures. They may be used 

 to anticipate the planting season in the spring from one to three 

 months and to prolong the season in the autumn for the same 

 length of time. Some short-season crops, such as lettuce and rad- 

 ishes, are brought to perfection in the hotbed. These structures 

 are also used, like cold frames, for starting, in advance of the 

 planting season in the open, some of the long-season crops which 

 require a high temperature early in their growth. Such plants as 

 muskmelons are often started in hotbeds, and when they require 

 more space the frames are either raised and propped up so as to 

 allow the plants to escape or are removed altogether. 



Hotbeds, as the name implies, require some form of heat, which, 

 in most cases, is supplied by fermenting manure. 



Manure for hotbeds. The best manure for a hotbed is produced 

 by grain-fed work animals horses or mules. A sufficient quantity 

 to meet the needs of the work must first be accumulated and then 

 piled in a broad, flat heap. When it begins to heat, it should be 

 repiled into the form required by the hotbed, care being taken to 

 scatter the hottest portions of the mass evenly through the heap, 

 to insure even heating. The duration of the heat will depend on 



