66 GARDEN FARMING 



Proper degree of heat. The proper degree of heat for a forcing 

 house depends upon the crop to be grown. For instance, lettuce re- 

 quires a temperature of from 40 to 50 F. at night and from 60 to 

 65 F. during the day, while tomato plants require from 5 5 to 60 F. 

 at night and from 65 to 75 F. during the day. It is obvious that 

 houses of the same area which are to be devoted to crops with 

 different heat requirements will need different amounts of radia- 

 tion. A given number of feet of pipe is capable, under given 

 steam pressures or temperatures of water, of giving off a definite 

 quantity of heat. Therefore, if the house be provided with suffi- 

 cient piping to maintain a night temperature of 65 F., it will be 

 difficult, unless proper valves are supplied, to reduce the tempera- 

 ture to 40 F. It is, however, much easier to change from a high 

 temperature to a low one than from a low to a high. That is, the 

 piping can be arranged for the maximum degree of heat, and by 

 supplying valves to cut out a certain number of runs of pipe the 

 lower temperature can easily be secured ; whereas, if the house is 

 piped for the minimum temperature, the higher temperature can 

 be secured only by supplying additional piping or by increasing 

 the temperature of the circulating medium. Table I gives the rela- 

 tive heating values of hot water at 160, 180, and 200 F., and 

 of steam at 5 and at 10 pounds pressure. 



There are certain definite rules governing the amount of piping 

 necessary to provide a given degree of heat in a house of given 

 dimensions. These have been carefully worked out by mechan- 

 ical engineers and experimenters under actual conditions in the 

 greenhouse and are concisely stated in the following paragraphs, 

 rearranged from Bailey's " Horticulturist's Rule Book." 



Radiating surface for heating greenhouses. Radiating surface, 

 that is, the length of pipe of any given size whether of steam or 

 hot-water pipes, is estimated in square feet of exterior surface. 

 All projections, ornaments, etc. on the exterior of pipes or radia- 

 tors are counted as efficient surface. Formerly cast-iron pipe of 

 about 4 inches in diameter was almost always used for green- 

 house work ; it is still used to some extent for hot-water 

 heating, but most houses are now piped with wrought iron, which 

 is made of standard size and thickness and is a regular article 

 of trade. 



