GREENHOUSE CONSTRUCTION AND HEATING 39 



greenhouses vary from 12 to 24 inches in width. Twenty 

 inches provides sufficient space under most circum- 

 stances. In solid plantings of lettuce it is customary to 

 omit two rows, or sometimes only one, while in cucumber 

 and tomato plantations the walks are 30 inches or more. 

 Special alleys (Figs. 5, 23 and 24) and roadways (Fig. 25) 

 are important in very large ranges. 



The walks in some of the best houses are made of con- 

 crete. These are especially desirable in heavy soils. They 

 are inexpensive and simple to construct. The ground 

 should be graded as level as possible before the walks are 

 outlined. Use 2 x 4-inch pieces for the sides. Care must 

 be exercised to get the sides straight. Tamp the soil in- 

 side the 2 x 4-inch pieces (the scantling may be laid flat if 

 desired) until within an inch of the top. Stretch a piece 

 of poultry netting over the tamped soil, and hold it in 

 place with bent pieces of old wire stuck into the soil and 

 hooked over the netting. Rather thin concrete is used, 

 and the top leveled and smoothed in the usual manner. 

 The poultry netting reinforcement greatly increases the 

 strength of the walk, and economizes concrete. The net- 

 ting should be permitted to bulge here and there over the 

 soil so that the concrete will settle all around the meshes. 

 Where freezing does not occur, as in the greenhouse, it 

 is unnecessary to use ashes under the concrete. The 2x4 

 pieces of lumber are removed after the concrete is 

 properly set. 



Steam vs. hot water heating. Modern greenhouses are 

 heated either by steam or hot water. Hot water is almost 

 invariably preferred for small greenhouses because the 

 boilers may be left for a longer period at night without 

 attention. About nine-tenths of the large establishments 

 are heated by steam, and the growers claim that the steam 

 system costs less to install and to operate, and that it 

 gives them better control of temperatures. But some of 

 the owners of very large ranges of recent construction are 



