160 VEGETABLE FORCING 



volume of water available as well as upon the rate of flow, 

 depth of bed and character of soil. 



During the winter months, one watering may last for 

 five weeks or longer, and in the summer it may not be 

 necessary to water oftener than once a week or every 10 

 days. In newly planted beds, especially if the plants are 

 small, it is necessary to make one or two surface water- 

 ings until the plants have become established. 



Thorne recommends that the tile be cleaned and relaid 

 every third year. 



Sub-irrigation is considered especially desirable for 

 lettuce and tomatoes. 



Overhead irrigation. Of the various systems of water- 

 ing greenhouse vegetable crops, overhead irrigation 

 (Fig. 54) is the most generally used in large commercial 

 establishments. Wright, in a recent survey of extensive 

 ranges used for the forcing of vegetables, found that 78 

 growers out of 100 were employing the overhead system. 

 The advantages of this system are as follows : 



(1) Comparatively small cost of installation. It is esti- 

 mated by the manufacturers that the cost will usually be 

 about $300 an acre. This is very much less than the ex- 

 pense involved in preparing the beds for sub-irrigation. 



(2) The water is applied more uniformly than is 

 possible with any other system except sub-irrigation. 

 There should be no wet spots anywhere in the house 

 when the overhead system of watering is employed. 



(3) The labor of watering is very slight. Only a few 

 moments are required to open the valves and to turn the 

 nozzle lines as may be necessary to water the entire 

 house. An automatic turning device has been invented 

 recently which should further reduce the attention 

 required by this system in the watering of greenhouse 

 crops. 



(4) The patented nozzles diffuse the water into an ex- 

 tremely fine mist, which then descends gently upon the 



