WATERING, HEATING, VENTILATING AND SHADING 155 



and the crop is being harvested. Ordinarily, it is prefer- 

 able to apply water in a fine spray or mist, and this is not 

 possible without the use of a special nozzle which may be 

 attached to the hose. Fig. 52 shows a most serviceable 

 nozzle for the watering of small plants, and it is also con- 

 venient for watering limited areas here and there which 

 may need water before the time of the next general water- 

 ing. Some form of hose watering is used more or less in 

 all commercial establishments. A combination of two 

 systems is ideal. For example, sub-irrigation is highly 

 satisfactory for lettuce, but many growers prefer the hose 

 or perhaps the overhead system for cucumbers, partly be- 

 cause they are valuable in controlling the red spider. 

 The hose and overhead watering also make a desirable 

 combination. Though the expense of installing two sys- 

 tems of watering may seem excessive, the advantages 

 thereof may more than justify the additional expenditure. 



Watering with a hose, unless carefully managed, may 

 result in the incrustation of the surface of the soil. To 

 avoid this difficulty, the spray should be as fine as 

 possible and the hose should not be held too long at one 

 place. 



Sub-irrigation. Several agricultural experiment sta- 

 tions have conducted experiments in the watering of 

 greenhouse vegetable crops by means of sub-irrigation. 

 The work of the Ohio station has attracted most atten- 

 tion. In the institutions where sub-watering has been 

 tried, the results have generally been favorable to this 

 system. For reasons, however, which cannot be satis- 

 factorily explained, sub-irrigation has not become popular 

 among commercial greenhouse men. A grower here and 

 there is using the system, but the rank and file of the 

 gardeners who produce crops under glass have not 

 adopted this plan of watering. It is presumably due to 

 the cost of installation. There is also some objection to 

 having the tile in the beds where they interfere more or 



