220 VEGETABLE FORCING 



head irrigation for greenhouse lettuce is practiced by 

 nearly all the large growers. The method is economical 

 and efficient. Supplemental watering with a hose may 

 be an advantage at times, but practically all watering 

 should be done by means of overhead nozzle lines. 



Lettuce requires a large amount of water. Probably 

 the tendency with this crop is not to water enough rather 

 than too much. The soil should be well supplied with 

 moisture throughout the period of growth. When the 

 crop is approaching maturity and making the most rapid 

 gains in weight, an enormous amount of water is lost by 

 transpiration from the leaves, especially during the spring 

 months when there is so much sunshine. Heavy .applica- 

 tions of water, just before the ground is covered with 

 the plants, are usually of special value. Special care must 

 be exercised in watering when the beds are well covered 

 with plants, for there is then very little circulation of air 

 among the plants and rot is more likely to appear than 

 at any previous time. For this reason, the watering 

 should be done early in the morning of bright days, if 

 possible, and then, if the house is properly ventilated, 

 the water will evaporate from the leaves before night. 



Temperature. High temperatures are favorable to 

 rapid growth, but excessive heat for this crop, associated 

 with high humidity, is certain to cause weak, spindling 

 plants, and it greatly increases the possibility of loss 

 from diseases, while low temperatures have the opposite 

 effects. Low temperatures, especially as the crop ap- 

 proaches maturity, are favorable to maximum weight 

 and compactness of heads. Nearly all growers allow 10 

 degrees higher temperature during the day than at night. 



Grand Rapids lettuce may be successfully grown at a 

 wider range of temperatures than either Cos or heading 

 varieties. A higher night temperature than 45 degrees 

 for head lettuce would not be permissible unless the 

 houses were ventilated all night. Many growers of head 



