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 
