222 VEGETABLE FORCING 
common to leave the ventilators open all night, to some 
extent at least. A safe practice is to ventilate every day 
as much as weather conditions will permit. 
Cultivation in growing lettuce under glass may not be 
as important as when the crop is grown in the open, 
though it is unquestionably beneficial. One cultivation, 
in a few days after the beds are planted, to break the 
crust of the soil, may be sufficient, but additional tillage 
will be valuable, especially if the soil contains much clay 
or silt. The most common practice is to use hand weed- 
ers or pronged hoes in narrow beds. Iron rakes reduced 
to a width of 6 or 7 inches are satisfactory for the cultiva- 
tion of wide beds. The tool shown in Fig. 75 consists. of 
a five-prong hand weeder secured to the end of a long 
handle. Wheel hoes are employed in some houses where 
the rows are far enough apart to permit their use. 
Intercropping is followed to some extent in the grow- 
ing of lettuce under glass. The various systems em- 
ployed are discussed in Chapter XXI. 
Frame culture.—The forcing of lettuce in hotbeds and 
various types of frames is treated in Chapter XXII, 
page 403. 
Pot culture——Studies have been made at several agri- 
cultural experiment stations to determine the value and 
feasibility of maturing lettuce in pots. Seedlings which 
are three or four weeks old are set in 2-inch or 21%4-inch 
pots, and the pots of large plants, as seen in Fig. 76, are 
then plunged at the usual distances for planting into the 
permanent beds. The rims of the pots should be at least 
half an inch below the surface of the bed. 
The crop is watered and cared for in the usual manner 
until the heads are large enough to market. Then the 
potted plants (Fig. 77) are lifted and sent to market in 
the pots, or preferably the balls of earth and roots are 
‘wrapped in paper and a dozen plants placed in a flat or a 
market basket. The plants will hold up better if the 
