404 VEGETABLE FORCING 
hundreds of frames in which lettuce is grown are heated 
by steam or hot water. 
Big Boston is the main forcing variety, especially in 
the Atlantic coast states. It is extremely hardy and will 
stand a large amount of exposure without serious injury. 
All of the varieties mentioned on page 207 are also grown 
to some extent in frames. 
Seed for the fall crop is generally sown from August 15 
to 20. Ifthe plants for the spring crop are to be wintered 
in the frames, the seed should be sown about October 1 
in the South, and the plants set in the frames later in the 
fall and covered with sash or muslin. In the North the 
Fig. 145.—Choice heads of lettuce saved for the production of seed. 
plants may be wintered in the frames or started in hot- 
beds or greenhouses in January or February, and then 
set in the frames in March, or even as late as April 1. 
The date of sowing and transplanting may depend largely 
on the uses of the sash and frames for other crops. Big 
3oston is set at various distances, probably 9 by 10 inches 
apart being the average. 
Muskmelons may be grown in frames by the same 
