394 VEGETABLE FORCING 
growth is not expected until March or even April, when 
the sun furnishes the required heat. Crops may be practi- 
cally matured in the fall, when they are covered with 
sash merely as a matter of protection until the vegetables 
are sold. 
Again, sash may be used for a period more or less 
definite in the spring, simply to advance the crops until 
no protection of any kind is needed, and if desired both 
the glass and the frames may be removed and all of the 
ground devoted to the crops. This plan is generally used 
from Norfolk southward. 
In the colder parts of the country it is often an ad- 
vantage to heat the frames. Ordinary hotbeds (Figs. 136 
and 137), varying in depth of manure from a foot to 3 
feet, are in common use for a great variety of purposes. 
A coil or two of steam or hot water pipes are often placed 
in frames, and this plan is gaining friends every year over 
the old plan of heating with manure. The temperature of 
the frames may be better controlled with steam than with 
manure, and the cost of heating the frames is often less. 
Fertilizing —The principles involved in the feeding of 
greenhouse crops (Chapters IV and V) are the same as 
Fig. 136.—Surface hotbed, Note notched block for supporting sash. 
