Building hotbeds for your garden From Gene Logsdon (1985) Garden
Farm Skills A hundred years ago, hotbeds were used profitably
near large cities to grow two crops of lettuce through winter,
and then a crop of bedding plants for setting on in the garden
in spring. As long as horse manure was available, and of course
it was in great quantities, these hotbeds produced lettuce at
the rate of forty to fifty heads per 3 by 6-foot bed at far less
cost than it takes today to ship lettuce from warm-winter states
or raise it in greenhouses. Farmers near such cities as Boston
and New York operated as many as a thousand beds, providing jobs
for many people and making a good profit, with the expenditure
of very little fossil fuel. The details of the design of a
hotbed for practical winter use are seldom given in modern
books, but they should be incorporated into any hotbed, even if
used only for starting plants in spring, and even if an electric
cable, rather than horse manure, is used for heat. But where
horse manure is