From Gene Logsdon
Excerpt from Practical Skills 1985
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 available, one can avoid the out-of-pocket cost of cable and electricity. This savings can be especially significant if one uses the hotbed all through the cold winter months. The cost savings is only part of it. Manure will also provide fertilizer for the hotbed plants and finished compost for the garden after its heat cycle has passed.
But whatever you use for your source of heat, a good hotbed should satisfy the four following requirements:
1. With a manure hotbed, the soil in which the plants are growing is some several inches below the outside soil surface. Therefore, it must be built on a high, well-drained location. The foot of manure will give good drainage for ordinary watering or rains, but obviously, if this hotbed is built in a low place, it might very easily fill with water after heavy rains or thaws.
2. It must be insulated from the soil around it. Otherwise cold will move through the soil and the frame of the hotbed into the growing space. Just as insulation is now placed around foundations of houses, so it should be put around the “foundation” of a hotbed. Styrofoam or other kinds of rigid-board insulation work fine set at least a foot deep around the perimeter of the hotbed. Manure works fine, too, so long as you place it at least 6 inches wide on the outside of the frame, extend it down to the heating layer of manure below the hotbed, and heap it up around any of the frame exposed above the soil surface. For this reason the pit for the traditional 3 by 6-foot bed should be dug 3 by 7 feet wide, to allow room for the manure that is used for insulation.
3. The hotbed should be protected from cold winds. This is provided by putting it in a pit, or what is now often called a “grow-hole.” The glass panes on top of the hotbed should be only a few inches above the soil surface. The surface of the loam in which the plants grow should be about 6 inches below the outside soil surface. Thus, the pit is dug a little more than 2 feet deep to accommodate a foot of manure, about 6 inches of topsoil, and then the plants, which then have about 8 or 9 inches of clearance under the glass.
The frame should be constructed with the back board a few inches higher than the front so that water drains easily off the glass, not, as so many think today, to allow better access to the sun. When the grow frame is built into the ground with the glass only a few inches above the soil surface, the slant does not allow any more sunlight in than a level glass pane would.
Next, the hotbed should be located to the lee side of prevailing winds; the south or east side of a building is excellent. If such protection is not available, a bit of board fence or a stone wall or several bales of hay or straw should be set up for wind protection.
4. Finally, and perhaps most important, the glass top should be covered at night with some kind of insulation. A piece of rigid-board insulation is ideal. A couple of straw bales work fine. Burlap sacks and old rugs will do. Plants will not grow well when night temperatures in the hotbed are coolish, no matter how much sunshine they get during the day. But on hot days, be sure to raise or remove the sash if inside temperatures rise much above 90°F. You’d be surprised how much the sun can heat a space under glass, even on a cool day if there are not clouds.
Packing in the Manure
Only horse manure is practical for hotbedding. Not only is it more available in quantity, but it heats up quicker and hotter than most other manures. The less straw or other bedding in it, the better, but mixed manure and bedding is adequate. The trick is to start the manure heating only just before you want to put it in the hotbed. This is easy if you have your own horse or access to a stable. Generally, the manure will not heat much as it lies in the stable (unless it gets quite deep). So, about three days before you want to start the hotbed, fork up the stable manure into a heap about 4 feet high and 4 feet in diameter, as if you were going to start a compost pile, which is exactly what you are doing. In a pile like this (larger is better if you can manage it) the manure will begin to heat right away, and in about three days it will be quite hot inside and really beginning to steam.
Wheelbarrow it to your hotbed hole (2 feet deep, 3 feet long, and 7 feet wide) and pack it in about a foot deep. For starting plants in spring, 8 inches may be enough if your hotbed is well-insulated as described, but 12 inches will be needed for winter heating and perhaps for spring, too. If the manure is quite dry, sprinkle a little water on it, but the manure will heat better if it has been well-moistened with horse urine in the stable. You will have to watch carefully so the the manure does not overheat the soil during the first two weeks of intense microbial activity. Keep a soil thermometer in the hotbed, and if temperatures climb much above 90°F., open the glass sash a bit.
Pack manure around the outside of the farm, too, running it under the frame into the manure layer in the bed. The 2 by 12-inch frame planks are nailed to treated or rot-resistant posts at each corner, so it is easy to pack the manure around and under them.
Then place a 6-inch layer of soil over the manure. Water lightly if dry. You don’t want it soggy. Place the glass sash over the bed and have insulative covering ready for night use. When the soil heats up to 70° to 75°F., plant your seeds. Regulate the temperature by raising and lowering the sash a bit.
For spring use, hotbeds should be started no earlier than March 20 in the north, and April 1 is better where plants cannot be set out until late May. The farther south, the earlier you can start, of course. If you contemplate experimenting with winter production of lettuce or similar crops, grow the first in November-December, then skip to February-March for the second, harvesting in time to grow bedding plants for setting out in the garden in spring. Skip the coldest times of the year: late December, January, and early February.
See also Gene’s An Ode To Horse Manure And Other By-Products Called Waste
and Hotbeds in Sweden