The hotbed, or forcing bed
A hotbed, sometimes; forcing bed, uses the heat from fresh, composting, horse manure to heat a soil bed during the cold spells in early spring. Put a thick layer of compacted fresh horse manure under a layer of soil and insulate the sides. To save space, we use LECA blocks for building and isolation, but straw is also useful. On the top, you put an ordinary cold frame in order to protect the plant from harsh weather. During cold nights, you may even protect the frame against freezing temperatures with a carpet or something similar. Since the hotbed creates its own heat, you can use it in any cold climate, if you only have enough light.
The hotbed - construction, management and growing
We build this hotbed in our garden in Lund, South Sweden every year. The method is versatile and is used as far North as Luleå in North Sweden (65 N lat.). The frame is constructed of LECA mortars in two sizes, 19*30*59 cm and 19*19*59 cm. The blocks are not mortared together, only stacked upon each other. It is possible to dig a hole, but the risk of flooding, lack of oxygen and the like is diminished if the pile is put on top of the ground. Therefore, we have only leveled the soil under the stones. To prolong the time of heating by reducing the air supply to the compost, we line the lower two layers of stones with a plastic sheet. By that, the combustion goes slower. Also, the cooling process goes slower. We use an inside-outside thermometer to monitor the temperature in the bed. The sensor is placed some twenty centimeters down in the bed.
This is the energy source, the horses. In January-February is it time to get the manure from a nearby stable. Very often, the owners are quite pleased to get rid of it. The manure should be as fresh as possible, preferably before it has taken heat, i.e. not started to be dark colored.The fresher it is, the more heat the manure can produce for the hotbed. The straw should preferably be from wheat or oat, Straw from rye or barley is worse. Shavings, sawdust or peat are the worst, almost impossible, alternatives.
The starting time depends on your preferred use of the hotbed. If you want it to get really early vegetables, one can start the bed in the beginning of February (in Lund, Sweden, 55 N lat.). For bedding plants, you can start later, February - March, since they should not be too large at transplanting, and they need a warm place outside the bed at transplanting time.
For growth in the hotbed, it is not the outside temperature that is limiting, but the amount of sunlight. Because of that, you can not start in the middle of the winter or in late autumn, because the plants will be weakened from lack of light in midwinter.
For this bed, outside size 240 x 160 x 80 cm, about two cubic meters of horse manure is needed. Fill the material in the bed and tread it carefully. This is actually the only time you will have odour from the horse manure.
If you use straw instead of LECA-blocks for insulation, the hotbed should be covered with at least one meter of loose material all around, somewhat less if you use bales. The use of strraw restricts the area of use. Your neighbours will probably not like your straw flying around.
During the composting process, the material will sink. If you use a thermometer, you can follow the temperature increase to about 70 degrees centigrade within a few days. It will stay the for some days, up to a week, then drop.
With good manure, the bed will be hot within a few days. When the temperature has dropped to about fifty degrees, it is time to put on a 10-15 cm deep layer of soil on the top of the composting manure, for sowing. After a week, there should be a good space above the shrinking bed. We use good, not so rich, loam for this. Measure the temperature in the soil layer to decide when it is cool enough for sowing. When it is lower than 25 - 30 degrees centigrade, it is fine. Cucumbers and melons like the upper part of the scale.
you have started early, it is sowing time in mid-February. In the picture,
you can see that the top of the soil layer is about 10 cm below the lid.
The soil level will go on sinking, preferably in the same rate as the
After sowing, it might be good to cover the lid with some insulating material, e.g. rag-carpets or quilts. They can stay there until the plants come up. After that, it is important that the plants get as much light as possible.
Be careful that leaves touching the glass during cold nights don't freeze. The amount of covering that is needed, naturally depends on the outside temperature.
On the top of the covering, it might be wise to have some plastic sheet, or the like, against rain and melting snow.
This stage demands most care. The seedlings need light not to etiolate (grow thin and weak). A rule of thumbs for successful growing is 'cool head and warm feet'. That is exactly what you get in a hotbed. Give the plants an airing as often as possible, and don't forget careful watering. We use to take away the insulation over the lid in early morning, replacing is when the sun goes down. It is better for the plants that the air is somewhat too cool in the air than that they suffer from lack of light. If it is very cold any singular day, you can leave the cover on, but remember that the plants don't easily recover from etiolating.
When the weather gets warmer, you need to increase the airing. Start with a tiny chink and increase it successively, but avoid blowing. Surprisingly early, you can take away the glass lid on sunny days.
Don't forget the watering. All composing is based on the fact that dry, cool air is drawn in from below, and warm, humid air leaves the compost from the top. By that, the compost is steadily losing water, and this must be replaced.
Also the spinach, sown in the end of February has small leaves at this time, and the Basil (Ocimum basilicum) has made some small leaves. The Basil will stay until fully grown in the bed, to take advantage of the sheltered environment there. Even if the glass is taken away, the stone rim protects the plants against wind.
It is a good idea to combine fast growing early vegetables
with those that grow somewhat slower. By that, the bed can be used fully
without any slacks in the harvest.
When the composting process in the bed slows down and it cools off, the bed can be considered as an ordinary cold frame, and used like that. The plants left in the bed, like squash, can look forward to a summer with a very fertile soil.
Most of what is growing in the frame at this time is quite large, and it is not possible to keep the lid on. On the picture, you can see squash, basil, tomato, cos (romaine) lettuce, kohlrabi, dill and spinach.
In the background our row house. The hotbed is situated in
a very small garden, only about 100 m2 , to
which it adds on a lot of produce.
The only time the neighbours might be annoyed by the odour of horse manure (and they aren't) is during the 2-3 hours in February the bed is filled. This is later replaced with the scent of warm soil and verdancy.
.. but the tomatoes are left in the bed for some more weeks.
November - December
In late autumn, you have a thick layer if the richest soil you can get to put out to the other plants in the garden. Note the celery plants in the foreground, sown in Mars and harvested until Christmas.
After that, it is only a few months until you can start over again!