How to make a hot bed?

Once overshadowed by heated shelters, the hotbed is making a grand comeback in our organic vegetable gardens.

And for good reason: this cultivation technique, which requires no energy expenditure, creates an environment conducive to the healthy growth of your outdoor plants, even in the depths of winter. A space with nutrient-rich substrate and ideal temperatures, ranging between 15 and 25°C.

In the following lines, discover why, when and how to create a hotbed.

Why create a hotbed?

Providing a warm, nutrient-rich environment, the hotbed can be used in three different ways:

1. As a nursery
   When temperatures are still very low, the hotbed aids the development of your vegetable seedlings. Once its temperature stabilises between 20 and 25°C, you can place your seed trays and pots (previously started indoors) inside. It’s also possible to transplant young seedlings directly into this bed.

2. For early crops
   The hotbed can also be used to accelerate your crops, allowing you to harvest certain vegetables earlier. Early varieties of carrots, turnips, radishes, spinach and lettuces can be grown here as early as January.

3. For the most demanding vegetables
   Finally, the hotbed allows you to grow the most nutrient-hungry vegetables. The decomposition of organic matter in the hotbed creates a similar process to composting. The resulting rich substrate is ideal for growing your aubergines, tomatoes or peppers.

When to create a hotbed?

The hotbed should be ready by January to accommodate your early vegetable seedlings. Since you can only sow seeds three weeks after its installation (once the temperature under the frame has stabilised around 20 to 25°C), you should prepare your hotbed around mid-November.

How to make a hotbed?

Although there are two distinct techniques, the general methodology for creating a hotbed remains the same: you must define a cultivation area before covering it with a glass frame, which is then filled with a mixture of manure and plant waste. It is this decomposing organic matter that generates heat.

Thus, the hotbed can be sunken or raised, and must be covered with a frame box 80 to 90 cm high. In both cases, the organic matter is covered with 15 cm of compost, which forms the substrate for sowing or transplanting your vegetables.

Creating a sunken hotbed under a frame

The sunken hotbed provides good thermal inertia. In fact, it is superior to the raised hotbed. However, its installation is more complex and requires more effort.

Start by selecting the "ingredients" for your hotbed. Fresh horse manure mixed with straw, dead leaves or BRF (ramial chipped wood) is generally recommended here.

Note: Sheep or rabbit manure can be used as a substitute for horse manure.

1. First, determine the size of your hotbed based on your needs and the dimensions of your frame.
2. Dig a pit 50 cm deep using a spade.
3. Place the frame of your hotbed at the bottom of the pit.
4. Add your mixture of organic matter to the pit.
5. Trample the mixture with your feet. This crucial step removes air pockets and promotes fermentation.
6. Water this first layer evenly with a hose.
7. Cover everything with a thick layer of compost, about 15 cm deep. The substrate level should now be level with the ground.
8. Attach the glass to your frame and close it.
9. Now, wait about ten days without disturbing your hotbed.
10. After this period, rake the surface of the hotbed, then level it with a flat board (without compacting it).
11. You can now proceed with sowing or transplanting your indoor-started vegetable seedlings.

Creating a raised hotbed under a frame

The raised hotbed has the advantage of being more comfortable, as cultivation work is done at waist height. Similarly, its installation is much simpler since you don’t need to dig a pit. This technique is preferable if you want to avoid constant bending while gardening.

1. At the bottom of the frame, add a 30 cm layer of organic matter composed of a mixture of horse manure, leaves, grass, straw, or green waste chippings.
2. Compact the mixture by trampling it with your feet and water it.
3. Cover the mixture with a thin layer of soil and mature, sieved compost.
4. Wait about ten days for the initial heat surge to subside.
5. Rake the surface of the compost and level it without compacting, using a flat wooden board.
6. You can now proceed with sowing.

Solenne’s tip: After sowing, keep the frame closed unless the weather is sunny and outdoor temperatures reach 20°C. Your plants will still need water, so water lightly without washing away the substrate, then close the frame again to protect your crops from cold and pests (especially cats, who may mistake your compost for their litter box).