Creating and planting a hotbed

In a hotbed, warm temperatures can be created even in winter, so you can start growing your first vegetables there – without the need for heating or a greenhouse.

Hotbeds produce heat through a rotting layer of manure. This allows vegetables to be grown and harvested earlier. In this aarticle, we explain how to properly create a hotbed, why horse manure is best suited for these cold frames and how to plant the hotbed.

Hotbed: explanation and benefits

Hotbeds are cold frames with built-in natural heating. So even those without a greenhouse can start growing vegetables early in the year. The heat generated by a hotbed is due to its structure, because there is a thick layer of fresh animal manure below the soil. There, a composting process takes place in which bacteria decompose organic compounds in the manure. These processes generate heat, which acts as a heater for the vegetables above.

Many vegetables need heat for good development, which is provided by the rotting manure at low cost and without additional energy input. Another advantage of a hotbed is the availability of nutrients. Vegetables that are in hotbeds year around draw nitrogen and other important nutrients from the compost layer, which means you have to use less fertiliser. Finally, in most cases, when disassembling the bed in autumn or winter, you end up with good, mature compost that can be reused in the garden.

Creating a hotbed: instructions

February is the optimal time to create a hotbed for growing vegetables. Basically, there are two different methods of building a bed like this. There are ground-level hotbeds, where the layer of manure is in the ground. Alternatively, opt for a raised hotbed.

You do not need a lot of material to properly create a hotbed:

• A cold frame or cold frame attachment for your raised bed
• Wire mesh
• Fresh manure
• Growing or planting soil
• For ground level hotbeds: Shovel and metre stick
• For raised hotbeds: Coarse material such as branches or wood logs; possibly garden fleece
• Optional: Thermometre

The right manure

Horse manure is most suitable as it generates the most heat. An important factor in composting is the so-called C/N ratio, i.e. the ratio of carbon to nitrogen in the starting material. Ideally, this should be about 30 to 1. Pure horse manure has a C/N ratio of about 20 – 35 to 1, so it is actually perfect. Straw, on the other hand, has a very unbalanced C/N ratio of up to 125 to 1. Therefore, make sure that your manure does not contain too much straw, as this would result in a slower composting process and consequently less heat generation.

Different types of manure differ in their nutrient content. This leads to different heat generation and different suitability for hotbeds. Chicken manure, for example, contains so much phosphorus that roots can be damaged by contact. It can still be used for hotbeds, but you should pay special attention to ensuring a thick layer of planting soil over the manure.

Manure from rabbits and guinea pigs produces sufficient heat and can also be used for hotbeds. However, you need a considerable amount, because the layer of manure should be at least 40 cm high.

Before you get started building your own hotbed, a suitable location should be selected. This is ideally south-facing with sun for most of the day.

Creating a hotbed in the ground

1. Dig a trench at least 60 cm deep. The length and width should match the dimensions of your cold frame.
2. As protection against moles and other critters, the bottom and walls of the trench can be lined with mesh.
3. Next, fill in a 40-50 cm layer of manure and tread it down. However, do not overdo it when solidifying, because oxygen and thus good aeration are essential for good composting.
4. The manure should ideally have a moisture content of 40-60%. So it may be necessary to water it a little. Use warm water for this purpose. This gets the composting going until the temperature can be maintained on its own.
5. Afterwards, the manure should be covered with a fleece, for example, to prevent it from cooling down.

Tip: The moisture content of the manure can be roughly determined using the “squeeze test”. To do this, take a handful of manure and squeeze it tightly in your hand. For this you can put on disposable gloves. If the material falls apart when you open your hand, it is too dry. If, on the other hand, water runs out between your fingers, it is too wet. The manure has a good moisture content if the lump retains its shape when you open your hand, but still breaks easily into smaller crumbs.

6. Wait a few days for the composting process to begin. Then it is time to spread the planting layer where your vegetables will grow.
   If it is very cold, the composting process may not start and you may not notice any heat development yet. Even then, pouring warm water over the manure can help.
7. What substrate you should use for the planting layer depends on the use:
   • For seedlings or weakly growing herbs: A rather nutrient-poor growing soil such as our Plantura Organic Herb & Seeding Compost is suitable.
   • For ripe vegetables: Here, a nutrient-rich soil such as our Plantura Organic Tomato & Vegetable Compost should be used. If your garden soil is of good quality, you can also use the excavated soil.
8. The planting layer should be at least 15 cm high. This is especially important if you want vegetables to grow to maturity in your hotbed – because fresh manure can damage roots. For deep-rooted vegetables such as carrots, the layer should be at least 20-30 cm high.
9. Cold frames are best as a cover for your hotbed.

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Creating a raised hotbed

Creating a hotbed in a raised bed differs from the procedure for ground-level beds in only a few respects:

1. Mesh should also be laid at the bottom as a barrier with the ground.
2. This is followed by an approximately 30 cm high aeration layer of coarse material. This can be, for example, branches, logs and brushwood.
3. We recommend putting garden fleece on top. This prevents material from the upper layers from falling between the branches.
4. Now it is time for the manure. Spread it about 40 cm high in your raised bed. Watering may also be necessary to achieve the correct water content.
5. Then wait a few days until the material has settled somewhat.
6. The manure layer is followed by the planting layer, which should be at least 15 cm high. This is created as described above under point 5.
7. Finally, the cold frame can be attached to your raised bed.

Tip: The better the hotbed is insulated, the higher and more constant temperatures can be achieved. Therefore, it helps to seal all cracks, for example, with straw, fleece or jute bags. You can also put bubble wrap on the outside.

Planting in your hotbed

About a week after you have finished filling the hotbed, you can start sowing. It is helpful to first test the bed with a thermometer, because the temperature should be at least 12 °C to 15 °C.

In principle, any vegetable type is suitable for planting a hotbed. For the first sowing in February, for example, you can rely on butterhead lettuce (Lactuca sativa var. capitata), kohlrabi (Brassica oleracea var. gongylodes L.), rocket (Eruca sativa and Diplotaxis tenuifolia), radish (Raphanus sativus var. sativus), spinach (Spinacia oleracea), celery (Apium graveolens), or carrots (Daucus carota subsp. sativus).

Then, starting in March, you can sow tomatoes (Solanum lycopersicum), cucumbers (Cucumis sativus), aubergines (Solanum melongena), and cabbage (Brassicaceae) in the hotbed.

In order for plants to grow well in a hotbed, there are a few things you should keep in mind:

• Water regularly: Rain and dew are kept away by the glass panes. Therefore, do not forget to water the plants even in rainy, humid weather.
• Ventilate daily: Open the cold frame for at least a few minutes a day to allow air exchange. However, be careful especially with freshly germinated young plants – the sudden onset of cold weather can permanently damage them. Therefore, at best, ventilate only at temperatures above 5 °C.
• Prevent freezing: On cold nights, it helps to spread a blanket over the hotbed to prevent the temperature from dropping too much.
• Avoid overheating: Even if it is still cold, it can quickly become too hot under the glass panes on sunny days. As soon as the temperature in the hotbed climbs above 22 °C, it should be ventilated.