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Unlock Winter Harvests: How to Make a Garden Hot Bed for the Earliest Spring Crops (Complete Step-by-Step Guide)

Garden hot bed

Embarking on a gardening experiment this winter, I discovered the incredible potential of a garden hot bed.

In this article, I’ll share my personal experience in setting up a garden hot bed. Using the heat generated from decomposing organic matter, I created a cosy microclimate for my plants, defying the chill of winter.

I’ll walk you through the entire process, from the initial setup to the maintenance, and show you step-by-step how I transformed a part of my garden into a warm sanctuary.

Key Takeaways

  • Extended Growing Season: Hot garden beds allow year-round cultivation, even in winter.
  • Simple Construction: Setting up a hot bed garden involves using compostable materials and a well-insulated structure.
  • Ideal Growth Conditions: Creates a warm microclimate, perfect for seed germination and growth.
  • Early Sowing Advantage: Start spring crops as early as January for a head start.
  • Grow a Second Crop: Early spring crops can be followed by nutrient-hungry summer crops.
  • Sustainable Year-Round Use: Repurpose the hot bed after the growing season for continued garden productivity.

What is a Garden Hot Bed?

A hot bed is a gardening technique that uses natural heat to create a warm growing environment. Hot beds are filled with layers of decomposing organic matter, like straw and manure. As these materials break down, they generate heat, raising the temperature in the bed.

This warmth is exactly what your plants need, especially during colder months or when you want to give them an early start. The decomposition process provides a steady source of heat, making hot beds an eco-friendly and cost-effective way to grow your favourite crops.

Hot beds can be created using different materials, but horse manure is the most common choice due to its high nitrogen content, which helps produce the most heat.

Building a hot bed is quite simple: start with a layer of manure, followed by straw, and repeat until you’ve reached your desired height. Lastly, cap it off with a layer of soil or compost in which to plant your seeds.

Building Your Own Garden Hot Bed

Here’s a step-by-step guide to building your very own hot bed.

Where To Place a Hotbed

The location of your hot bed is crucial. It can be set up outdoors, directly in your garden, or inside a polytunnel or greenhouse for added protection and heat retention. The key is to choose a spot that receives ample sunlight during the day and is shielded from harsh winds.

Inside a polytunnel or greenhouse, the hotbed benefits from an additional layer of insulation, making it ideal for extremely cold climates.

Positioning it near the house can also be helpful if you want to avoid trekking down the garden on the coldest days.

For me, however, it had to be at the far end of my garden: this is the spot that gets the most sun during winter.

Materials for Your Hotbed

The materials you choose for the edges of your hot bed impact its heat retention and durability. Options include:

  • raised beds,
  • bricks,
  • stones, and
  • reclaimed concrete.

Materials like bricks and stones are particularly effective as they naturally retain heat.

For my hot bed, I decided to use what I had available: a galvanized metal raised bed, 60cm high.

Constructing Your Hotbed

When building your hotbed, choose a design that is tall and skinny rather than wide and shallow. A height of at least 80cm is recommended to generate enough heat. My raised bed is slightly shorter, but fear not, I have a secret plan!

For easy access, your hotbed should not be wider than 120cm (4ft).

For my experiment, I used a rather small bed: 1m x 60cm. This is dictated by the size of my cold frame that will need to sit on top.

Filling Your Hotbed

Horse manure is ideal for filling a hotbed

A hotbed is essentially a compost heap, but with 3 key differences:

  • a hotbed is built at once, instead of adding materials gradually throughout the season,
  • it contains plenty of nitrogenous materials that help generate a lot of heat,
  • it is topped with a layer of soil/compost mix in which to grow.

Traditionally, hotbeds are filled with layers of horse manure and straw. Horse manure is very high in nitrogen, which is why the bed heats up so fast.

To fill your hotbed, the first step is to create a blend of manure and straw, using alternating layers.

If you live near horse stables, you can easily obtain fresh manure, as stables are usually happy to give some away. However, I encountered my first hurdle: my garden is located in central Cambridge, with no easy access to fresh horse manure!

Luckily, other materials can be used instead to increase the nitrogen content of your pile:

  • chicken manure,
  • grass clippings,
  • coffee grounds,
  • comfrey leaves,
  • nettles leaves,
  • pee.

These materials need to be layered with “browns” such as:

  • wood chips,
  • garden prunings,
  • dried leaves,
  • straw.

The idea is to utilize organic materials that you have readily available.

These materials will decompose over time, generating heat that warms the bed.

For my hotbed, I used a mixture of dried leaves and garden prunings, along with grass clippings, comfrey leaves, and chicken manure generously provided by my friends who raise chickens.

Compressing Hot bed Materials

To generate consistent heat, it’s essential to compress the materials once you have filled your bed. The compression process helps to initiate and maintain the decomposition process.

I used my rake to tamp down on my pile, aiming for a depth of 60cm of compressed material. This technique will help in increasing the heat generation.

Topping Your Hot Bed With Growing Medium

Once your hotbed is filled and compressed, top it with a 20-30cm layer of soil and compost mix.

This mix serves as the growing medium for your plants. It should be fertile and well-draining.

Creating a Cover For Your Hotbed

Finally, I covered my hot bed with a cold frame. This step is important to conserve the heat. This cover can also be an old glass window, a mini plastic polytunnel or greenhouse.

The cover should be easy to remove or open for ventilation on warmer days, ensuring that your plants do not overheat.

It can be useful to further isolate the edges of your hotbed, to seal any gaps between your cold frame and your raised bed. Straw is perfect for that.

No Raised Bed? Go Underground!

If you don’t have materials for a raised bed, you can still create a hotbed by installing it in the ground.

To do so, begin by digging a trench, at least 60 cm deep, that matches the size of your cold frame. Fill the trench in the same way as a raised bed: 40-50 cm of your manure and straw blend, topped with 20 cm of compost and soil mix. Place a cold frame over the filled trench.

And there lies my secret weapon: since my raised bed is a little short, I extended it underground by digging a shallow trench (approximately 20cm), before filling it with my decomposing materials.

Using Your Garden Hot Bed

That’s it, we’ve built a garden hot bed! Here’s how I make the most of it throughout the growing season:

First Check the Temperature

Monitor the temperature just below the growing medium, about a foot deep, with a soil thermometer. As soon as you notice the temperature stabilizing for a few days or starting to drop, it’s time to start sowing your seeds!

Using Your Hot Bed To Grow Early Spring Crops

The hot bed is perfect for growing early spring crops like beets, carrots, lettuce, other salad leaves, onions, spinach, chard, and radishes.

Greens are amongst my favourite crops, so this is what I initially used my hot bed for. I have successfully sown a variety of salad leaves such as lettuces, rocket, claytonia and pak-choi as early as February, about a month earlier than usual!

Using Your Hot Bed As a Propagator for Extra-Early Sowings

A hot bed is also useful as a propagator for starting seeds early in the season. The warmth it generates creates an ideal environment for germination and early growth. I’ve found it ideal to give a headstart to heat-loving plants such as tomatoes, peppers, aubergines and melons.

Growing a Second Crop

After harvesting your early spring crops, the hot bed’s heat will have diminished slightly, but it’s still a valuable resource.

This is the ideal time to plant tender and nutrient-hungry crops such as tomatoes, squashes, zucchini, cucumbers, and even melons. These plants will thrive in the warm, nutrient-rich environment that the hot bed provides.

Discover the 9 Best Vegetables to Grow in Raised Beds (Plus Your Garden’s Best Kept Secret!)

At the End of the Season

As the growing season draws to a close, you have a couple of options for managing your hot bed.

You can keep it as a raised bed, continuing to grow plants in it as you would in any other part of your garden.

Alternatively, you can spread the contents of the hot bed onto other beds around the garden. This enriches the soil, feeding next year’s crops.

Then, you can refill your hot bed with fresh materials to restart the cycle for the next season.

Using your garden hot bed effectively allows you to extend the growing season, maximize your garden’s productivity, and enjoy a diverse range of crops that would otherwise be impossible to grow during certain times of the year.

Historical Perspective

Back in time, hotbeds played a significant role in gardening. The Arabs, living on the Iberian Peninsula (Spain and Portugal), were among the first to create true hotbeds for early forcing of vegetables. They used donkey, horse, and occasionally pigeon dung to construct their raised hotbeds, usually about two feet in height.

During the Victorian era, gardeners developed hotbeds as a solution to provide heated growing conditions for delicate crops like melons and cucumbers. Victorian gardeners often had grand-scale hotbeds in large walled gardens of their country houses, with an army of inexpensive workers tending to the gardens. Sir John Griffin Griffin enclosed Lady Portsmouth’s kitchen garden with a wall around 1766. He constructed heated glasshouses, producing exotic flowers and fruits like melons, pineapples, grapes, and figs throughout the year.

Frequently Asked Questions

What is the purpose of a hot bed garden?

A hot bed garden is designed to create a warm environment for plants during colder seasons, allowing you to grow vegetables and other plants earlier in the year. It generates heat through the decomposition of organic matter, like manure.

How do I build a hot bed for my plants?

To build a hot bed, start by building a raised bed about 2-3 feet deep. Fill the bed with layers of fresh manure and straw, then add a 4-6 inch layer of soil mix or sifted compost on top. You can plant your seeds directly in this soil, which will provide them with the warmth they need to thrive in colder temperatures.

What are the benefits of hot beds compared to cold frames?

Hot beds produce a warmer environment for plants than cold frames, making it easier to grow heat-loving plants even in cold weather. They also generate natural heat through decomposition, which means no additional heating sources are needed. On the other hand, cold frames help protect plants from frost and wind but do not provide the same level of warmth as hot beds.

How can I heat my raised garden beds?

To heat your raised garden beds, you can use a variety of methods such as adding decomposing organic matter like manure or compost, using heating mats, or installing a greenhouse structure to trap and retain warmth.

There are many hotbed designs to choose from, but some popular choices include a basic trench filled with manure and covered in soil, a more elaborate structure made of wood or bricks, and an insulated hotbed for added warmth. You can also convert a cold frame into a hot bed by adding a layer of decomposing organic material.

How long do hotbeds typically last?

The lifespan of a hotbed depends on the materials used and the level of maintenance. Generally, a well-constructed hotbed with a layer of manure can provide warmth to your plants for about 3-4 months as the material decomposes.