Making a Garden Hotbed | Benefits, Plants To Grow & How-To

If you live in an area with a limited growing season, implementing various methods to extend your season can significantly affect your gardening achievements.

Building a hotbed is a great way to begin your seedlings before the last spring frost or continue growing into the fall and winter seasons. 

What is the purpose of a hotbed? The purpose of a hotbed is to utilize natural methods of heating to extend your growing for longer than the seasons regularly allow. Natural thermal reactions that occur from decomposing manure or compost keep the soil warm enough to avoid freezing temperatures. 

If you’re interested in the basics of a hotbed but think that it’s too complicated, this article will prove the opposite!

Continue reading for details on growing in a hotbed, how you should build your hotbed, and the hotbed’s most important uses for growing!

Garden Hotbed – The Basics

Many times hotbeds are used when the seasons begin to show unfavorable temperatures.

Utilizing the natural heat source composting manure provides to help insulate your seedlings or semi-hardy plants can be your best option concerning freezing temperatures.

They are easy to build and can be utilized season after season for growing in those colder months.

Difference Between Hotbed and Cold Frame

There are a few differences between a hotbed and a cold frame in terms of their construction and their use.

Cold frames are built-in planters that utilize the sun to keep crops warmer during the cooler months.

A hotbed, on the other hand, has a source of heat other than the sun and is best utilized for germination and plants that must be hardened off but require heat to keep them alive. 

Garden Hotbed Pros & Cons

Hotbeds are great for that bumper season of growing when there is still enough sunlight but the temperatures are too varying to continue growing outdoors without protection.

Advantages of a Garden Hotbed

  • Stabilize the temperature, reducing fluctuations in the overall growing and soil temps. 
  • Allow for a greater growing window.
  • Utilize composting manure as a cheap heat source.

Disadvantages of a Garden Hotbed

  • The overall cost and time to build.
  • Will provide a comfortable environment for critters to make a home in when the season gets cold. 
  • Not as useful during the hot summer months. 

How Big Should a Hotbed Be?

There are numerous people who have built hotbeds approximately 6 feet wide on each side and others who prefer 6 feet by 3 feet wide.

This is completely customizable to your space or preferences, but just keep in mind that the larger and deeper the bed, the more challenging it can become to reach the center and maintain it. 

How Deep Should a Hotbed Be?

Since the amount of compost you will need is significant, you’ll need to ensure you have ample space to place it.

Build a hotbed with at least 1 to 2 feet of space for compost to fit under your hotbed. At a minimum, your bed should be 3 to 4 feet deep to include room for plants and the compost bedding.

Garden Hotbed Temperature

The composting manure will generate heat between 140℉ and 155℉ at the center.

This will significantly warm your growing environment, so you must be sure to monitor the heat in your hotbed so you don’t fry your plants on a warm day!

How Warm Do Hotbeds Get?

The interior temperatures of your hotbed can fluctuate significantly. Depending on your heating source as well as the temperatures outside, your hotbed should reach temperatures between 65 and 150℉.

The soil will remain cooler but can still reach temperatures of 90+ degrees. It is recommended to wait until the soil cools to 85℉ before planting.

Can You Use a Hotbed in the Summer?

Hotbeds can be utilized in the summer for certain crops that prefer higher humidity, but they can still get too hot.

It may be necessary to stop adding composting manure and just use the hotbed as an unheated bed to grow small crops. 

Can a Hotbed Get Too Hot?

Yes, a hotbed can reach a temperature too hot for your plants to find advantageous. Most plants will find the high heat of an unvented hotbed too extreme and suffer or die.

Cold season crops prefer temperatures to not exceed 75℉ but find it most suitable at 50-60℉. 

Do Hotbeds Protect From Frost?

Since frost will only last on a surface that is of freezing temperature, it will not accumulate on plants in a hotbed.

The plants and ambient temperature will be greater than freezing, so your plants will not be affected by frost. 

Can You Use Hotbeds in the Winter? 

Hotbeds can be utilized in the winter for semi-hardy plants that require warmth.

It can be tricky sometimes to get ample heat from your composing bed if in an exposed area, but most microbes will still continue to do their job at lower temperatures.

You can also place a heat lamp inside to keep the temperature warm enough to allow the manure to continue its composting. 

Plants for Hotbed

Plants that will thrive in a hotbed environment include any seedlings or starts for plants since they require extra warmth and humidity.

Other crops like lettuce, spinach, broccoli, cauliflower, bok choy, beets, and other greens are great candidates for hotbeds. 

How To Build a Hotbed

Building a hotbed is easy! It should take no longer than a weekend, but it will lead to a much longer growing season. Check out the list of materials below and a step-by-step guide to building a hotbed!

Materials 

  • Cinder blocks or bricks
  • Rebar
  • Concrete or mortar (optional to make it a permanent structure)
  • Roof material (translucent plastic, plastic roofing)
  • Hay
  • Animal manure or used animal bedding

Steps To Build a Hotbed

  1. Create a plan. Most hotbeds are 2 to 3 feet high and wide while being anywhere from 3 to 6 feet long. It can be made into any shape you desire as long as the walls are 2 to 3 feet tall. 
  2. Lay out your cinder blocks or bricks in a rectangular shape on the ground in the location you will build the hotbed. You’ll want to locate an area with southern exposure to help heat the box naturally.
  3. Dig a trench to set your bricks into to help create a sturdy foundation. 
  4. Stack your cinderblocks or bricks into a wall in the shape of your hotbed. 
  5. Pound rebar into the ground to keep the bricks aligned. If you wish to make a permanent structure, use concrete or mortar to build permanent walls. 
  6. Mix the manure, hay, and bedding to create a compost layer at least 1 foot thick. Then lightly wet the material throughout.
  7. Monitor the composting mixture. If it begins to warm and its composting is causing a thermal reaction, you’ve created good bedding. If it does not begin to heat up over a few days, mix in more manure.
  8. Once you have a manure layer that is heating, fill your hotbed foundation with the mixture at least 1 foot thick. 
  9. Flatten the manure and place landscaping fabric over the compost.
  10. Place your seedlings inside and allow the bed to work its magic!
  11. Add the roof to keep the heat in. Be sure to open it every so often to allow oxygen flow and regulate the temperature. 

Maintaining a Garden Hotbed

Maintain your hotbed garden just like a regular gardening plot with a few additional tasks.

You’ll need to ensure that the interior temperature is not too high and the air does not become stagnant, so it will be necessary to vent the hotbed occasionally. 

You’ll also want to keep an eye on the composting manure. It will break down over time and stop heating the bed.

Depending on the mixture and activity of microbes, you’ll need to change the bedding every 2 to 4 months.  

Since the bed is heated, plants will use more water at a higher temperature. Be sure to always check your plants to ensure the hotbed hasn’t become too hot and dried your plants quickly. 

Using a Hotbed for Seedlings

The seedling stage requires the most warmth to help seeds open and encourage root growth, but remember to allow the soil to cool slightly (85℉) before planting.

Use the hotbed early in the season while frost is still an issue to get your transplants started and ready to plant outdoors once temperatures reach a comfortable growing level.

Using a Hotbed for Greens in Winter

Greens prefer cool temperatures and usually germinate well even with chilly temperatures. They will be your best bet at getting a solid harvest during the winter.

Lettuce, spinach, bok choy, kale, and some cabbages will all grow well into the winter. The crops will take longer to mature due to a lack of sunlight and may require additional supplemental lighting to grow. 

Related Questions: 

How Long Does a Hotbed Last?

Depending on the materials you use, your hotbed can last 10-15 years if you build it with quality materials. If you use wood rather than bricks, it will deteriorate quickly, perhaps lasting only a few years. 

What Is the Best Manure for a Hotbed?

The best manure for hotbeds is horse manure and bedding. It will consist of hay, manure, and urine from the horses and will help to produce a thermal reaction upon decaying.

Other mixtures of livestock manures with hay also produce decent thermal-heating compost mixtures. 

Conclusion 

Hotbeds are a great way to grow in the colder months and supply some extra heat naturally. It’s also a great way to compost and reuse bedding from animals.

Give it a go, and create your own hotbed to get your seedlings the warmth they need and your growing season off to a great start!