The True Facts About Cold Frames and Hotbeds

cold_frame_greenhouseA great way of extending the growing season or getting a jump on the garden is by using a cold frame.  But are you aware that a cold frame and hotbed is the same thing?  If not, come join me in the world of cold frames, hotbeds and sustainability.

Whether you are talking about Outdora’s Redwood Cold Frame Starter Box Greenhouse or the Solexx Deluxe Cold Frame, the general definition is the same.  A cold frame is solely heated by the sun and is normally used for cool-season or Cole crops.  A hotbed is artificially heated in some way and is typically used for warm-season crops, such as tomatoes, peppers, and eggplants.

Both of these structures can be directly planted into or seeded flats can be placed inside each.  To maximize the amount of sunlight that is typically available during the spring and fall when these structures are used, the cold frame/hotbed is normally painted white or silver on the inside.  Both of these colors reflect the light back onto the plants, which aids them in the photosynthetic process.

When sitting up a cold frame or hotbed the location is very important.  The front of the structure needs to face the south. This will allow for the maximum amount of sunlight to hit the structure.  Also make sure that the top can freely open.  This allows for ventilation of the structure along with a way of getting to the plants for picking and maintaining.

Regardless of whether you are going to use your mini greenhouse as a cold frame or hotbed, it is important to decide if you are simply going to plant in the soil or in flats.  A good quality garden soil will work if you plan to plant in either structure.  Another approach is to create your own soil.  This is easy to do and only requires a few ingredients.  For a bushel of soil, you will need ½ bushel shredded sphagnum moss, ½ bushel vermiculite, dolomitic limestone 5 tablespoons, 20% superphosphate 2 ½ tablespoons, 5-10-5 7 ½ tablespoons, iron NaFe 138 ½ teaspoon, and ½ teaspoon of dish soap in ½ gallon of water. Mix thoroughly and place inside structure.  Seed as usual.

Hotbed Construction

If you want to create a hotbed, a heat source will be required.  There are several different types of heating systems that can be used.  These heating systems can be very simple to very complex.  The type you choose depends on your time, expertise, and budget.

The simplest type of heating source consists of plastic milk jugs painted black.  These painted milk jugs are then filled with water.  As the sun hits the milk jugs, the dark color absorbs heat that is transferred to the water.  When these jugs are placed in a hotbed, they will release their heat during the night and absorb additional heat during sunny days.

This technique should only be used to take the chill off.  The amount of heat produced is not enough to keep plants from freezing in cold climates.

The next heat source comes from manure.  This type of hotbed is called a pit hotbed and consists of digging a pit that is 16 to18 inches deep.  Then, lay a 4-inch layer of gravel in the bottom.  On top of that, place 8 to 14 inches of manure and pack down.  Top this manure layer with 3 to 4 inches of weed-free garden soil or homemade soil, as described above.  Then, place the hotbed frame on top of the pit.  Push two thermometers into the soil about 2 inches deep and monitor the temperature.  When the temperature reaches 75 degrees F, the bed is ready to plant.

The third heat source comes from an insulated heating cable.  This type of hotbed is referred to as an electric hotbed.  When utilizing this technique, one will need to dig down into the soil about 2 to 3 inches.  Then, lay 4 inches of gravel over the surface and smooth out.  On top of this gravel place the electric cables.  You will need about 2 feet of cable per square foot of bed.  Cover the cable with a wire mesh and then top with 4 to 6 inches of soil.

Plug the cable up and let the soil prewarm before planting.

Another heat source is through a flu that is created by burying a steel barrel along the side of the hotbed.  A door is cut out on one end and two heat-conducting pipes are attached to the other.  The pipes are then sent under ground around the hotbed.  The heat radiates to the surrounding soil.  When using this approach, always remember to preheat the area 3 to 4 days before plantings.

The last way of heating a hotbed consists of using an existing greenhouse heating system, such as steam or hot water heating.  Steam from the circulating system is funneled into the hotbed through buried pipes underneath the hotbed.

Cold Storage

While cold frames and hotbeds can help one extend the garden season, a little known fact about these structures is that they can be used for cold storage of certain vegetables.  Heat is not required for this technique and starts out by removing 12 to 18 inches of soil inside the structure.  A layer of straw is then placed on the soil.  Vegetables, such as turnips, rutabagas, beets, and carrots, are then placed on top of the straw in a single layer.  Top the vegetable layer with a thick cover of straw.  Place the top on the structure and then cover with a tarpaulin.

Utilizing this technique should keep these vegetables fresh all winter long.

Cold frames and hotbeds are a great addition to any garden space.  And with added heat, they can help you grow vegetables year round for the health and sustainability of you, your family, and your community.

Mindy McIntosh-Shetter has been an Agricultural Science educator, and is a horticulture and/or environmental blogger who earned a degree from Purdue University in Agriculture Education with a minor in biology, and natural resources. Presently she is finishing up her Masters in Environmental Education and Urban Planning for the University of Louisville while working on her own agriculture/environmental blog.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.