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Best 5 Ways to Warm a Cold Frame

Avatar for Jodie Weston Jodie Weston  |  Updated: June 12, 2018
Best 5 Ways to Warm a Cold Frame

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Where I live in California, the growing season is generally from mid-March until Mid-October. This has more to do with the intensity of the light which shifts around the fall equinox. However, of equal importance is the temperature, which in winter can drop to 10°F – just cold enough that many things freeze.

We are quite lucky here that we do not have harsh winters and that our natural growing season is quite long. With a little help, we can actually grow a garden year-round and a true cold frame, for us, is generally enough to ward off the damage from our winters. However, there are times when some added warmth is necessary, even in this warmer climate. In this blog we discuss five ways to warm your cold frame.

Cold Frames vs Greenhouses

A cold frame and a greenhouse are not the same thing. The big difference is in the quality and predictability of the interior environment. A greenhouse is designed to offer the grower far more control over the interior environment than is a cold frame. In really cold areas, a greenhouse is often more appropriate than is a cold frame. There are other differences too.

A cold frame is a basic structure that helps to shield plants from the worst of weather, but not much more than that. It will stop wind damage, some freezing damage, and plant damage from driving rain and hail. Will it protect your plants from the harshness of winter? Maybe.

cold frame

The idea of a cold frame is basically as a tool that extends the growing season for a short time. Maybe, if you are lucky, you will gain an extra month of growing time in the fall. They are generally not great for starting plants early in the spring as they only raise the outside temperature a few degrees – generally less than 10 degrees and seeds need a warm area to germinate and young seedlings need a specific soil temperature to thrive.

An old window over a wooden box is great for starting seeds 1-4 weeks before your garden is hospitable enough to sustain them. That is the perfect idea of what a cold frame does. It extends your start and growing season a few weeks or maybe a month.

Focus on Need

If you are planning on raising the interior temperature of a “grow building” more than 5 – 10°F then you really need a greenhouse rather than a cold frame. With that being said, there are some tricks that you can use to warm a cold frame, but in general we are talking about a couple of days at the most.

Also, some improvements in design can help offset shifts in temperature. Cold frames also come in a wide variety of sizes and styles. Not every trick listed here will work for your cold frame.

The Sudden Cold Snap – If a cold frame has worked well for you in the past, but the weather is calling for a sudden cold snap, then you can take some actions to help ward off the cold. These situations tend to be short in duration and the solutions are not suited to the long-term as they are not cost effective for more than a week or so.

  1. Add heat – literally. There are a number of ways to safely add heat to your cold frame. A small earthen oven with a smoldering fire can warm your cold frame. The idea is to build a small brick box with a small opening for smoke to escape and a small opening by which the unit draws air. In the box you would build a smoldering fire much like you would in a traditional smoke house. As the fire smolders, it warms the bricks which then radiates heat. You can either let the smoke meander about the cold frame as it too adds warmth or vent the unit so that the smoke can escape. Remember that where smoke can escape, so can heat. This is not an open fire, but a bed of coals that burn gently for a long period of time. You will need to tend the fire periodically and because we are talking about fire, you will need to make sure that the unit is safe.

fire wood

If your cold frame is large, then consider installing an old wood stove complete with a chimney.

Wood heat is probably the most affordable and certainly more available during a disaster. I also like that wood heat is fairly simple. You could even use a BBQ or Hibachi

  1. Gas Burners – Propane mounted burners can also be useful to heat a cold frame, but they are going to be costly to operate for any length of time. The plus side to these is that they are ready at a moment’s notice and while they require some safety considerations, they do not need to be tended to as often as a smoldering fire.

gas burner

I also like that there are a variety of styles of these burners available so you have a wide range of options that allow you to pick the best one to fit your growing needs.

  1. Electric Heat – This is the option that I like the least. In storms and winter, power failures occur. No power means no heat unless you have a solar option. Or a generator. Like propane heat, there is a wide array of electric heaters that are suitable for a cold frame. Some even come with blowers that help to distribute the heat evenly throughout the cold frame and that is a plus.
  2. Solar Heated Water – Sounds complicated, but the process is really fairly easy and has almost no cost. Take gallon containers of water, fill them, and then leave them in the sun to heat. When they are their hottest, move them into the cold frame and put them into a container with a lid on it. The container has to be something that will radiate heat. The idea is that the warm water in the enclosed space cools much more slowly then it would in an open cold frame. That added increase in warmth can be enough to undo the damage of a sudden cold snap.

These work best in smaller cold frames that are sealed tightly.

The problem with solar heated water is that it might not put off enough heat to save your plants. In fact, the amount of heat it produces is not really predictable and the condition and weather proofing of your cold frame play a huge role in how well this system works.

sun glowing hot

The original idea comes from snow caves. Inside of a snow cave the temperature never drops below freezing, even when the outside temperature is much colder. It is the same principle that keeps igloos functional during the arctic winters.

  1. Manure and Compost Troughs – Both manure and compost piles give off heat. Even in the dead of winter, a hot compost pile is often free of snow. It is the biological decomposition of the compost or manure that causes the natural heat. In an enclosed environment decomposition can help to maintain an even and sustainable temperature inside of an enclosed space (cold frame.) The rawer the manure and fresher the compost the more heat it produces. Well-aged manure or composts produces very little heat.

In a more sophisticated system you can combine #4 – solar heated water – with manure and compost by creating a decomposition stack with an inner core that is tubing or pipe. The decomposition of the stack warms the water in the tube or pipe. That heated water is then pumped into a radiator where it emits heat. The water circulates so that as the radiator starts to cool, the heated water is refreshed. Some systems are set up, so the water circulation is constant.

Other Fixes that Help Warm Cold Frames

As mentioned, a true cold frame is not really meant to be heated. Mostly what people buy are cold frames even though they are sold as greenhouses. In short, they are not very energy inefficient.

If you live in a colder area where the winter temperatures are consistently below 20°F then make an informed decision about the type of cold frame kit you buy. If you are making one on your own, then pay close attention to how well insulated the unit is. For sustainable growing in colder climates, opt for a greenhouse, not a cold frame.

greenhouse plant

Tip: A cold frame that is built to accept an outer layer will help maintain indoor air temperatures better than one that is only a single layer. If you design your cold frame so that there is an air gap between layers, then that gap will help insulate the unit and slow the loss of heat.

The greenhouse here on our property is designed so that the outer lay is added in winter and removed in summer. In its place during the warmer months of the year is shade cloth. That kind of set up allows you to have more control over the interior temperature. It also moves you farther away from what a cold frame is and closer to what a greenhouse is.

Tip: Position the cold frame so that it receives the most light possible during the winter months. Winter is the time of year when we in the Western Hemisphere receive the least amount of light from the sun – thus the colder weather. However, plants need light to grow and if you can take advantage of the available light you are also taking advantage of the available incoming solar radiation, which is a form of heat – solar energy.

Long-term Growing

If your plan is to grow crops year-round rather than just extend the growing season then what you need is a greenhouse not a cold frame. Cold frames are useful for starting plants early and adding time to your growing season. They are not designed to create growing space year-round, though they do help us to preserve plants that would perish in during colder weather.

cactus plant growing gardening

Remember that a cold frame or a greenhouse is a tool. The rule of thumb for tools is that they are designed to perform a specific set of tasks. Start your quest to heat your cold frame by asking yourself what you need a heated cold frame to do for you and your growing projects.

Then start with a cold frame design that helps to offset the problems you are trying to solve. By doing so, you can create a growing space that is more cost effective to operate and that does a better job of meeting your growing needs.

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3 Responses to “Best 5 Ways to Warm a Cold Frame”

  1. By the way, I’ve killed seedlings in a cold frame a time or two by not providing enough ventilation on a sunny spring day. The rule for managing the frame is not to capture as much warmth as possible, but just to provide protection against freezing temperatures (and wind, hail, and varmints). Also, be aware that a gust of wind can flip the lid on a cold frame if it’s simply propped open (as illustrated), and not tethered to the box. That’s a quick way to shatter a glass top, by the way, and tear the hinges off.

    • Very good points that most “cold frame beginners” don’t know, especially that they are not to collect heat, but basically to prevent extremes like: heat/cold; freeze/thaw; wet/dry; dehydrating wind; hail and snow. If you can prevent these extreme weather changes your plants may survive. Another good point you make is that no matter how cold it is outside, on a clear sunny day your cold frame can gain 60 degrees of additional heat, which will kill many plants. Wish I knew where you lived Lathechuck we could be garden buddies!

  2. I have found that some crops are tolerant of extremely cold weather, as long as they are completely protected from movement while frozen. Lettuce, spinach, and kale don’t grow very fast (if at all) in cold weather, but they can freeze and thaw without deteriorating in an unheated cold frame. Sown in late summer, even if not big enough to harvest through the winter, they’ll get off to a really early start in the spring. They’ll also go to seed (“bolt”) as the weather warms, so don’t wait too long to harvest. You can reserve a few plants to provide seed for the next crop.

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