14 effective ways to keep a cold frame warm at night

Photo by Ofer El-Hashahar CC by 2.0


A cold frame is a transparent outdoor frame to keep your plants warm. Cold frames do that without using any source of energy other than sunlight. But now you’re probably thinking, “If a cold frame uses sunlight, what happens at night?” fret not. This guide will tell you all about how to keep a cold frame warm at night. And ensure you get the best use out of it through all seasons.

How to keep cold frames warm at night? You can keep your cold frame warm at night by adding heat-absorbing materials, improving the frame’s insulation, and increasing sun exposure. Cold frames should always face south at a light angle to make the most out of the daytime sunlight.


Cold frames – How do they work?

As you probably already know, cold frames work by absorbing and storing heat from the sun during the day and retaining that heat through the night. The heat retention depends upon the weather, material of the cold frame, and location where it is installed.

Cold frames are perfect for growing cold weather plants. Vegetables like broccoli, spinach, and different kinds of lettuces thrive inside the frame. Root veggies like carrots, beets, radishes, onions, and leeks also nurture well when grown in cold frames.

Unlike greenhouses, cold frames have no sophisticated heating system. They can be a permanent fixture or removable. The construction determines the degree of insulation a cold frame can provide. You can tweak and tinker with your cold frame to make sure it protects your plants during cold, harsh nights.


How to properly set up a cold frame

Another great tip: Place your Cold frame next to a wall! The Stones will store heat and release it during the night keeping your cold frame a bit warmer.


While a cold frame is capable of providing suitable nighttime heat to plants, you can improve that for some of those extra chilly nights.

But before we take a look at how to keep a cold frame warm at night, we’d like to give you some essential tips to get started. Follow them, and you won’t go wrong.

Always have your cold frame facing south

I bet you already knew that one. And if you didn’t, now you do. If your cold frame is facing south, the sun will keep in front of it all day long. This way, you’ll be able to make the most out of the daytime sunlight.

Whether you have a raised cold frame or a sunken one, make sure nothing comes in the way of it getting sunlight all through the day. For a simple cold frame, sunlight is the only source of heat. You have to capitalize on it as much as you can whenever it is available.

Ventilation is important

To avoid cooking your plants inside the cold frame during summer and fall, ensure that your temperatures don’t exceed what your plants can handle. During the warmer days, prop the glazing of your cold frame up to let the air in and keep things from getting too hot and humid.

We talked about tough love before. Come springtime, ventilating your cold frame for a little longer every day, as the weather warms, can toughen up your young plants before they get planted outside.

Let the cold frame heat up your soil first

Before you move your seedlings to the cold frame, the soil needs to be sufficiently warmed up. Waiting a couple of days after setting up your cold frame will ascertain that the soil is optimal temperature before moving your baby plants to it.

Ideally, the soil temperature in the cold frame should consistently hover around 45F before you move your plants in there.

Have your sash at a slope

My Cold Frame – without an angled lid

For best sunlight penetration, have your sash, aka the lid of your cold frame, at an angle. The placing of your cold frame should be such that the sash slants towards the south.

An angled sash allows the sun rays to fall at right angles all through the day. The angle allows rainwater to slide right off. It also prevents the top from being covered with snow or autumn leaves from nearby trees, thereby ensuring that your cold frame warms up adequately during the day.

Have the back wall of your cold frame higher than the front wall. Angle the walls on the sides at a slant to line up with the sash. Use cement, clay, paper, soil, or any other material to fill any gaps.

Water your plants at the right time

Like we said above, water cools the surrounding area quickly. If you want your cold frame to stay warm during the night, you need your plants to be dry. During spring, it is vital to water your plants at the right time.

The ideal time for watering your plants is early morning. This way, your plants will dry out during the day. Come nightfall, there won’t be any excess water to draw the heat away. In addition to keeping frostbite at bay, having your leaves stay dry prevents fungal growth as well.

There, you know some of the basics to ensure that your cold frame functions right. We will now move on to some methods you can employ for the best results in the absence of sun.


Proven methods to keep a cold frame warm at night

Increase the sunlight inside your cold frame

To have plenty of heat stocked in your cold frame for your plants to survive the night, you need to have the insides reflect maximum sunlight all around the frame.

How to do it

Paint the hind walls of your cold frame with a bright color. White should be the obvious choice here. If you want to take it up a notch, line the insides of your frame with tin or aluminum foil. The increased sunlight will warm the cold frame up nicely through the day to last the night.

Store the heat to be released at night

In addition to increasing the sunlight inside your cold frame, what if you could store that sunlight during the day to be released during the night?

How to do it

Paint some plastic bottles black (some dark-colored stones also do the job). Fill em up with water and place them in the brightest spots of your cold frame. The dark-colored bottles will absorb the solar energy throughout the day and release it slowly at night, keeping the temperature up all night long.

If one day you forget to put the bottles in the frame, or if there wasn’t enough sun to heat the bottles during the day, worry not. Pour some hot water into plastic bottles and place them in your cold frame. Just don’t let the leaves touch the bottles.

Use manure or compost

Photo by Ted – CC by 2.0


Manure releases heat when it decomposes. That heat is enough to keep all the plants in your cold frame warm at night. Decomposing manure is an age-old heating method used by farmers of yesteryears.

How to do it

Pile up fresh manure to about 1-1.5 feet in your cold frame. Pour warm water over the manure and even the surface. Add 3-4 inches of soil on top, and you should be good to go.

Make sure the bed of manure has provisions for drainage. Without it, the decomposition may stop. This method is messy but cheap and natural.

Use light bulbs for heating

All right, the gloves are off. For some extra chilly nights, natural methods may not be enough. In that case, you are free to use modern science to help you out. Use the power of electricity to have mini suns heat your cold frame.

How to do it

All you need are some Christmas lights (of the non-LED variety) and a nearby electrical socket. Line them adequately spaced inside your cold frame and hit the switch. You can rest easy knowing your plants will stay snug and warm during the sub-zero temperature.

Convert your cold frame into a hotbed

If you really want your plants to stay safe from the blistering cold and you don’t believe any other option would work, you can turn your cold frame into an electric hotbed. You can do that using an insulated heating cable. Get the insulating cables which automatically regulate the temperature, so you don’t have to worry about cooking your plants.

How to do it

Excavate the bottom of your cold frame and layer it with sand or ash. Lay a mesh of heating wire, cover the wire soil, and you’re done. Power that wire up and let it take care of your plants through the night.


Make your cold frame cold resistant

You can surely do many things at night to make sure your plants don’t freeze to death. But as said before, cold frames are self-sufficient when it comes to managing temperature on their own. That being said, here are some additional steps you can take to improve the heat absorbing and cold resisting properties of your cold frame.

Make a sunken cold frame

A sunken cold frame is one in which the base of the structure is the earth, or you can say it has no base at all. Aside from being relatively simple to construct, it also offers the most consistent insulation from the elements.

A sunken cold frame also offers better drainage. Excess water can cool the surrounding area fast at night. With plenty of earth beneath the plants in your cold frame, excess water will drain away swiftly.

All you need is a glass or plastic sheet for the sash (top), some bricks or lumber to make the walls, and some free space on the ground. Surround that area with walls and put on the sash. Use mulch or clay to fill up the gaps, et voila! You have yourself a sunken cold frame.

Have a clear top

A transparent sash will allow the most sunlight to warm up the insides of your cold frame. Glass, plastic, Plexiglass, fibreglass, and acrylic can be used to make the perfect roof of your cold-frame. However, each material can have flaws.

Plastic-based materials can deteriorate from continuous exposure to ultraviolet light. This deterioration may result in yellowing or cracking of the sash, which may block or hinder the sunlight. They are also lightweight, which means heavy wind may blow them away, exposing your plants to the elements.

Glass isn’t affected by UV rays, and it is weighty enough to withstand substantially heavy winds without flying off. However, it is brittle and can shatter under a massive pileup of snow or during a hailstorm.

If your area doesn’t get hail, then glass is the way to go. Make sure to slant your sash to prevent snow buildup. If you are using plastics, then check whether they are resistant to UV rays or not, and secure them properly during windy conditions.

Use heat-trapping material for walls

Photo by Laurie M – CC By 2.0


Since sunlight is the only source of heat for your cold frame, you need to make sure that it doesn’t escape once it gets in. You can take care of that by using the right material to build the walls of your frame.

Making your cold frame out of wood, cement, or bricks can provide rudimentary insulation to your plants at night. It can be significantly enhanced by lining the walls with a couple of inches of Styrofoam.

You can also place bags of dead leaves, old clothes, bubble-wrap, etc., along the outside of your cold frame to keep the insides shielded from the night cold.

Another way is to make your cold frame out of hay. Thick bales of straw are readily available, and walls made of it will keep your cold frame nice and warm through long winter nights.

Prevent heat from escaping out of the glazing

A transparent top allows for the best sunlight penetration. However, come nighttime, it may also let heat escape quicker.

A permanent solution is to use double-layered glass or plastic glazing. The air between separates the two sheets from meeting and conducting heat. Thus the inside temperature stays constant for a longer time.

For a cold frame with a single-layered sash, you can improve insulation by covering the top at night with an old blanket, bubble-wrap, or tarp


Importance of consistent temperature for young plants

Temperature can make or break your harvest before the seeds grow. For the best results, you need to check on the temperature inside a cold frame at different times. You can do this by installing a high/low thermometer in your cold frame. Make sure to keep it in the shade, so the sun doesn’t mess with its readings.

You should also try not to go overboard with the heating. Temperatures above 90F might damage seedlings and make seeds not sprout at all.

The temperature inside a cold frame can be 10-25 degrees above the outside temperature. However, for seeds to start growing, sometimes even this temperature is not high enough. It is recommended to start your seeds indoors, using grow-lights to give them the right temperature in the absence of natural sun. You can move them to the cold frame after they shoot out of the soil.

Of course, every plant has different needs. Seed packets usually tell you how warm the environment needs to be for that plant to sprout.

Since the temperature inside a cold frame is uniform, you’d be wise to plant seeds with matching temperature requirements in your cold frame.


The optimum temperature for cold frames

For planted seeds, the ideal temperature should be around 70 to 75 degrees F. You need to maintain this temperature at all times until your seedlings are about an inch high. After that, you can cool your cold frame down to about 55-65 degrees F.

You wouldn’t want your cold frame to be too cozy; plants raised with a little tough-love will be able to withstand the harsher outdoor environment once they leave the cold frame. Acclimating seedlings before planting them in the open is one of the best uses of a cold frame.


The wrap-up

A cold frame provides an excellent environment to acclimatize your seedlings for harsher conditions of the outside. It is perfect for overwintering your plants and for extending the gardening season by a good month or two.

Cold frames can be used in all seasons for different results. You can use them in spring to harden off your baby plants for the outside weather. You can use them early in the summer to plant some of the more heat-hungry plants before their actual planting season. You can use them in winter to protect your plants from freezing. You can use them in the autumn to elongate your harvest season by almost a month.

Despite being perfectly capable of retaining heat during the night, cold frames may not be enough for frigid winter nights. For this, they may use a little bit of help. With adept gardening and correct heating, your plants can grow and flourish in your cold frame no matter how cold it gets.


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