8 Ways to Keep a Cold Frame Warm at Night: Insulation and Heating Tips

8 Ways to Keep a Cold Frame Warm at Night: Insulation and Heating Tips 2

A cold frame is a great way to extend your growing season. It can keep your plants warm at night and protect them from frost. If you are using a cold frame, it’s important to make sure that it stays warm throughout the night. In this blog post, we will discuss 8 ways to keep your cold frame warm at night!

What are cold frames and how do they work?

Cold frames are mini-greenhouses. They are constructed to enable cold-weather plants to survive and thrive outdoors even when the temperatures go really low. They do this by absorbing the heat from the sun throughout the day and retaining the heat at night to keep the plants in them warm.

Cold weather plants that do very well in cold frames include lettuce, spinach, broccoli, leeks, onions, radishes, beets, and carrots. While a well-constructed cold frame will always ensure heat is retained, sometimes the heat retained is not enough to keep plants warm, especially during extremely cold winter nights. This can lead to plants dying.

If you do not want your cold frame plants to die due to extremely low temperatures, check out the section below. It outlines eight different ways to keep your cold frame warm at night.

8 ways to keep your cold frame warm at night

1. Paint the back wall white

One of the easiest things to do to keep your cold frame warm at night is to paint the back wall white or with any other bright color. Doing this will reflect sunlight and heat around the frame throughout the day. In other words, it will heat up the frame and make it warmer than usual during the day. This increased heat or warmth will last into the night and keep your plants safe at night.

To paint the back wall of your cold frame white, simply purchase white paint and paint the entire back wall inside your cold frame. The white wall inside your frame will help to increase sunlight and warmth inside your cold frame. It will make your cold frame warmer in the day and warm at night.

An alternative to painting the back wall white is to line it with aluminum foil. Doing this will also increase the sunlight in your cold frame during the day. The warmth will protect your plants at night.

2. Install light bulbs for night heating

If you feel as if the cold nights are becoming too cold for your plants, you should install light bulbs for night heating. Make sure you install traditional light bulbs (tungsten light bulbs). These light bulbs become very hot and release heat unlike energy-saving bulbs.

By installing a few traditional or conventional light bulbs in your cold frame and switching them on at night, you will ensure your plants stay warm even during frosty nights.

The only issue with installing light bulbs for heating is that the installation process may be a bit challenging. However, if you love DIY projects you should find it easy to install light bulbs. If you cannot install light bulbs in your cold frame, you should hire someone to do it for you. It should not cost more than fifty bucks to get light bulbs installed in your cold frame.

When you install light bulbs for night heating, you should never forget to switch on the bulbs at night. Failure to switch on the bulbs could lead to disastrous consequences for your plants.

3. Increase the insulation of your cold frame

If you have cold frame plants that are thriving, they are probably thriving because they are receiving plenty of sunlight and heat virtually every day from the sun. The heat from the sun usually lasts long into the night in cold frames. However, in some cases, the heat does not last very long at night. This is frequently the case when cold frames are not well-insulated.

If your cold frame is built using bricks, cement, or wood, it has some sort of rudimentary insulation that will keep it warm for longer at night. Whatever material that was used to build it, you can increase its insulation in a number of ways.

The easiest way to increase its insulation is by placing bags of old clothes around it. These bags of old clothes should be loosely packed. Doing this will make them trap air and make them super insulating. To take advantage of these insulating bags, you should make sure they cover your entire cold frame. This will insulate them totally.

Another way to insulate your cold frame is to line it with Styrofoam. Styrofoam is a very good insulator and a few inches of it around your cold frame will make sure your cold frame plants never get too cold.

4. Trap the heat from the sun

You can keep your cold plants warm at night by trapping the heat from the sun. So how exactly can you trap heat from the sun? Well, get plenty of plastic bottles and paint them black. Then put dark rocks or pebbles in the bottles and fill them with water. Put these bottles in your cold frame where the sun shines brightest.

The dark bottles and the dark rocks in them will trap heat from the sun throughout the day. And then at night the bottles will release the trapped heat to the cold frame. The bottles will make sure your cold frame plants never get cold.

In case the sun does not shine a lot on a particular day, you can still make sure your plants do not freeze at night. You can do this by putting hot water in your heat-trapping bottles. The bottles will release the heat from the water slowly at night and ensure your plants never get cold.

5. Add compost to your cold frame

When you add compost to your cold frame you will be achieving two things. You will first be giving your cold-frame plants natural nutrients to make them grow faster. Secondly, you will be ensuring your cold frame plants do not get too cold at night. This is because compost, just like manure, continues decomposing even when mixed with the soil. And the decomposition causes the release of heat.

So adding compost to your cold frame has two benefits – it makes the ground nutrient-rich and ensures plants do not get too cold at night.

To ensure the compost you add to your cold frame is thick enough to produce a good amount of heat, you should add about 1-feet of compost and water it until it is moist. The water will ensure the compost continues decomposing.

6. Install a heating cable

When you have done a lot to ensure your cold frame is warm at night and whatever you have done is not working, you buy and install a heating cable. A heating cable will turn your cold frame into a hot frame. If well-installed, it will ensure even peak winter conditions do not freeze your plants to death.

Heating cables are usually insulated so they are quite safe to install in cold frames. They also normally come with a temperature regulator. The regulator allows you to control the heat produced to ensure it is not too much to cook your growing plants.

7. Double glaze your cold frame

Most cold frames have a glass top to allow sunlight to penetrate easily and get to the plants. What many people do not know is that cold frames usually lose most heat through their glass tops. They also get cold largely through cold air from their glass tops.

The solution to preventing cold frames from losing heat through their glass tops is to replace the tops with double-glazed glasses. So replace your cold frame glass top with a double glazed glass top. The double-glazed glass is a wonderful insulator that will reduce heat loss significantly. It will also significantly reduce the quantity of cold air getting in.

8. Sink your cold frame

Positioning your cold frame inside the earth will protect it because the ground will largely act like an insulator. Just make sure the frame of the cold frame is lumber and your cold frame will be fully protected from losing a lot of heat.

The only issue with building a sunken cold frame is that it takes a lot of time and effort to sink a cold frame that is already built. Nevertheless, sinking your cold frame will certainly ensure your plants are warm at night.


Cold frames provide the perfect outdoor environment for cold weather plants to thrive. They absorb and retain heat during the day to ensure cold nights are not too cold for the plants growing inside them. When well-constructed, they can be used in all seasons. However, during winter, the weather can get so cold at night that it can kill even cold weather plants housed in well-constructed cold frames.

Nevertheless, there are things that can be done to ensure cold frames stay warm at night despite cold weather. You now know exactly what to do to keep your cold frame(s) warm at night. Act now to ensure your plants/vegetables survive even the coldest nights of the year.