Using Cold Frames in the Garden

cold frames in garden
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Using Cold Frames in the Garden

Have you ever planted seeds or put plants into the ground, only to lose them a week later to a frost? It’s happened to the best of us. We check our calendars for the frost date and as soon as it passes, we eagerly put our plants or seeds into the ground. Winter is a hard season for gardeners, because all you can do is wait for the next growing season. What if we told you there was a way that you could extend your growing season and even stop fearing an unexpected cold snap?

Cold frames are a nifty little invention that can do just that. Let’s talk about cold frames and why you need some.

What Is a Cold Frame?

People have been trying to extend the growing season for thousands of years. We might think of cold frames or greenhouses as fairly new ideas, but in reality, some variations of these have been used for hundreds, if not thousands, of years. It’s thought that the Romans were the first people to use cold frames. They built up boxes with straw or stone walls and they used sheets of mica as lids.

The cold frames used today aren’t much different to the ones the Romans used; we just use different materials. The term ‘cold frame’ is a recent invention. When greenhouses started to become popular, they were often heated. Cold frames were used as an extension of the greenhouse. Therefore, there were two distinct growing spaces; the heated greenhouse and the cold frame (which wasn’t heated).

How to Use a Cold Frame

Cold frames sit on the ground and create a small greenhouse effect for your plants. Think of them as a miniature greenhouse. They will help to keep frost off of your plants and can warm the ground and air around your plants to create a longer growing season. Cold frames are a great way to plant sooner in the spring and later in the fall.

The best plants to grow in a cold frame are plants that are smaller cool-season plants. Large cool-season plants like Brussels sprouts will outgrow a cold frame. Instead, grow plants like lettuce, greens, chard, radishes, beets, celery, cabbage, broccoli or carrots in your cold frames. Cold frames are not a substitute for a greenhouse. If you want to grow summer crops all year long, you won’t be able to do that in cold frames. You’ll need to invest in a greenhouse that is heated if you want to grow summer crops all year long.

When you’re setting up your cold frame, you’ll want to consider where you’re putting it. One of the most common mistakes people make with a cold frame is to put them right up against the house or a garden shed. It’s tempting to put your cold frame close to your home or shed, since that generally makes it easier to tend to the plants. However, consider the amount of sunlight that your cold frame will get each day. The cold frame works because it allows solar energy to heat the interior. If the cold frame doesn’t get enough sunlight, the interior will stay cold, defeating the purpose.

Wondering how you’ll heat your cold frame at night? You won’t. The heat and warmth that is absorbed during the day will remain trapped in the cold frame at night. While the interior of the cold frame will get cooler, it won’t get as cold as the surrounding air will.

What happens if your cold frame is covered with snow? When the cold frame is covered with snow, it can’t get the sunlight it needs to warm up inside. Brush the snow off of the top of the cold frame to allow sunlight to get in. This will help ensure your plants stay nice and warm, even with snow on the ground around them.

A cold frame is a great place to start seeds. The small greenhouse effect can create an environment that is safe from frost and warm enough to start seeds. Starting seeds in a cold frame can also help reduce the amount of time you need to harden off your seeds. Seedlings that are grown in a cold frame are exposed to plenty of natural sunlight unlike seeds started indoors under grow lights. When you get ready to harden off your plants that were started in a cold frame, you’ll need to adjust the seedlings to the outdoor temperature. Use caution when starting seeds in a cold frame. Although this creates a good environment for starting seeds, be careful not to start seeds too soon in the cold frame. Your seedlings will be protected from frost, but they may still experience cold temperatures, especially at night. Also, don’t start seeds directly in the ground in your cold frames. This will mean you’ll have to dig them up and potentially damage the roots. Instead, place your seed starting trays into the cold frame.

Where to Get Cold Frames

Although you can buy cold frames from a few select places, it’s often cheaper to build them at home. Cold frames today are often constructed from simple materials and use solar energy to heat them. You can construct a cold frame using wood as the frame and clear plastic for the sides and top. Some people have even repurposed old windows to turn into cold frames. Old windows make excellent top doors for cold frames that are sturdier than wood and plastic.

The clear plastic that you can use to create your cold frames is the same clear plastic used to construct greenhouses. This inexpensive plastic can be purchased from greenhouse supply stores or farm supply stores. If you use clear plastic from hardware stores (such as painter’s plastic) make sure that it is transparent enough to allow ample sunlight in. Without ample sunlight, your plants won’t thrive.

A well constructed cold frame will last years with good care. It’s a good idea to anchor them to the ground somehow. Landscaping fabric pins are a cheap and easy way to do this. Anchoring the cold frame to the ground will help to prevent it from blowing off of your plants during a strong wind. Cold frames are lightweight and can be blown around if they aren’t anchored. This will not only remove the protection from your plant, but it will often damage your plant when the cold frame is blown off.

If you want to increase the warmth in your cold frame, you can double insulate it. Large commercial greenhouses use this same principle to increase the warmth inside. Instead of stapling one layer of plastic on your cold frame, add two layers of plastic. Staple a layer of plastic on the inside of the frame and one on the outside of the frame. This will create a pocket of air in between the two layers, creating an additional layer of insulation.

Your cold frame does not have to be airtight. You want the cold frame to have as few air leaks as possible, but a slight leak somewhere won’t destroy the purpose of the cold frame. An imperfect cold frame can still be quite effective. A large leak will have consequences. A large leak will create a draft of cold air that may cool the air inside of the cold frame too much. Large leaks will also allow bitterly cold air to enter the cold frame at night and can lead to frost damage. Build the cold frame as well as you can to avoid large air leaks. If your plastic becomes ripped or damaged, use clear greenhouse tape to repair the damage and prevent air leaks.