Cold frame gardening is an effective strategy to extend your growing season. Whether you want to protect your plants from fall frosts or seedlings from spring cold snaps, these boxes are easy and inexpensive to build and simple to use.

Our guide to cold frame gardening will help you decide whether to try this technique to protect your plants when temperatures drop.

What is Cold Frame Gardening?

Cold frame gardening involves positioning a bottomless, low–to-the-ground box over tender plants or seedlings to protect them when temperatures drop. Frames come in various materials, but the sloping top is always transparent to allow light in. 

Cold frame use is common in the late fall or early winter to extend the growing season of cool-season crops by up to a month. They’re also helpful in the spring, providing early-planted perennials with protection from late frosts.

Some gardeners use cold frames to help harden off seedlings grown from seeds indoors, which need gradual acclimation to outdoor conditions.

Temperatures in a cold frame are typically around 5°F to 10°F higher than those outside the box, helping to create a controlled microclimate. One side of the frame is usually higher than the other to capture as much sunlight as possible.

This slow-release solar energy heats the air inside the box, helping to prevent extreme fluctuations in temperature (especially overnight) and maintain moist, warm soil. 

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Types of Cold Frames

It’s possible to make a cold frame or buy one. While the DIY options are often wooden, you might want something more flexible to be able to move it to different spots in your yard or fold it away when you don’t need it. 

Not sure whether to opt for a portable, permanent or temporary cold frame? We’ve outlined the pros and cons of each one below.

Permanent

Some cold frames are designed to be left in a suitably sunny spot in your yard year-round. Often, they are sunk into the ground for better stability and insulation. Permanent cold frames are commonly made from durable and aesthetically pleasing materials such as brick, cinder block, poured concrete or treated, rot-resistant wood with a glass or plexiglass lid. 

Temporary

If you’re experimenting with a cold frame or don’t want it using up yard or shed space in the warmer months, a temporary cold frame could work if you don’t mind putting in the work for a single season’s use.

Using old recycled materials, such as scrap wood, bricks, straw bales, old window panes, or plastic sheets, means you won’t be wasting money or materials if you get rid of the cold frame at the end of the season. You can even use old milk jugs as cold frames for individual plants.

Portable

Portable cold frames are lightweight and might fold down or stack to make them easier to store in your garden shed after frosts have passed. Garden supplies retailers have a wide range of collapsible frames (often made from aluminum).

However, portable frames aren’t as substantial as permanent designs. They don’t always have such good insulative qualities and aren’t as durable. 

How to Build a Cold Frame

Building a DIY cold frame is within the reach of most handy homeowners. Techniques vary depending on the materials and design you opt for. The steps below will help you understand if building a traditional wood cold frame is something you want to tackle.

1. Use wooden deck boards (or other suitably-sized scrap wood) to create a 2-foot by 4-foot rectangular frame. 
2. Build the frame’s back edge higher than the front to allow the lid to slope downward. 12 inches at the front and 18 inches at the back is common. Avoid anything taller than 3 feet; otherwise, it can become tricky to reach inside.
3. Insert an old glass window into a grooved wooden frame for the lid. You can drill plexiglass directly into the wood.
4. Mount the lid using door hinges to make it easy to open and close.
5. Treat the wood to weatherproof it. Select an exterior sealant or paint that is safe for use alongside food crops. 

Best Crops for Cold Frames

Cold frame gardening is great for hardening off seedlings or protecting seeds sown outdoors in the spring from unexpected late frost. You can also use them to extend your vegetable garden harvest calendar. Below are some of the best crops for cold frames.

• Broccoli: You can plant cool-season brassicas like this in a wide range of climates and they cope with light frosts. Growing in cold frames means you could plant them as late as September in some locations, with some broccoli varieties being ready to harvest in 50 days. 
• Lettuce: Consider fast-growing, adaptable salad greens that mature quickly. Even when temperatures drop, you can enjoy a healthy harvest within 40 to 60 days in a cold frame.
• Tomatoes: After germinating the seeds indoors, harden off the tender seedlings in a cold frame. The same goes for peppers.

Cold Frame Growing Tips

It’s not just the build you need to get right when trying out cold-frame gardening. Keep the following tips in mind for the best chance of success.

• Pick a sunny spot: A site with a southern exposure maximizes the heat from sunlight.
• Offer shelter: Position the north end near a home, wall, fence, or in front of an insulating bale of hay to reduce cold wind exposure.
• Well-draining soil is a must: A flooded cold frame is a recipe for disaster. Consider adding aerating soil amendments if yours is heavy and poorly draining.
• Monitor temperature and ventilation: If the cold frame gets too hot on sunny days, it can quickly kill off your plants. Consider attaching a thermometer and, when necessary, prop open the lid during the hottest part of the day to promote air circulation and cooler temperatures.