The lower temperatures of fall don’t have to mean the end of your vegetable garden. One of the best ways to extend your growing season is by using a cold frame. This article gives an overview of what a cold frame is and does and how to make one for your garden.

A cold frame is a simple structure that uses the sun’s warmth and insulation to protect your plants from cold weather, including wind, rain, snow, ice, and cool temperatures. It is a raised bottomless box with a clear cover that works much like a small greenhouse. The ground and frame of the cold frame are heated by the sun during the day and radiate that heat during the night to keep the inside warmer. 

A cold frame can extend the growing season by two to four weeks in both the fall and the spring, allowing you to start plants outdoors sooner in the spring and continue to grow plants later in the fall.

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What’s the difference between a cold frame and a hotbed?

Cold frames and hotbeds are similar in how they are constructed but differ in their use. A hotbed is basically a heated cold frame. 

Whereas a cold frame relies on the sun for warmth, a hotbed uses external heat sources, such as electric heating cables. Another difference is you might use a hotbed as a warm place to start seeds while using a cold frame to protect young starts from freezing temperatures or harden them off before moving them to your vegetable garden.

What grows best in a cold frame?

Compact, non-fruiting plants are best for cold frame growing. Most fruiting plants grow too big for a cold frame and require more warmth than they will get in a cold frame. Also, fruiting plants require pollinators, which are not around in cold weather.

Here is a list of plants that are your best bets:

• Arugula
• Beets
• Bok choy
• Broccoli
• Cabbage
• Carrots
• Chard
• Chinese cabbage
• Endive
• Kale
• Leeks
• Lettuce
• Mizuna
• Mustard greens
• Onions
• Radicchio
• Radishes
• Spinach
• Turnips

If you are starting your cold frame plants from seed, you’ll want to plant them before the days are too short. Plants tend to stop growing from mid-November to mid-January.

How to Make a Cold Frame

You can purchase a pre-assembled cold frame or a cold frame kit, but many gardeners find it easy and more affordable to make one on their own. The best part of going this route is that you can repurpose old window frames, shower doors, and wood pallets for the project. You can also get creative with old picture frames, plastic storage bins, or an unused aquarium.

The two main components of a cold frame are the box-like frame itself and a transparent cover.

The Frame

The traditional cold frame is made of wood. Avoid using wood that has been pre-treated with chemicals, as the chemicals can leach into the soil and, thus, into your plants. If you wish, you can paint untreated wood to reduce decaying from contact with the soil. 

White latex paint is a good choice as it reflects light. However, it’s a good idea to also paint the inside walls black, for the way the color radiates heat. In addition to wooden doors, window frames, and pallets, you can build the frame using bricks, cinderblocks, or straw bales.

The Covering

You can repurpose many materials to serve as the transparent covering. An old shower door or storm window works well. You also could use clear plastic sheeting or fiberglass panels. You will attach the covering to the frame using hinges.  

The materials you use will dictate the size of your cold frame. However, there are a few rules of thumb to keep in mind:

• The frame should not be more than three feet deep so you can reach the back without having to step inside. 
• The dimensions of 12 inches high in the front and 18 inches high in the back allows more sunlight to enter the protected area. If you have taller plants, you can go higher, but if the cold frame is too large, it will be hard to reach inside to care for the plants.
• For more use next season, consider using pins or eye hooks that allow the cold frame to be easily un-assembled.

Placing and maintaining a cold frame

The ideal spot for a cold frame has well-draining soil. For maximum sunlight, orient the cold frame so it faces south. 

Position the north end of the cold frame near a home, garage, or fence to maximize warmth. And don’t forget to consider the availability of a water source when you choose a location.

It’s crucial to monitor the temperature inside your cold frame. Temperatures can rise suddenly – even on cloudy days – potentially killing your plants.

Use a thermometer to know when you’ll open the cover for ventilation. Using a small board or a dowel to prop the cover open, you should raise the cover when the air temperature inside the cold frame goes above 85 degrees Fahrenheit. You should close it again before sunset to retain heat. 

Many gardeners have had success insulating their cold frames with mounds of soil or straw bales.

Another factor to consider is whether you want your cold frame to be permanent, portable, or temporary.

• A portable cold frame is smaller and more lightweight than a permanent structure. Kits for these cold frames typically have aluminum frames and polycarbonate tops and sides. You can move them from bed to bed as a way of protecting veggies from light frosts or brief cold spells. Along with a heavy layer of leaves, you can also insulate root crops such as carrots and parsnips. Most portable cold frames are easy to take apart and store during the summer.
• A permanent cold frame is an in-ground structure that is made to stay put growing crops throughout the year. These frames are built with wood or cinder blocks, and light can only reach the plants through the clear tip covering. The ground helps provide insulation. 
• A temporary cold frame can consist of straw bales and an old window. For example, you can use it to protect your tall leeks and then take the structure down in the winter.

Here are a few videos that provide instructions for building a cold frame on your property.

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