In our last post we discussed making the most of fall’s cool weather by adding perennials to your garden. Another way to enjoy fall in the garden is to build a cold frame to help extend your growing season. Traditionally cold frames were placed against the south facing wall of a heated green house. They were the perfect place to grow cool weather crops in the fall and harden off seedlings each spring before transplanting them out in the garden. Cold frames are a wonderful option for people without the space or money to set up a larger greenhouse. They can be beautifully crafted permanent garden beds or temporary ones made from scrap materials.
Choose a Location
The first step is to select the best spot for your cold frame. We growers in the northern hemisphere should face our frames south to get as much light as possible. You may also want to choose a location close to your home, which will give you easy access to the plants for care and harvesting.
Create a Top
When using salvaged materials, you’ll want to create the top of your frame first so you can build your bed to fit the lid, not the other way around. The lid should fit snugly, avoiding gaps, but you should also be able to prop it open on sunny days to avoid burning your plants.
Even in chilly weather, the interior of a cold frame can heat up surprisingly quickly. Great options for a top include an old window or sliding glass door. You can also purchase a polycarbonate roofing panel anywhere that sells greenhouse supplies and cut it to whatever size you desire.
Build the Frame
Depending on your goal, the frame, or bed piece, can be built with a variety of materials, and the more insulation the frame can provide, the better.
If you’re looking to create something quickly and on a budget, you can use straw bales to frame a bed. They provide great insulation for the plants, but they probably won’t last more than a season. Bales are also a good option if you’re a renter or are planning to move in the near future.
Another option is to make a frame out of wood planks. Wood is an excellent economical material that will last a long time, especially if you use a rot-resistant wood like cedar and add a layer of bricks or blocks underneath to keep the frame off the ground. Avoid pressure-treated wood, as it will leach chemicals into your soil.
Also, unlike an ordinary raised bed, your cold frame should be designed with a higher back, or northern-facing, side compared to its front, southern-facing side. This will allow your lid to sit at an angle, which encourages the most light into the frame when the sun is relatively low in the sky.
Put It All Together
If you’ve opted for simple straw bales as your frame, this step is as easy as setting an old window on top. However, with a more permanent construction, it’s probably worth building a frame for your lid and attaching it to the bed frame with hinges for easy opening. You should make a “prop stick” too. This can be an actual stick or a hinged rod with notches used to hold the lid up at different heights, for both venting and access, and that you can fold out of the way when not in use.
If you’re not putting your cold frame on an existing garden bed, you’ll need to break up the soil beneath it, which you can do with a garden or broad fork. Regardless of whether the soil is an existing bed or a new plot, add some good quality compost. This will contribute important nutrients and encourage abundant harvests.
What to Plant
Depending on your gardening zone, you can still sow a few crops in your new cold frame this fall. Hardy greens, like kale, arugula, collards, and spinach, make excellent fall and winter crops, as well as green onions, turnips, beets, and radishes. It may be tempting to sow plants close together in a small frame, but overcrowding stresses the plants and should be avoided.
Also, be patient: even if your cold frame stays warm, you may not see a lot of fast growth due to the reduced daylight hours of fall and winter. However, your efforts now may still pay off in the spring and deliver a super early crop.
Follow these steps, and you’ll see how easy it is to make use of a cold frame to extend your growing season and provide you with a wonderful fall and winter harvest.
Jordan Charbonneau is an organic farmer and writer from West Virginia. She holds degrees in ecology and environmental humanities from Sterling College in Vermont.