This weekend we got the first tantalizing taste of spring as the weather was clear and bright and temperatures rose well above freezing for the first time in months. Snow melted (though not entirely yet), and started the thoughts of summer gardens in mind. But nighttime temperatures are still falling below freezing, and it’s far too early to put plants in the ground, unless you provide a little assistance.
If your garden has a spot with good access to the sun throughout the day, you can use a cold frame to start your plants earlier in the year than you would otherwise. A cold frame is a very simple item. It is really just a small greenhouse. Daytime sun will warm the air and the ground inside, making it easier for plants to start growing. Nighttime temperatures inside the cold frame may fall back close to outdoor ambient temperature, but the extra heat gained during the day and the wind protection the encosure provides will help keep the plants alive even if there is an overnight frost.
Building a cold frame should be a simple project. An elaborate structure is not required. It should cost little or nothing to build and nothing to operate. Plants can be started close together while they are small, and then, as they get bigger and the weather gets nicer, they can be moved out of the frame and put into the garden.
Last year, I built a pair of new windows for my garage using architectural glazing samples. When I took out the old windows, I salvaged them rather than breaking them up and tossing them in the trash, so now I have two windows which are the basis for my cold frame. You can also build a cold frame using acrylic or other plastic sheets held in a wood frame. As I said, cold frames don’t need to be elaborate.
Alternative: Check out steel building kits
This article explains how I built my cold frame, but you should be able to easily adapt it for your own materials and needs. I would be very interested to hear from other DIY builders about their cold frame projects. It would be very useful to gather other hints and compile them for a future edition.
- (2) salvaged windows, 22″ x 28-1/2″ each
- 24″ x 24″ sheet of plywood, 1/2″ thick (for sides)
- 18″ x 44″ sheet of plywood, 1/2″ thick (for back)
- (3) 1×4 or 1×6, cut to 43″ long (2x4s or 2x6s would also work well)
- scrap wood or 2x2s for corner blocking
- nails or screws
- (2) 1×2 strips, about 12″ long (for cleats)
1. Cut the sides for the cold frame from the 1/2″ plywood. The window needs to be angled toward the sun to gather the light. My plywood had already been ripped with a 6:12 slope. But cutting a 24″ x 24″ sheet of plywood at an angle with the smaller side matching the height of the front cross piece makes efficient use of the material.
2. Nail or screw the blocking to the ends of the long pieces of wood.
3. Nail or screw the long pieces to the side pieces. If you are using 2x4s or 2x6s it is possible to omit the corner blocking and directly fasten through the side pieces into the middle of the 2x. But nailing into the end grain of the wood is not a strong connection, so screws should be used.
Cleat holds window onto frame4. Fasten cleats to the top of each window with 2 or 3 screws. The cleat needs to overhang the window frame at the bottom so that it will catch the top of the cold frame and hold the window in place. I prefer using cleats to hold the windows in place because this way they can be completely removed and set aside in order to work inside the frame. The windows can also be attached with hinges, which makes it easy to crack the windows open to allow ventilation and prevent overheating.
5. Prepare the soil in the garden. It is also possible to set the frame over seedling trays or starter pots, particularly if you plan to relocate all of the plants elsewhere in your garden.
6. Set the frame in place. Most wood exposed to ground contact will start to rot, and most gardeners don’t want to use treated wood near the soil they are using for growing food. Even painting the wood can help protect it somewhat. I used paver bricks set in the ground around the garden to keep the frame from direct contact with the soil. Other options would be to use cedar or another wood that resists rotting.
7. Plant your plants.
- corner brackets
- rigid insulation
Metal corner brackets and screws, instead of nailed connections, could be an alternate way to put a cold frame together. This could also make it possible to disassemble the cold frame for storage when it is not needed, and then quickly reassemble it later on. Hinges can be used to fix the windows to the frame and still allow access to the plants.
Hinges can be used to connect the windows to the frame. But, as noted above, having windows that can be completely removed makes it easier to work inside the frame.
If there are clear sunny days, it is quite possible for a cold frame to overheat during the day. To help moderate the temperature inside, you can prop one window open slightly to let the warmest air out at the top. Just be sure to close it back at night, especially if cold overnight temperatures are expected. Also, since you are putting a roof over the soil and heating the air inside, you have to remember to regularly check and water the plants in your cold frame.
If you want to do some data gathering and monitoring, you can put a thermometer inside the frame to measure interior temperature. It’s best to put this on the front of the frame (on the inside) where it will be shaded as much as possible, so that your readings will not be affected by direct sun on the thermometer.
Put the cold frame into storage during the summer, but the cold frame can also be used in the fall to help extend the season for some plants as frosty nights return.
Putting insulation on the sides of the cold frame doesn’t do much for it, because so much of the heat captured during the day will be lost through the uninsulated window once the sun goes down. However, building a back wall for the cold frame from stacked bricks can serve as a heat sink to store more heat gathered from the sun, to help keep the temerature up through the cold night. Building a cold frame against a wall, or into a hillside, where there is more thermal mass to store the heat, can also be a strategy to push it even further. But for the purpose of just extending the season for a few weeks, a simple cold frame will be more than adequate.
If you want to try growing hardy, cold tolerant plants throughout the winter using your cold frame, you could put insulation on the sides of the frame and leave space for an insulated lid which could be installed inside to cover the plants during overcast days. I recommmend looking at the Brines Farm site (which I wrote about earlier) for more information and suggestions about year round food growing. Shannon has also posted a video in the Convenient Truths contest where he discusses some ideas about growing local food.