If you ever wished the growing season in your area was longer, cold frames might be of interest to you. Cold frames are like mini-greenhouses that warm the soil and trap heat around plants and they can extend your growing season or even allow you to grow some plants over the winter.
Cold frames rely on the heat of the sun, and so location is very important. Your garden will need to be in an area that gets lots of light so the cold frame can store heat. Consider placing it so your house or building will protect it from the wind. Also, putting it against a heated building (house) can add additional heat.
It is also important that your cold frame gets good drainage. An area with a slight slope helps. If you do slope your frame, point it towards the south for maximum sun exposure.
Cold frames can be an easy do-it-yourself project or they can be bought as cold frame kits to be assembled at home. Cold frame designs can be found in a number of places online, or even at your local nursery.
Old window sashes are excellent tops for cold frames, as the panels of glass let in light. Your cold frame could be as simple as cinder blocks stacked in a square around your plants with an old window sash as the top. It is preferable that the top be hinged, however, to make it easier to access your plants. Cold frames can get as sophisticated as you want. For those who are ambitious, an automatic opener for ventilation could be built and attached to a timer. A good place to check for these automatic timing devices is in gardening catalogs or your local nurseries.
If building wooden cold frames, make sure the wood will not decompose. Don’t make the frame bigger than you are able to comfortably reach across, so you are able to access the rear plants to weed and harvest.
You can also attach plastic window-well covers to a hinge. Use a storm window or recycle an old paned window. Plexiglas or plastic sheeting works well too. Build the frame from cinder blocks, bricks, or lumber – even bales of hay may be used.
Your garden cold frame will need some ventilation, so don’t build it air tight. On days that are above 45 degrees, you will want to prop the top, so keep this in mind as well as you build. Not opening the top on warm days could kill your plants if they get too hot. Don’t let the interior of your frame go above 80 degrees. If the temperature gets too low, you can throw additional blankets over your cold frame overnight, making sure to remove them in the morning.
Ventilation is important both so plants don’t overheat and so excess moisture is allowed to escape, preventing rot and disease. Paint inside of cold frame white to reflect sun onto plants. Prop the lid about 6 inches above 40 degrees and remove the lid above 50 degrees. Don’t wait too late in the afternoon to replace the lid, however – you want to trap some heat inside the cold frame for overnight.
Some snow piled on top of your frame is okay, and will act as insulation. Beware of heavy snow, however, as the weight could break your lid.
The most important thing to keep in mind is the temperature inside your cold frame greenhouse. It is better to err on the cool side rather than the warm side. Summer plants should not go much above 75 degrees and cooler-weather plants not much above 60 degrees. A good way to monitor the temperature is with a wireless electronic thermometer. You can place the probe in the cold frame and check the temperature from the comfort of your home.
When watering your plants, use water at least as warm as the soil. This will go a long way to maintaining the right temperature for the plants, and keep the plants from drastic temperature changes.
Cold frames greenhouses can also be used to harden seedlings started indoors. You can start your spring garden earlier and continue your fall garden later. It also allows you to continue growing some plants over the winter, when others are forced to pay for store-bought greens and herbs.
Tips to improve your cold frame gardening:
- Build the cold frame foundation into the soil.
- Insulate the walls.
- Use a double-glazed top.