What Is A Cold Frame?
A cold frame is a protected plant bed, like a small greenhouse, that helps extend the growing season. Historically, cold frames were built in addition to greenhouses. After seeds germinated in the heated greenhouses, they would be moved to a cold frame before the final planting in the ground. It has no artificial heat added, which is how it got its name. It only uses the heat from the sun to warm the inside temperature for your plants to grow.
The temperature difference between the inside and outside of the frame is generally not more than 5 to 10 degrees. A mat or blanket can be placed over the frame on cold nights to conserve heat, but that will increases temperature by just a few degrees. There are times, however, when a few degrees can be very important.
In cold climates they allow plants to be started earlier in the spring and to survive longer into the fall and winter. It has a transparent roof to let the sunlight in and prevents heat from escaping. It’s built low to the ground and protects plants from the wind and the cold winter months. The back of the box should be taller than the front (usually by 3 to 6 inches), which lets the top of the cold frame sit at an angle. This slope allows maximum sunlight to enter the structure and will help it shed rain and snow.
Presently they’re mainly used in home gardens and vegetable farming for starting seedlings to later be transplanted into the ground and can also be a permanent home for “hardy” vegetables grown for fall or winter harvest.
By providing increased warmth in early spring with the cold frame, you can harvest vegetables ahead of their normal season when they are extremely expensive to buy. Lettuce, parsley, onions, spinach, radishes and turnips are great vegetables for a cold frame. You can plant one vegetable for the whole cold frame or a combination of vegetables to get a wide variety throughout the year.
Types of Cold Frames
Portable – A portable cold frame is smaller and more lightweight than a permanent or temporary cold frame. You can purchase kits for portable cold frames at most garden centers, catalogs and the internet auction sites. They are usually made from aluminum framing and polycarbonate tops and sides.
Permanent – Permanent structures are built in the ground. Therefore be sure it’s placed where it’ll get the most sun. These are generally made from wood, brick, or concrete blocks to provide a more solid structure. The only light that reaches the plants is from the top. By being in the ground it provides more insulation and helps protect the crops.
Temporary – A temporary cold frame is the best way to experiment with cold frames
before building a permanent one. Simply use a few straw or hay bales, stacked around your plants. For the lid you can use an old window, or sheet of rigid plastic. When you don’t need the hay any longer you can use it for mulch in the spring.
Bulb Frame- A “bulb frame” is a cold frame designed for growing bulb plants particularly in wet winter climates. It’s generally raised further above ground level than a normal cold frame, so that the plants can be seen better when they flower. They’re often used for the cultivation of winter-growing bulbs which flower in the autumn or spring. The covers are used in winter to provide some protection from very bad weather, while allowing good ventilation. Then in the summer, the covers provide dry, warm conditions which many such bulbs need.
Things to Consider About Cold Frames
Probably the most important thing to consider is the location for your cold frame. How much sunlight will it get? It should be positioned with the lid facing the south. In the winter when the sun is low and the days are short you will still get the most sunlight.
You will also want to avoid the bottom of a slope because it can cause a problem if there isn’t good drainage. The ideal spot would be if you have a spot that slightly slopes to the south. That would have good drainage and maximum sunlight.
Size is another consideration. How much winter gardening do you want to do? How much south-facing space do you have available? If your goal is to garden all the way through winter, then build the largest one your can. Do you already have an old window or glass door that you can use, make the frame the same size.
Also consider where your water source is and how much work it will take to irrigate it. Will you have to carry water?
Pros and Cons of Cold Frames
Pros–You can have vegetables all winter long. They are easily moved (unless permanent) and a lot less expensive than a green house. They will protect your plants from the wind, cold, sleet and snow. Your winter garden with a cold frame will save you money on vegetables.
Cons – Lightweight frames are cheap and easy to move but easily damaged in the wind. The temperature is harder to manage than in a larger greenhouse. Because of the smaller space, the temperature will fluctuate more.
A Hot Bed is a heated cold frame, basically with the same benefits as a mini-greenhouse but with limited space.