A cold frame is like a mini-greenhouse. It is made of wooden sides, a glass cover that can be vented, and holds rich soil. The cold frame can be used in early spring to start seedlings, to harden-off plants started indoors, or to protect vegetables in the winter. A cold frame will keep vegetables 10°–20° warmer than the ambient temperature.
Savoring the rich taste of vegetables just picked can be addictive. Many families have recently begun vegetable or victory gardens to save money and to have local produce available throughout the warm weather months. The pleasure of growing your own and enjoying the health benefits of unprocessed foods makes a gardener wish that summer would never end.
But, of course, it does end and cold weather comes eventually. However, cold weather does not have to mean the end of growing vegetables. A simple cold frame allows gardeners to have fresh vegetables into the winter and for those in hardiness zones 7–9, a cold frame may be viable throughout the entire winter.
Just imagine, going out to your yard in December and harvesting greens for a dinner salad.The fresh picked lettuce and spinach will taste so much better than the store-bought products shipped from Mexico or California. The cost of fresh produce will be less also. You will be measuring it in terms of pennies and not dollars.
How Does a Cold Frame Work?
A cold frame traps the sun’s heat through the glass cover and holds the heat in place with its wood or cinder-block frame. Plants are protected from the elements and the soil retains heat to keep the plants thriving. A well-positioned cold frame will have a sloped roof with a southern exposure. The roof will be made of framed glass.
Cold frame covers or roofs can be made of old, framed windows or storm doors. If you do not have framed glass, acrylic can be placed in a frame and used.
The size of a cold frame is dictated by the dimensions of the cover. If wood is used for the sides of the frame, choose a wood that tolerates dampness such as cedar or redwood. Most people paint their cold frames to add protection to the wood surface.
Once the frame is built and the cover installed, add well-composted soil deep enough to provide growing room for roots—usually 12”–18”. Cold frames are built on an angle with the back of the frame being 50% higher than the front thus providing a slope for the glass cover.
It is essential that your cold frame have the ability to be ventilated because even on winter days when the ambient temperature is above freezing, it can become too hot.
Wood pieces or concrete blocks can be used to prop the cover and allow hot air to escape. Or, if the cover has sliding panels, just open them during the heat of the day. Don’t forget to close the cover at night or when the temperatures are below freezing. If the temperatures are very cold—low 20’s or teens—use a blanket or burlap cover at night.
What Crops can be Grown in a Cold Frame?
- Leaf Lettuce
A cold frame for winter vegetable gardening or protecting new plants is an excellent investment as well as a source for fresh, healthy produce. A cold frame is relatively easy to build or a kit can be purchased relatively inexpensively.
Once your building project is completed and the cold frame has been filled with rich, composted soil, you will be able to grow and harvest a wide variety of salad greens in the fall, winter, and early spring.