Unfortunately, a personal greenhouse remains an unattainable dream for most people, as they are expensive to build and maintain, and take up a lot of space. However, it’s possible to build a low-cost cold frame that performs all the functions of a greenhouse, with little or no heating required.
The cold frame can be seen as a miniature greenhouse that extends the gardening season, whether in spring or autumn. It takes its heat from the sun and the ground, as well as from the back wall if it’s built against the house, protecting the plants it contains from frost from March until late November.
It’s very easy to build, and requires no special tools. First, find an old double frame with intact glass. This will serve as the lid. Then, using plywood, build a sloping box the size of the lid. The front panel should be 30 cm (12 inches) high, the back 45 cm (18 inches), and the sides sloped. To make the corners, drive four 2″ X 2″ pieces into the floor, cutting them to the required height after the box has been built. Treat the wood with a plant-safe preservative or buy pressure-treated wood.
After assembly, paint the whole with two coats of matt white paint to reflect even more light. To retain as much heat as possible, add polystyrene or polyurethane insulation to the inside of the box, and caulk where the lid comes into contact with the box. All that remains is to attach the lid to the back of the structure with hinges, and provide a small piece of wood to hold the lid open on warmer days.
How the Cold Frame Works
If possible, place the cold layer in a south-facing location, protected from strong winds and preferably a little higher than the surrounding ground to facilitate drainage. Leaning it against a wall will provide even more protection against the cold. During the day, when the temperature inside the layer reaches over 21? (77°F), open it to air it out. On very cold nights, cover it with an old blanket. You can even turn the “cold” layer into a “hot” one by installing a thermostatically-controlled heating cable under a layer of sand. A real mini-greenhouse!
What to Grow
The cold layer can be used in autumn to sow a last crop of lettuce or spinach, to force bulbs, to stimulate cactus flowering, etc., or, with a little heating, to extend the harvesting season for tender vegetables. Because of the excessive snow in our region, the bed will be condemned in winter, but from March onwards it will still be used for sowing annuals. In this case, start the seedlings in the house, where they will benefit from the warmth they need to germinate.
When they have four true leaves, they will appreciate the cooler temperatures and more intense light of the cold layer. They can also be used to acclimatize houseplants that have to spend the summer outdoors, or for early harvests of lettuce, radishes, beet, etc. In summer, it’s the ideal place for “chilly” plants like melons and squash… as long as you keep the lid open, except in cool weather.
Larry Hodgson published thousands of articles and 65 books over the course of his career, in both French and English. His son, Mathieu, has made it his mission to make his father’s writings accessible to the public. This text was originally published in Le Soleil on October 1, 1988.