An insulated cold frame for winter crops with the plastic windows removed. (Mary Lowther photo)

An insulated cold frame for winter crops with the plastic windows removed. (Mary Lowther photo)

Mary Lowther column: Cold frames extend the gardening season

A cold frame is a solid structure that can sit over crops to extend their lives

By Mary Lowther

Gardeners who make the most use of their land would do well to invest in a cold frame. Usually we make these simple structures ourselves since they don’t require fancy carpentry skills. They do take some planning, time and maintenance, but if you want fresh salad greens in the winter, it’s worth the effort.

A cold frame is a solid structure that can sit over crops to extend their lives when weather gets cold, can be sown now for vegetables that will grow on to produce crops that can be harvested all winter, and can be a place to grow early spring vegetables or a transition area to harden off potted seedlings in the spring, preparing them for the rigours of the less pampered life ahead in the open garden. I recommend making one wide enough to span the width of the garden bed and short enough so that one person can move it without having a hernia. The sides are usually made of wood with the back side about 12 inches high, the front about eight and the sides cut on a slant to join the front and back. We tried using glass windows for the top but not only were they too darned heavy, they cracked in the cold weather, so I re-used their frames and covered them with clear plastic. I hold them on with bungee cords and open or remove them on warm days.

Author Eliot Coleman suggests putting a thermometer inside and keeping the temperature around 21 C until winter and then around 16 C during the day. He says to err on the side of coolness in order to produce plants hardy enough to take on cold weather in winter, so we need to check the temperature and open the tops on hot days. On cold winter nights it’s a good idea to cover the frame with a blanket to help keep it warm inside, and I’ve added Styrofoam insulation to the inside of mine for extra insulation. Coleman recommends putting a thermometer in a slatted box on the soil in the centre of the frame as this area will give the truest reading.

Crops we can plant now inside a cold frame include arugula, Italian dandelion, lettuce, mizuna, radish, spinach, carrot, kohlrabi, mache, scallions and sugarloaf chicory. I’m going to keep a dry clean 12 inch path around the cold frame in hopes it will deter voracious insects from crawling under the frame to gobble up my carefully tended plants in their cosy warm abode.

During these last warm months we still need to water to get the plants underway, but once cold weather hits they shouldn’t need much water, if any. I plan on watering by hand with a rosette attachment. My soaker hoses will be put away and I don’t use a sprinkler in the garden, so hand watering will suffice.

We can also extend the lives of cold-sensitive crops already in the ground, like carrots and chard, by covering them with plastic tunnels. I haven’t been successful growing snap peas this way, but I’ve read that some gardeners have. Tomatoes and cucumbers can be taken down from their trellises, tops nipped off so their energy goes into ripening fruit instead of growing more vine and laid over straw on the bed and covered with plastic as well to extend their season. Once it gets too cold for them and before they start freezing I bring them inside and either find a place to hang tomato plants upside down, or make sauce with the tomatoes and pickle the cukes.

Please contact with questions and suggestions since I need all the help I can get.