Cold frames are often seen around town and last week I was talking to one of our experienced local gardeners about the cold frame they began using 10 years ago to extend the growing season into the winter for various types of greens. The frame is designed as a single row of concrete blocks in the front facing south to southwest, and three stacked rows of blocks in the back. It has double-walled wood sides that slope from the top of the three back blocks to the one front block. The top of each side is covered with heavy pieces of plastic that move on and off smoothly.
What was especially interesting to me was the use of multiple layers of row cover, in this case Agribon, of varying densities to keep the plants protected from both insects and temperature fluctuations. This gardener has been able to pick greens such as arugula, spinach and lettuce throughout each winter. When day temperatures are below 40 degrees they use all the layers and usually don’t open the cold frame. In past years, during a winter with heavy snow, they have gone for over a month without uncovering their frame and once temperatures warmed were able to pick good fresh greens.
What is a cold frame
and how does it work?
Essentially, it’s a box with four sides and a removable top or lid. It has no bottom since that opening sits on the soil. When built and positioned properly, a cold frame — which is an enclosed structure — harnesses solar energy through its top, which allows in sunlight. It sits on the soil absorbing heat. Depending on type of material used to construct them, the sides might also gather heat. The heat that is absorbed is then slowly given up during the colder nights. Cold frames with thinner walls or sides, say just wood rather than blocks, would cool off faster inside. Those using thinner materials can use milk jugs or other containers painted black and filled with water as solar heat absorbers, or even stones painted black and placed here and there in the cold frame. The insulating abilities of the top will also have an effect on how warm it stays inside.
Benefits to having a
For many the benefit is to extend the growing season, whether by starting seeds earlier in the spring or allowing plants to grow further into the fall and winter. Equally important is that you can start seeds in pots or flats inside the frame, or harden off seedlings you started indoors. If you build a moveable cold frame you can place it over seeds you plant in the garden to give them that extra warmth and time to grow while air temperatures are still cooler. Just move it off the planted area once temperatures warm up.
Materials for a cold frame
If in a permanent location consider building the cold frame using concrete blocks or other types of stone blocks, all of which absorb heat well. Less permanent would be wood planks, which can still be pretty permanent depending on the wood and how you build the frame. Bales of hay work well and are also easy to move as needed.
As for the top, there are many options, including old tempered glass or shatter-resistant storm or shower doors. You could use regular glass doors or old framed window panes, but you might want to buy some clear contact vinyl to place on both sides as protection to prevent the glass from breaking. You can also frame up plexiglass or even multiple sheets of plastic. I like the use of multiple layers of row-cover material, such as Remay or Agribon, which lets in light and moisture but provides an extra amount of heat retention depending on the mix of row cover densities you use. Of course, even with glass or plastic tops you can throw blankets or other material over the top on cold winter nights to help keep the heat in.
Next time, we will look at size, placement, planting and issues when using a cold frame.
Thought for the day: “People don’t notice whether it’s winter or summer when they’re happy.” —Anton Chekhov.
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