The simple anatomy of a cold frame
Cold frames are just little boxes that trap and retain heat. Just like a greenhouse, these little garden tools are used to create an interior growing environment that is warmer than the exterior environment. It's all about the sun. In the northern hemisphere, cold frames face the south and they are constructed and positioned in such a way that they collect as much direct light as possible during the day. Notice that in the picture below the back wall is higher than the front wall. When the transparent lid is placed on top of both these walls it sits at an angle facing south, toward the sun. Inside the cold frame the soil heats up and the air is kept warm throughout the day. Any insulation that is added to the cold frame helps to keep the air warm after the sun goes down. In Pound Ridge, NY the climate allows us to grow a wide variety of 'cold hardy' greens, whiche are varieties of greens (like kale and lettuce) that can grow at cold temperatures and tolerate freezing temperatures. In general, your summer varieties of greens will do poorly in a cold frame in January - stick with the 'cold hardy' greens. Here is a list of what we are currently growing:
Forcea lettuce
Red Russian kale
Green kale
Arugula
Purple mizuna
Escarole
Italian dandelion
Pac choi
Tat soi, and
Hardy white scallion
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| A cold frame is essentially a small, south facing green house used for extending the growing season. The picture above shows the basic design, on which more bells and whistles can be added |
The following pictures were taken at the Armstrong House on New Years Eve, 2012. You can see that snow is no problem for our cold frames - the little plants inside continue to thrive.
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| Our rectangular cold frames sit on the southern side of the Armstrong House. You can see there that we removed some snow from the plexiglass on the cold frame's right side. |
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| A close up picture of the cold frame lid. Can you see the condensation on the inside of the plexiglass? That is a great sign because it means that the temperature inside the cold frames is warm enough to evaporate water from the soil. |
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| Harvest time! My partner Sarah Bush picks kale from one of our cold frames. Examine the cold frame's construction: we used straw bales as insulation and lined the cold frame's inside with a thin reflective insulator. Currently, our lid is not attached to any part of the cold frame - it just rests on top of the straw bales. This method is not ideal for the long term but it seems to be working for now. |
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| Sarah Bush harvesting greens from our second cold frame. You can see that on this 30 degree day, Sarah (a Tennessee native) is dressed for the cold. The 'cold hearty' greens in our cold frames do just fine through the winter. |
There are hundreds of cold frame designs out there - many of which can be made with recycled materials. Just look at all the cold frames on Google images. Your cold frame design will vary depending on your growing goals, your space, your winter climate and your materials. As long as you are achieving a warm temperature inside your cold frame, you are doing it right. The straw bale cold frame pictured above is not a permanent structure because the straw will eventually rot and loose its ability to insulate. In the future we may continue to use temporary straw bales or we may choose to design and construct a permanent cold frame. The sky is the limit.
Want to learn more?
The book Four Season Harvest by Elliot Coleman describes how to successfully grow food through the winter. Elliot has mastered season extension through his work up in Maine. Here in Westchester County, Stone Barns uses and demonstrates many different season extension methods to grow food through the winter. In fact, it was the Stone Barns Vegetable Farm Manager Jack Algiere who assured Sarah and I that cold frames would work here in New York.
Happy growing
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