Simple, Heated Cold Frame

Use a birdbath heater or a crock pot to warm up a small cold frame, extending your growing season even further.

| February/March 2011

  • Blue Pot
    The warm water in this ceramic pot keeps the cold frame warmer on chilly nights. 

  • Blue Pot

We love our organically grown vegetables, but in Montana we have only four months to grow our garden. To extend the growing season, we use a cold frame and a simple heating device. 

Our cold frame is basic: a rectangular wood frame topped with recycled thermal-pane windows and insulated on the sides with straw. Inside the frame we keep a medium-sized ceramic pot of water, which contains a birdbath water heater. 

The submersible heater is designed to keep water just above freezing. It has no controls — just plug it in (use a GFI outlet). Birdbath heaters are thermostatically controlled to automatically turn on when needed (35 to 40 degrees Fahrenheit) and shut off to save energy. 

To minimize evaporation, we keep the lid on the pot (the electric cord for the heater enters through a convenient ladle hole). The water acts as a heat reservoir. We also monitor the temperature in the cold frame with a wireless indoor/outdoor thermometer. Even though there is snow on the ground, a sunny day in March or November can drive the cold frame above 85 degrees, prompting us to prop open the glass top. 

As fall progresses to winter, we cover the cold frame every night with a tarp. This, coupled with the heater, gives us another three months of cold-hardy salad greens and herbs. In the spring, this process allows us to grow plants March through May even as frequent freezing temperatures occur. 

Ron Krager
Bozeman, Montana

10/10/2020 8:06:50 PM

Geat Idea! Im thinking a thrift shop coffee cup warmer, crock pot, an aquarium heater in a bucet of water could do as well in my small-ish (4x4) cold frame.

10/10/2020 8:04:28 PM

Geat Idea! Im thinking a thrift shop coffee cup warmer, or even a crock pot, could do as well in my small-ish cold frame

Gale Green
4/20/2011 9:15:51 AM

Hi. I like this idea! Could you add a diagram to show the size of your cold frame (or describe it)? How much space will this method heat? Thanks. gg



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