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.


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

  • 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
 

Pat
12/25/2020 8:51:54 AM

I built a 6 x 8 green house and covered it with 6 ml greenhouse plastic and a 12 ml white poly tarp to insulate it better.I use an old crock pot filled with water to heat it .If temps are above 40 I set it on low but if the temps are expected to drop lower I set it on high. When its set on high once a day I have to add water to the crock pot(less than 2 quarts) I use a ceramic heat emitter for back up(connected to a thermostat power plug).If the temp falls below 35 for any reason inside it kicks on too(hasn't yet)My 6 Rosemary plants are thriving here in Zone 6 using this method.A sheet of plastic separates the front from the back (makes it easier to heat)Milk jugs cover my plants for extra protection in temps below 50 ( cut holes in the bottom for air) Mylar sheets also trap heat but because they block light I only use these at night .I barely open the vents if its going down in the 30's I close them if the temps are going down in the 20's.During the day when the sun comes out I can open the vents without worrying as much about losing heat.Hope this helps!


dayhut
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.


dayhut
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






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