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I live in Northern Virginia on the cusp of USDA plant hardiness zones 7a and 7b which means I can get annual extreme minimum temperatures between 0 and 10 degrees Fahrenheit. We just had a very cold night where the temperatures were forecast to dip to 6 degrees in my neighborhood so I wanted to make sure my more tender plants had some extra warmth.

Open structure with the door above.

In past posts I have shared the cold frame structure I have built up against my house. It easily provides 5 to 10 degrees of additional warmth just by being closed and up against a heated house. You can see in the image above that a basement window is inside the structure, and this is one easy way to boost the temperature in the structure when needed.

View from inside with the window open less than half an inch

Last night I closed up the shelter and cracked the basement window just a fraction of an inch to allow air from the heated basement to warm the space. When I checked this morning, the outside temperature was at 8 degrees, and the thermometer inside the cabinet read 20 degrees. This is exactly what I was hoping for. The trees stored there are experiencing freezing temperatures, but not the single digits they would have experienced without the shelter.