Cold Frame Calculator

How It Works

A cold frame is essentially a miniature greenhouse -- a bottomless box with a transparent lid that sits on the ground over your plants. It traps solar heat during the day and insulates at night, creating a microclimate 10-20 degrees F warmer than the outdoor air. This lets you start seeds earlier in spring and keep harvesting later into fall or even winter. The best cold frames face south, have a sloped lid (higher in back) to maximize sun exposure, and are easy to prop open for ventilation on warm days. Overheating kills plants faster than cold, so always vent when temperatures inside exceed 70 F. You can build one for under $20 using scrap lumber and a recycled window.