Are you looking to harvest more out of your same garden space? Want to get growing and keep growing longer on each of the shoulder seasons? Adding a cold frame can be part of the solution. Using passive solar energy to warm soil sooner helps northern gardeners get a jump on the season—naturally!
After considering existing microclimates in your yard (like noticing those spaces that melt first, where bulbs or perennials pop up first) my next go to are cold frames or low tunnels.
Cold Frame & Low Tunnel
Both of these structures provide heat-trapping benefits that warm both the soil and air temperatures to get you planting cool weather crops weeks earlier. Low tunnels are easy to construct and move around, following your crop rotation. Cold frames are stationary and built out of heavier, more insulating materials, which typically equates to warmer temps inside those structures.
Cold frames are basically a bottomless box, with an angle on top, facing south, to capture solar energy. I suggest going no more than 3 feet front to back because you’ll need to be able to easily prop up the window or plexiglass piece.
The exact materials you use to craft a cold frame with are up to you. Most likely you can use what you’ve got! We’ve built ours out of both wood and windows, and wood and plexiglass.
Using concrete blocks and even straw bales works, too.
You’ll want the top of your cold frame 18 inches to 3 feet off the ground so you can angle the ‘window’ section to capture the most sun. The exact angle isn’t crucial. Try for between 25 to 55 degrees, and make sure it faces due south for the most intense warming benefits.
My cold frame warms up sooner than my low tunnels. These structures tend to warm up soil temps between 10 to 20 degrees F more than uncovered soil. Air temperatures will always fluctuate more than soil temps, but hitting the timing right you’ll have seeds that can germinate in 40 degree F soil and grow happily in 50 degree F air temps.
For me, the trickiest part of growing early-season crops in a cold frame is not letting them overheat! Once the sun becomes more powerful, you’ll have to prop open the window to keep air temps comfortable for the cool-weather crops.
My favorite crops for starting out the season in the cold frame include those that start germinating around 40 to 45 degrees F, such as:
- All the spring salad greens
(Note: If you start playing with season extension, consider getting a dedicated soil thermometer.)
My favorite way to get a jump start on growing the earliest spring veggies is using my cold frame. But I not only plant directly into my cold frame, I also place my winter sowing jugs inside my cold frame to double on the layers of ‘cover.’ Season extension pioneer Eliot Coleman introduced this idea, estimating that the double layers moves your garden beds about 1,000 miles south.
And up north, that 1,000 miles makes weeks of difference in our first harvest!