Before I built my cold frame, I was a garden risk taker for years. I went by my gut when determining when to plant. After a stretch of good weather, if the long term forecast looked favorable, I’d get started. The problem was that, as often as not, I’d get over-committed way too early. Lots of stuff would be growing and then a late cold snap would hit. Last spring was a perfect example. I was scrambling around the third week of April trying to find stuff to cover and insulate 12″ tomato and pepper plants, as well as a recently planted sweet potato bed, against 20 degree weather that was coming in. The results were mixed.
One of the lessons I think I’ve finally learned is not to plant too early. The other is to plant the early, cold tolerant crops with a cold protection plan in place. So I built a cold frame out of lumber salvaged from nursery deliveries and a scrap piece of greenhouse plastic. The coldframe allows me to plant early and not have to worry about freezing weather moving back in (because it will).
The Cold Frame
A cold frame is a small, unheated structure that protects plants by keeping frozen precipitation and drying winds off. It only raises overnight temperatures marginally, but it radiates the heat of the day well into the night and warms up quickly when daylight comes. Though it may get nearly as cold inside the structure as outside, the cold does not persist as long. When a cold frame is built in a sunny, protected area, it can radically improve cool and cold weather gardening results.
Cold frames are completely adaptable to your climate and site. My cold frame is a 3′ x 6′ x 18″ freestanding structure built completely above ground, like a raised bed with extra high sides and a sloped lid. In colder climates, it may be beneficial to build a cold frame so that the earth insulates it on the sides. This can be done by digging a pit, then framing the sides and putting a lid on it; or by building above ground and ramping soil up around the sides. Straw bales would be another good insulator.
How I’m Using It
This weekend I planted lettuce, kale, spinach and cilantro in the cold frame. I started the seedlings a month ago and have spent the past week hardening them off. I chose this weekend to plant because the ten-day forecast (for what it’s worth) is predicting very mild temperatures and overcast skies. I would not plant them out if freezing temperatures were expected.
I could just as easily have sown my seeds directly into the cold frame, but that would have slowed production a bit more than I wanted. The cold frame is also a good place to harden off seedlings before they are planted in the open. In the summer, I can remove the lid and use the cold frame to grow whatever I like.