Building a Cold Frame From Scrap
Okay. We've talked about watering deep, and before that we talked about getting your seeds and transplants in the ground without them dying of cold or rotting from bad timing. This lesson is about buying yourself some insurance for those in-between weeks, the ones where it's warm enough to plant something but you know a cold night could still show up and ruin your evening.
A cold frame is just a box with a clear lid. That's it. It sits low to the ground, the sun heats the air inside during the day, and the box holds onto some of that heat overnight. It buys you three or four degrees, sometimes more if you build it right. Here, where our last frost can sneak into late May, that's the difference between a tray of seedlings living and a tray of seedlings not living.
What you need
You do not need to buy anything for this. That's the whole point.
- An old window or storm window with the glass still intact. Garages and sheds around here are full of these. Ask around before you buy one.
- Scrap two-by-fours, or really any lumber you've got sitting in a pile. Doesn't need to match.
- Screws, a drill, a saw. A miter box helps if you want clean corners, but honestly mine aren't clean and it still works.
- Hinges, if you want the lid to open easily. Not required. I didn't have any the first time and just propped mine open with a stick.
Building it
Mine's built out of an old storm window and scrap two-by-fours I had left over from a shelving project. Took me a weekend, and I mean a real weekend, not an afternoon, because I kept stopping to figure out angles I hadn't thought through. It's ugly. The corners don't match. I have eleven photos of it on my phone from four different angles because I was so pleased with myself when it worked.
Here's the basic shape. Build a rectangular box with no top and no bottom, sized to fit the window you've got. Make the back taller than the front, four to six inches taller is plenty, so the lid sits at a slant. That slant matters two ways: it sheds rain and snow, and it angles toward the sun so you get more light in during the day. Set the box directly on the soil where you want it, or on top of an existing bed. Lay the window on top as the lid.
That's the whole build. People overthink this. You're not building furniture, you're building a box that keeps warm air in and cold air out for a few extra weeks a year.
Using it
Cold frames are for hardening off transplants, protecting young seedlings on a cold night, and stretching the season on both ends, getting things in the ground a little earlier in spring and keeping things going a little later in fall. Prop the lid open partway on sunny days once it warms up, because a closed cold frame in direct sun can cook whatever's inside. I learned that one the hard way with a tray of lettuce that basically steamed.
This connects to something I've said before: our real enemy here isn't the hot afternoons, it's the cold nights nobody's watching for. A clear night in early May can drop into the high 20s while you're asleep, and a cold frame is one of the cheapest tools you've got against that.
One more thing while we're on timing. A few seasons back I direct-sowed carrots too early because I was impatient and the calendar said I could. The seeds just sat in cold ground and rotted, half the row never came up. The half that did come up were fine, which was almost more annoying, because it meant the timing was the only problem. I write my sow date on a popsicle stick now, stuck right in the row, so I stop guessing and start trusting my own notes instead of my gut. A cold frame doesn't fix bad timing, but it gives you a little more room for error while you're still learning your own yard.
Watch the wood if you're using reclaimed lumber. Old boards sometimes have nails still in them, or paint you don't want to be sanding without a mask. Check before you cut.
Before next time: see what you've got sitting in your garage or shed that could become a cold frame. An old window, some scrap wood, anything. You don't need to build it yet, just take stock.
- D