Why cold frames beat hoop houses for most of us
We've spent eight lessons getting the bed ready. Good soil, right depth, drainage sorted, prepped for shoulder-season planting. Now we get to build something.
Before we start cutting scrap wood, I want to talk you out of the thing you probably want to build.
Everyone who gets serious about season extension eventually looks up hoop houses. The big arched tunnels, PVC or metal conduit bent into ribs, plastic sheeting stretched over the top, sometimes tall enough to walk into. They look impressive. There's a reason every gardening magazine cover shot is a hoop house and not a cold frame. A cold frame is a box with a lid. It doesn't photograph well.
Here's what actually happens, though, once people build the tunnel.
You've got a structure that's harder to vent (you're usually rolling up plastic on the sides by hand, which means you're out there twice a day adjusting it). It's more exposed to wind, which out here means it's more exposed to wind, full stop, especially if you're anywhere that catches the canyon gusts. It costs more. It takes longer to build. And because it's a bigger commitment, people tend to either underuse it or get frustrated with it and let it sit.
A cold frame is a box, usually about knee height or lower, with a clear lid that opens. That's it. You can build one out of scrap lumber and an old window in an afternoon. You vent it by propping the lid open a few inches. You close it by lowering the lid. There's no learning curve.
Honestly, this is a mildly contrarian opinion and I'll own that. Everybody wants the impressive tunnel. But for a home garden, for a bed that's four feet by eight feet, a cold frame does the job better, costs less, and you'll actually use it because it's not a project every time you need to adjust it.
What you actually need
A cold frame is three things: a box with no bottom that sits on your soil, a clear lid, and something to prop the lid open with. That's the whole design. Everything else is preference.
For the box, use what you've got. Scrap lumber, old deck boards, cinder blocks if that's what's lying around. Back of the frame should be taller than the front — six inches taller is plenty — so the lid slopes and sheds water, and so it catches more low winter sun at an angle instead of sun bouncing straight off flat glass.
For the lid, an old window is the classic move, and it's what I'd point you toward if you can find one. Check the classifieds, check with anyone doing a remodel, check the free section before you buy anything new. A storm door works too. Clear corrugated plastic roofing panels work if you can't find glass. The lid just needs to let light through and be rigid enough to stay put in wind.
The build, step by step
- Measure your bed first. Build the frame to fit the bed you already have, not the other way around. If your raised bed is four by eight, build a frame that sits inside those dimensions or matches them.
- Cut your side and back boards, back taller than front by four to eight inches, depending on what depth of window or panel you're working with.
- Screw the corners together, don't nail them. Screws hold up better against the frost heave and the seasonal expansion the wood does out here with the dry air.
- Attach the lid with hinges at the back, so it opens like a car hood, hinge side up top.
- Add a prop arm. This is not optional. A notched piece of scrap wood, or a simple hook-and-eye chain, anything that holds the lid open at two or three different heights so you can vent a little or a lot depending on the day.
That last step, I want to slow down on, because I learned it the hard way.
My oldest, Bryce, helped me build our first cold frame years ago, out of an old window we got off a classifieds listing for free. Good window, heavy glass, real wood frame. We built the box, hung the lid, felt pretty proud of ourselves. Didn't think hard enough about how much that lid weighed. It came down on his fingers before either of us had a hand free to catch it. He was fine, nothing broken, but it scared both of us plenty. Every cold frame I've built since has a prop arm built in from the start, no exceptions, and I'd tell you the same even if it wasn't my own kid's fingers that taught me.
If you've got a heavy lid, especially glass, treat that hinge like it wants to slam. Because it does.
Why the venting matters more than the cold
I've said this before and I'll keep saying it because it's the thing people don't believe until it happens to them: more cold frame crops die from cooking on a sunny 45-degree day than from an actual freeze. Glass and clear plastic trap heat fast. A cold frame closed up tight on a bright winter afternoon can hit 90 degrees inside within an hour, even with snow on the ground outside. Your spinach doesn't care that it's cold out. It cares that it just got steamed.
So check it. Prop the lid an inch or two on any day with real sun, more if it's above 40. Close it back down as the sun drops. This is the part of cold frame ownership that actually takes discipline, more than the building does.
Before next time
Go find your scrap lumber and see what you've actually got to work with, and if you don't have an old window sitting around, start keeping an eye on the classifieds now. Next lesson we build.