How to make a cold frame

Very useful things, cold frames. They provide a half-way house accommodation for plants while hardening off from the greenhouse; they provide some protection for seeds undergoing cold stratification; or even for somewhat tender plants that need a little cossetting. I have had one for a couple of years, but felt constrained by lack of cold-frame space. I resolved to make more – two, in fact. Here is how to make a cold frame, largely from scrap wood.

The cold frame will just sit there on the ground, so it doesn’t need to be hugely robust in construction. I made a simple box frame using pallet boards. To save mucking about with too much cutting, I just made the long sides the same length as the boards, in this case 110cm. The rear needs to be higher than the front. I used four boards to make the back, if you want a taller cold-frame, use more.

I used some scrap wood, cut to size, to fix the four boards together centrally, using screws rather than nails. On its own this wouldn’t be enough but the ends will be secured later on.

I repeated this process for the front side, using just 3 boards this time. Next the sides. As the back is taller than the front, the sides need to slope so that the lid can sit flush with the frame when closed. I found it easier to make the sides like the front first, then cut the slope.

Notice I’ve left enough room at the top of each side to make a cut for the slope. To help me, I made sure to position the top horizontal board at the correct distance – the width of one pallet board – from the top. This makes the cut easy, it just needs to go from the top corner to level with the horizontal board. I ended up using a hand saw as I found this easier to control.

I was left with these.

To give the frame a little bit of rigidity I braced the corners using a bit of offcut pallet rail, any bit of battening or similar scrap wood is fine. In the below picture you can see one in the only visible corner, back right. I screwed it to the side panel first, flush with the edge, then screwed the back panel to the battening, repeating the process for each corner. I am so used to having the electric drill – if I had to screw this together by hand I might not bother, too much like hard work. However, if you don’t have one, perfectly OK to use hammer and nails. This will be a little less robust, but honestly it’s just going to sit there, it’ll be fine.

That is the basic frame finished, just the lid to do. For my first cold frame I had an old shower screen door, a big heavy piece of safety glass. I constructed a heavy duty frame for it – it works fine, but the whole lid is very heavy. This time around I had some spare polythene sheet which will do the job just as well. It’s purpose is mainly to keep the weather off.

To make the lid, I measured the width of the frame from side to side. I cut two pieces of scrap wood to length and placed them on the worktop. I then adjusted their position until they were the right distance apart – ie to match the front-to-back dimension of the frame, taking care to ensure they were square.

I cut the polythene sheet to be a couple of inches larger in every direction then just stapled it to the wood, taking care to make sure it was fairly taut. I then just screwed three pieces of wood across the lid to make it a frame – one at each end and one across the middle. I folded the spare polythene over the edges of the board and stapled it to the other side.

Lastly I fitted the hinges. I found it easier to attach the hinges to the back of the frame first, then sit the lid on the frame and having positioned it correctly for the hinges, screw the hinges to the lid.

That’s the finished article. I ummed and erred about treating the wood. As it is made from pallets, which are pressure treated at some point in their manufacture, I thought I might get away without. In the end I had some wood preserver handy in the shed so I did give it a coat of that both inside and out.

Here it is in position, with some resident seeds. You’ll note that seed trays fit – that wasn’t an accident, I measured them and made sure the sides were long enough to cater. A rare example of me actually following the old saw “measure twice, cut once”.

And here are the pair, both made in the last couple of weekends. The first one took an afternoon to make, including thinking through how I was going to make it, the second was a lot quicker, just a couple of hours, but I knew what I was doing by then.

They are a bit tucked away between the shed and the greenhouse – ideally they would be in a sunnier spot, but this’ll do for what I need them for. This area already has weed membrane and gravel, so I didn’t have to worry about making the floor weed proof, I just plonked them both down.

Why not make one? Give me a shout here in the comments, or on twitter, if you have any questions.

I’ll be back soon with more propagation and mucking about in the garden.