When I first built a cold frame I felt very pleased with myself – especially since I had managed to salvage almost all of the materials involved.
All of the wood came from skips and disassembled pallets. It took a while to work out the angles at which the various sections needed to be cut so that the lid would close smartly, but eventually I figured them out and cut the bottom section using a jigsaw with its bottom plate set on a tilt.
The glass came from an old 1950s drinks cabinet that had been languishing in a relative's garage. Because it was already ridged it helped diffuse light more evenly over the planted area. I made a slotted frame for the glass by routing a 5mm groove in some batten and slotting the glass into it. I then added glazing putty to form a ramp all around between the glass and the edge of the batten.
In retrospect I should simply have made a conventional frame for the glass - it would have been stronger and simpler. But my thinking was that water needed to run straight off the bottom edge of the glass, without collecting against a wooden frame, to avoid rot. I'm sure there is a better way!
The wood was painted with some left-over shed preservative. Not very environmentally friendly I'll admit – if anyone has any eco alternatives that provide similar protection I'd be interested in hearing about them.
When the cold frame was finished, I used it successfully to preserve salad crops over the winter months and raise seedlings in early spring. As soon as sunnier days arrived, however, I inevitably forgot to prop the frame open one day and returned to a cold frame full of frazzled plants! I realised that I needed a simple, automated solution.
I had seen greenhouse window openers previously, but had never seen one fitted to a cold frame. I found that they were relatively inexpensive online (certainly in comparison to the number of seeds and plants I stood to lose), so I ordered a Bayliss Autovent XL.
The openers themselves are the kind of technology that is so simple it gives you instant faith in its operation. They consist simply of an articulated arm, a pair of springs (which close the window in the evening) and a sealed cylinder containing a wax/oil mix. As the cyclinder heats up, the wax melts and expands, pushing up a piston which opens the window.
When I went to fix the cylinder to the coldframe I discovered there wasn't enough front-to-back clearance, so I had to take a router to the back panel of the frame to give the vent enough room open and close fully. Once that alteration had been made I simply had to use a screw setting on the vent to make sure it didn't open too early in the morning, or close too late at night.
The automated cold frame was a great success and gave me the confidence to be able to leave plants in it without scurrying home at the end of the day to discover them turned to toast. I eventually built a wooden trough to fit underneath the cold frame that could hold grow bags, giving me the soil depth to grow bigger plants, and also increasing water retaining capacity.