Build a self-opening cold frame

Mark Anslow
Thursday, 31st May 2012

Coldframes are essential garden equipment in temperate climates, but forget to prop them open sufficiently in hot weather and you've just built yourself a plant oven! Mark Anslow describes one solution that he came up with...

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.

Richard Gay |
June 16, 2012 - 6:22pm

Dimension the frame to the glass. Mount the glass right up to the edge of the frame with a question-mark shaped metal tang: the long end screws to the inside of the frame, whilst the rest reaches out to hold the edge of the glass.

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