Weekend project: Cold frame

Justin Newcombe with a cold box, used to keep sprouting plants warm. Photo / NZ Herald

Justin Newcombe shows how to improve your seedlings by building them a simple cold frame

A cold frame is a great project for any avid gardener, experienced or not. The purpose of a cold frame is to increase the quality, germination speed and volume of your seedlings: in effect, it's a small greenhouse. The cold frame provides a constant internal temperature several degrees warmer than the outside air temperature, which does not fluctuate every time a cloud rolls by. You can improve the efficiency of the cold frame by positioning it next to a thermal mass such as a brick wall. The stored energy (heat from the sun) in the wall will slowly be released for a time after sunset, adding another layer of temperature stability for your young seedlings.

The technology is an old one, using a window angled toward the sun and popped on top of a box to trap the heat.

Recycled windows, fence palings and paint can be formed into something with a lovely utilitarian aesthetic. I've made cold frames out of old pallets and plastic bottles but this time I wanted to re-interpret the garden classic. I used an 18mm thick plywood offcut I had left over from a previous project. The best thing ply has going for it is its versatility, but it does have a tendency to warp so it needs reinforcing. And that's when I had the idea of using words. The script will block out some light but the cold frame's heat-retaining properties won't be disrupted. I also cut a very rough fretwork out of the sides of the box to give the frame a nice light look. Finally I've painted the rest of the box black. This is a good choice of colour because as well as being heat-absorbent, it looks rather fine with my fresh baby green, germinating shoots.

Step 1
Measure and cut the pieces for your cold frame. You can make a box to fit any situation where you have the space. The two end panels give you the basic shape. Mine has a flat bit on top approximately 50mm, an angled cut at somewhere between 30 and 45 degrees and a vertical drop of about 100mm which will make up the front of the box. My box is 250mm high at the back and 250mm deep from front to back. The frame can be as wide as you wish, simply measure and cut the front, back and top panels to the required length.

Step 2
Glue and screw the pieces together. I used Gorilla polyurethane glue which expands into any gaps.

Step 3
Once the box part of the frame is glued together, measure and cut the lid.

Step 4
Draw any adornment or message you like on the lid of the box. Make sure enough light will get through your design for the plants.

Step 5
Drill starter holes for your jigsaw then carefully cut the design out. My cutting out is a bit organic and I added to this by whittling some of the edges with a craft knife.

Step 6
Paint with primer and trim difficult-to-paint edges with a craft knife.

Step 7
Paint with acrylic paint. I gave my box four good coats. The cold frame is an outside object and needs to be protected from the elements.

Step 8
Measure and cut the plastic for the inside of the box then screw to the cold frame and lid.
Step 9
Attach the hinges to the lid then attach the lid to the cold frame. I've hinged mine from the front. If you hinge yours from the back small adjustments may be necessary.