My mother in law is an avid gardener, and on several occasions complained about rodents, woodland creatures and even her own dog eating or otherwise destroying her plants. The growing season in Canada is also relatively short compared to more southern countries, limiting what you can grow every year.
The solution? A cold frame! Basically a greenhouse with an open bottom, it can protect your plants from the elements and from animals. Because it acts like a greenhouse, you can start plants earlier in the year and keep them growing later into the year than normal.
It's easy to build one in a weekend using common tools and materials.
Step 1: Tools and Materials
A hand-held drill (cordless or corded)
A miter saw (powered is better, but you could do it with a hand saw and miter box)
A jig saw, circular saw or table saw (really, anything that can cut large sheets of plywood)
A screw driver
A Tape Measure
A marking implement of some sort
A paint brush
Two, Three or Four windows, with at least one dimension in common
Some 2x2s (as required)
Some 2x3s (as required)
Some 2x4s (as required)
Some inexpensive 1/8" or 1/4" plywood
Exterior wood screws (deck screws)
Exterior wood-sealing paint
Some galvanized or brass hinges (may not be necessary if the hinges on the windows are still good)
Some big, sturdy exterior-grade handles
A tube of latex sealant
A common heating vent (optional)
I was fortunate enough to have a lot of the materials just lying around already. Some were left behind by the previous owners of the house, other pieces were left from tearing apart my basement. Use reused materials were you can!
Step 2: The Basic Plan
The basic cold frame consists of hinged or fixed windows on top of a base. You can either hinge the windows or affix them permanently (and lift the entire frame off the plants to tend to them.) Be aware that the cold frame may end up being too heavy or awkward to conveniently lift, so plan accordingly.
The frame dimensions will depend on the dimensions of the windows. With four windows placed side by side, my cold frame was about 6 feet wide and 2.5 feet deep.
The height of the frame is up to you. It can be low, with just enough room for plants to sprout, or much taller so they can reach nearly full height. I chose to make mine pretty tall, over two feet, so that taller plants could still enjoy the protection the frame provides. Remember that if your plants outgrow the cold frame, you can simply leave the windows open all the time.
Lastly, the top can be either flat or on an angle. I put mine on an angle to facilitate drainage when it rains.
Step 3: Clean and Lay Out the Windows
With four windows to work with, I had a number of options when it came to layout. I could hinge them all against the back edge of the cold frame, I could bolt them together in pairs and hinge them on the back or sides, or even leave off the hinges completely and add handles to remove them completely.
Ultimately, I decided to group them in pairs, hinged together and at the sides, bi-fold style. This had a distinct advantage over hinges at the back. Namely, that full access to the interior of the cold frame could be given, while needing to move the heavy windows only a small distance.
Lay out your windows on the ground and shift them around until you find an arrangement that works the best for you. Take note of the position of each window, marking each one with a number and roughly indicating where the hinges will go.
Finally, measure the outside dimensions of the windows as a group, so that you can build the frame based on that.
* The windows have since weathered one Canadian winder, and the paint on the windows is peeling. I guess they'd been painted with oil paint, and I didn't strip them to the wood before repainting them. Oooops! If your windows are painted with oil, and you don't want to strip/sand them to the wood, then make sure you paint them with oil again.
Step 4: Build the Window Frame
This is pretty straight forward. The 2x2 pieces form a perimeter around the outside edges of the windows. The 2x3s sit directly beneath the 2x2s, the extra inch of width providing a ledge on which the windows rest.
Cut the 2x2s and 2x3s to fit, allowing about 1/8" gap on all sides for wood expansion. Try to make sure the pieces of wood overlap at the corners for enhanced strength. Screw them together with deck screws.
Test the fit of the windows before continuing. The rest of the frame is based on these dimensions, so it's important to get them right!
Step 5: Use Math to Calculate the Bottom Half of the Frame
First, measure the depth of the window frame you just built, when laid flat on the ground.
You can either guess at what angle you want the top to be (10 or 15 degrees from horizontal, maybe?) or you can grab a few scraps of wood and prop up one side. Propping is definitely useful for visualizing exactly how much slope you want - I recommend it. Take note of either the angle you've selected, or of the height your frame has been raised to.
With the depth of the window frame and either the angle or rise you've determined, plug some numbers into your calculator or the handy utility I posted above (hint: one of the angles must be 90 degrees!) You should now have a third measurement slightly less than that of the window frame depth - this will be the depth of the frame itself.
Okay, so now you know the width of the cold frame, and the depth. One last thing to figure out before building the bottom - the height. As I mentioned earlier, I deliberately made my frame extra tall to accommodate taller plants. Take a moment to think about what you'll be planting, or what the frame will be used for. Do you want to shelter tomatoes until they're 2 feet tall? Or are you only interested in herbs? Will you plant things directly in the ground or in boxes? By all means, go outside with a measuring tape and figure out exactly how tall your plants are. I don't recommend much taller than 30 inches at the front though, or it will be a big strain to bend over and work the soil.
Remember, the height of the rear of the cold frame will be the height you just determined, plus the rise you decided or calculated earlier.
Step 6: Build the Sides of the Frame
I built the sides out of 2x4 lumber, so that I'd have enough space to fasten the front and back sections as well as the plywood walls.
Here's where a miter saw comes in handy. You'll need to make a few angled cuts to match your calculations, so that the sections line up perfectly. Dial in your intended angle, and try not to change it until all the angle cuts are finished. That way, at least they'll all line up with each other...
Attach the pieces together with 2.5" or 3" long deck or construction screws. Since you'll be driving screws into the ends, through the 3.5" wide boards, it will be necessary to first drill a suitable counter-bore hole. Use one screw per joint.
Step 7: Attach the Front and Back
The front and back sections of the base are simple rectangular frames.
Measure the length of the window frame, and subtract the thickness of two 2x4s (about 3 inches). This is how long the rectangular frames should be.
The front frame should be as tall as the shorter half of the side frames you just built. Measure pieces of 2x3 to fit, and assemble the frame using deck screws. One screw per joint should be enough, as they will all be screwed together later.
The back section of frame will (obviously) be taller than the front, but slightly shorter than the tallest part of the side frame. The top-forward edge of the back frame should not be higher than the top edge of the side frame, or there will be a gap when the window frame is installed.
Now, hold the frame sections together with clamps and screw them all together with deck screws. Two or three screws per edge should be enough.
Step 8: Attach the Window Frame
Drill some countersink holes in a few locations around the frame, then drive screws into each hole. Three screws along the sides and four along the front and back should suffice.
Now, flip the frame right side up again.
Step 9: Add Walls
With a circular saw or jigsaw, cut pieces of plywood to fit the insides of the frame. Ideally you should try to use complete pieces for each side, but you may use multiple pieces as long as they fit tightly.
Screw each piece of plywood to the frame using shorter 1" screws. Use a few around the perimeter of each piece.
If you like, you can "winterize" your cold frame at this point by filling the spaces inside the frames with Styrofoam house insulation, then screwing another layer of plywood on top. With this additional insulation, you may be able to grow some heartier plants year round.
Finally, drill a few drainage holes along the bottom edge of the window frame. Three should be enough. These holes will allow rain to drain away from the bottom ledge, rather than collecting and rotting the wood.
Step 10: Paint Time!
Start with the exterior. I chose a nice grass-coloured green paint, to help the giant box blend in a bit. But ultimately the colour is really up to you! You could paint it to match your house, or maybe a cool camo paint job with spray paint. Or, make it a family affair and let your kids decorate it however they wish (just make sure you apply a weather-resistant base coat first!)
Paint the interior of the frame with white paint. The white paint will help reflect light towards the plants inside. Apply two coats at least.
The windows must also be painted. I recommend sealing all of the glass with a high quality outdoor silicone sealant first. Once the silicone is dry, patch any holes that were left behind from removing the old hardware.
Paint the windows with a paint compatible with what's already on there - I found this out the hard way. If the windows are old they're probably painted with oil. Make sure you repaint them with oil, or a latex paint designed to adhere to oil. If the windows are painted with latex or you managed to strip all the old paint from the wood, go ahead and use a regular exterior latex.
Step 11: Attach Hardware
For ventilation, I cut a hole in the front of the frame and installed a heating vent I had lying around. This probably isn't necessary at all. On hot days the heat inside the frame is so ferocious that a little vent like this has little effect. It's easier to just open the windows a bit.
Attach strong handles to either end of the cold frame, so that it can be transported more easily. Screw the handles into the thick framework and not the thin plywood walls.
Next lay down the windows and screw on the hinges. You can get fancy with a chisel and dig out indentations for the hinges, if you like. I do suggest drilling pilot holes for all the screws, to avoid splitting the wood. Mount the hinges as square as you can - if they're out of alignment the windows will be difficult to open, and even crack the wood if forced. Also, make sure your hinges are designed for outdoor use or they'll rust before the summer is over.
Finally, attach handles for the windows so that you can actually open them!
Step 12: General Use Instructions
According to a number of websites, the best place is one with lots of sun, preferably facing south. Placing the back edge of the frame against a house or building will help capture some of the heat from the building during colder months, and provide some protection from wind.
It's also a good idea to place it on a slight slope, so that water drains away and your plants don't get water logged. My mother in law has a great hill in her back yard for this purpose. A bit of soil was removed at the back of the frame to make sure it sat flat on the ground.
The cold frame should sit directly on the ground, with no gaps where air, rodents or bugs can get in. You could even to bury the bottom an inch or two into the ground (If you do this you might want to wrap the bottom edge in plastic for better rot protection.) If burrowing rodents are a problem, you should also install galvanized metal mesh 4-6 inches into the ground around the perimeter of the cold frame.
General Use Instructions
A cold frame acts like a miniature green house, greatly extending the growing season. You can plant things earlier in the year and keep them growing much later than normal. In areas where it doesn't get too cold (say, -10C at the lowest) you may even be able to grow heartier plants year-round. Your best friend in this case is a thermometer: install one inside the cold frame to monitor the temperature for a few days. Choose your plants based on what temperature conditions are available. You can even get fancy with a wireless thermometer and monitor things from inside your house!
During the summer, it's likely that the interior of the cold frame will get hot enough to bake your plants to a crisp. Don't feel bad if you need to leave the windows open all the time, even at night, to keep the temperature down. It is the summer, after all! If rodents (raccoons, especially) or deer are getting in because of this, you can cover the top of the frame with metal mesh.
Apart from that, proceed as usual. Make sure your plants get the water and fertilizer they need. Pluck out any weeds that somehow got in.