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Garden Construction Coach on How to Build a Cold Frame

March 08, 2009

In an attempt to prolong the growing season I built this cold frame. It is easy to build and not too expensive and is a good size to start plenty of plants as well as winter over a few.main_large

The most expensive part of the cold frame is the glazing. For my project I just used acrylic glazing purchased at my local home store, but you could also use old windows, glass panels or polycarbonate greenhouse panels.

Along with some kind of glazing, all you need for this cold frame is a sheet of 1/2″ plywood, 3 – 2x6x10s, 2 – 2x2x8s,   3″ corner brackets, 5″ T-hinges, plenty of 2 1/2″ #8 wood screws, and some 3/4″ #6 Round head screws.

I started by cutting the plywood. You need two pieces that will form the sides. First,  I turned the plywood on it’s long edge and cut a section off that is 37 1/2″. This piece is plenty to get the two sides.

Since the width of the cold frame is 37 1/2″, I measure up 11 1/2 inches on one side and then 23 inches on the other. I drew a line from each point to mark my angle cut. I did the same thing from the other side of the cut piece. Now you have your two sides of the frame.

Next, I took the 2x6x10s and cut them in 1/2 giving me 6 5 foot pieces. 2 of these will be the front and 4 of them will be the back. I line up one of the 2×6 with the front or short end of a plywood side and screw them together with the #8 wood screws. Then I did the same on the other side. I repeat this stacking one more 2×6 in the front and a total of 4 for the back. Now we have the main box.

To strengthen everything I cut two pieces of 2x2s that will run on the inside of the plywood at the base. Then cut another set that will run along the inside of the plywood on the top edge. These pieces not only strengthen things, but will also give the lids a place to rest on.

Cold Frame Filled With Veggie Starts

Cold Frame Filled With Veggie Starts

With the main cold frame done, I cut the 2x2s for the two lids. Each lid is long enough to go from the top of the back wall to the front about 39 inches and is also about half the width of the cold frame, just under 30 inches. Each corner of the lid frame is connected with the 3 inch corner brackets.

Next, I cut the acrylic glazing to size of the frame. I made it about 1/4 inch shorter and thiner then the frame. The pieces are attached with the #6 wood screws and it is best to pre drill the holes through the glazing. The Rounded head screws will hold the glazing now and snug to the frame.

Next I attached the lids to the back with the T hinges. First i put both lids on the cold frame and positioned them between the sides so the could each open individually. then I attached the T-hinges to the back of the cold frame and then the back of the lid.

The cold frame is basically done. The last thing I did was get some perforated straps from my local hardware store to help hold up the lids for venting. Attached a screw to the front of each lid in the center and allow it to stick out 3/8th of an inch. Then half way down the top front 2.×6 in line with the screw in the lid I attached a L-hook. Hook one end of the strap to the L-hook. To prop up the lid, open it and connect on hole on the other end of the strap around the screw in the lid.

-Bob

main_small side_open front_open

11 Comments

  1. rhaglund says:

    Since Bob built this, we have modified the box a bit. I didn’t like the metal poles that hold the box open. They’re great for venting, but they were a bit of a hazard for holding the box open when working in it. To modify, Bob added rope holds. Now the entire lid can be pushed back and held open by the rope. Also, to help with keeping the box warm, Bob lined the “floor” with scraps of rigid foam insulation. Since our box is on a cold patio, the extra ground insulation helps keep the soil and roots warm and protected. And, don’t forget to put a thermometer inside the box. You might be surprised how hot they get even on a cold day!

  2. Margaret says:

    What did you use to cut the glazing without cracking it or chipping the edges?

  3. balbert says:

    The glazing was cut on my table saw with a fine toothed blade. I also fed it through at a slower rate. Personally I don’t think this was the best technique and I understand it can be cut with a plexiglass tool similar to a glass cutter. I would check with the store you buy it from on the best way to cut the material.

  4. Bob Olesen says:

    Why did you opt to build a rigid cold frame (which I will grant is the style used for decades in U.S. gardens) versus the style that uses plastic sheeting spread over hoops (as was done by the gardener on Mother Earth’s web site)?

  5. Catherine says:

    It looks great. I’d love to have some that would fit over our raised beds.

  6. rhaglund says:

    We’ve come up with some design ideas to do just this. Get in touch if you’d like a gardenmentors consultation to help you with yours 🙂

  7. rhaglund says:

    Good question. We actually have both. The hoop house is over raised beds in one part of the garden, and we’ve been using that for several years. The cold frame, which stays a bit warmer, is great for potted seedlings that need some time hardening off after being started indoors. It allows us to start hardening tender seedlings but not put them into larger beds/early where they may struggle in the beginning. West of the Cascade Mtns we have lots of soil microbes that can take down tender seedlings rapidly. This just gives us an extra insurance policy.

    By-the-way, the cold frame was almost 100F when I went out to vent it this morning. That was from sunlight. The air temp outside the frame was hovering around freezing and frozen rain was covering the exterior of the cold frame.

  8. […] you don’t have room or budget for a greenhouse. Maybe you don’t have room for a coldframe. Instead, consider adding a simple hoophouse over your exisiting vegetable beds or plot. They are […]

  9. […] of the heated greenhouse. You can accomplish this at home by keeping starts in a cold frame (learn how to build one here), greenhouse or even hoop house. Or taking them outdoors for longer and longer periods of time over […]

  10. […] Cold frames are bottomless, slopped greenhouses on a much smaller scale, with four sides and a clear lid (glass, fiberglass, heavy plastic, or frost cloth) that opens for ventilation, and gardening. A typical frame is about 3 x 6 feet with an 18-inch back and a 12-inch front wall. You can purchase a kit, or make your own from scrap wood by using the instructions at, http://gardenmentors.com/garden-help/greenhouse/garden-construction-coach-on-how-to-build-a-cold-frame/ […]

  11. […] of the heated greenhouse. You can accomplish this at home by keeping starts in a cold frame (learn how to build one here), greenhouse or even hoop house. Or taking them outdoors for longer and longer periods of time over […]

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