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Main Content
cold frame

How to Build a Cold Frame

Benjamin Kilbride, Editorial Assistant at The Old Farmer’s Almanac

Learn how to construct a miniature greenhouse to protect plants from frost and extend the beginning of the growing season.

What Is a Cold Frame?

A cold frame is a bottomless container with a skyward-facing window used to protect plants from frost and speed up their growth during the early spring months. Basically, it’s a miniature greenhouse with easy access to the plants inside. A cold frame design can be as simple as a brick-edged trough in the ground and an old window covering the top or as intricate as a wooden box with a greenhouse-plastic–covered hinged lid. 

Why Use a Cold Frame?

A quick and easy way of providing extra protection to a limited number of plants in the garden, a cold frame is the perfect size for a crop of microgreens, a few herbs, or some starters in spring. Start small before making a large investment of time and resources on a full-size greenhouse or row covers.

A Simple Design

Cold frames can be as simple or complicated as you want to make them. This design is for a 24x12-inch, lightweight version that can be picked up, moved, and placed over plants in need of protection as needed. It is perfect for frost-sensitive plants such as tomatoes and peppers.

You Will Need . . .

A total of 184 inches (15.3 feet) of 3/4-inch-diameter PVC pipe cut into:

(3) 24-inch-long pieces

(6) 12-inch-long pieces

(4) 8-1/2-inch-long pieces

6 three-way fittings (an “L” with the third leg extending upward at 90 degrees)

4 angle fittings (45-degree angle)

a rectangle of grow tunnel plastic (72x54 inches) 

16 snap clamps (each 4 inches long)

Assemble the Base

1. Insert a 24-inch length and a 12-inch length of PVC pipe into two slots of a three-way fitting so that they are at right angles to each other. Lay the joined piece on a flat surface with the remaining open slot on the three-way fitting facing up. It should look like an “L.”

2. Repeat step 1 to create a second “L.”

3. Using two more three-way fittings, connect the two “L” pieces to form a rectangle. The base of the structure is now complete.  

Build the Wall and Roof Supports

1. Insert a 12-inch length of pipe into each of the four upward-facing slots of the three-way fittings in the base of the cold frame.

2. Fit a 45-degree-angle fitting onto the top of each of the four 12-inch lengths of pipe. Ensure that the fittings face toward their closest neighbor.

3. Insert a 8-1/2-inch length of pipe into the open end of each of the four 45-degree-angle fittings.

4. Use the final two three-way fittings to connect the two pairs of 8-1/2-inch lengths of pipe. Ensure that the open ends of the fittings face toward each other.

5. Insert the last 24-inch length of pipe into the open ends of both of the inward-facing three-way fittings. The wall and roof supports of the structure are now complete.

 Attach the Plastic Cover

1. Lay the rectangle of grow tunnel plastic over the cold frame structure. Orient the plastic so that the length and width of it match the length and width of the frame 

2. Pull all of the edges of the plastic down over the cold frame structure. Fold up any excess plastic and secure it to the base of the frame using a total of six snap clamps, two for each of the long ends and one for each of the short ends (ensure that the bottom of the frame is left uncovered). Further secure the plastic to the frame with one snap clamp on each of the remaining pipe supports, and two clamps on the main roof beam.

old farmer's almanac