How to build a small PVC cold frame

A cold frame traps the sun’s warmth over young plants. Set over a bed in early spring, it lets seedlings go out a couple of weeks sooner; in autumn it keeps a crop producing a couple of weeks past the first frosts. How much time it buys depends on the climate and the crop, and on the covering. Thin plastic sheeting lets through the most light but holds little heat. Thicker film insulates better. Shade cloth does the opposite job and keeps cool-season plants out of the summer sun.

The classic cold frame is a wooden box topped with an old window, and building contractors will often hand over scrap lumber and salvaged sashes for free rather than pay to dump them. This design takes a different route: a light A-frame of plastic pipe that snaps together without glue. Because nothing is cemented, the same pipe and fittings can be taken apart and rebuilt in another size whenever the garden changes.

About the materials

Use schedule 40 furniture-grade PVC in 3/4-inch nominal size. Furniture-grade pipe is smooth, unprinted and UV-resistant, and it comes in colors, though plain white is the cheapest. Hardware stores sell the pipe; the fittings are harder to find locally and are usually ordered online from furniture-PVC suppliers.

The frame described here measures about 26 inches long, 14 inches wide and 20 inches tall at the ridge. That is enough room for two or three plants to grow without crowding. The whole thing weighs around two pounds.

You will need

Pipe: 3/4-inch furniture-grade PVC. Four 5-foot lengths cover this cut list with a few feet to spare.

• 3 pieces at 24 inches (two for the long sides of the base, one for the ridge)
• 6 pieces at 12 inches (two for the short ends of the base, four for the corner posts)
• 4 pieces at 10 inches (the rafters)

Fittings:

• 6 three-way corner fittings (the kind with three outlets at right angles)
• 4 fittings at 45 degrees

Covering and fastening:

• Grow-tunnel plastic, around 2.4 mil; one 25-foot roll, 6 1/2 feet across, covers this frame many times over
• About 10 snap clamps, each 4 inches long
• A short length of paracord or garden twine

Tools: a fine-tooth saw, miter saw or PVC cutter, tape measure, marker, dust mask and safety glasses.

When this frame was first built in 2015, the parts came to roughly $32 before tax and shipping, and a larger frame costs very little more because the fittings are the main expense.

A note on snap clamps

Snap clamps are C-shaped plastic clips that press over the pipe and grip the sheeting against it. They can be bought pre-cut or as a long strip to be cut to length, which is cheaper. Clamps sized for 3/4-inch pipe grip the plastic very tightly and are hard to remove. Clamps sized for 1-inch pipe hold well enough and are much easier to take on and off, which matters on a frame that will be adjusted and packed away.

Cutting the pipe

PVC throws off a surprising amount of fine plastic dust even on a small job, especially with a power saw. Wear a dust mask and eye protection, and cut outdoors on saw horses rather than in the house. A ratcheting PVC cutter makes no dust at all and handles 3/4-inch pipe easily.

Mark every piece before cutting, then check the ends are square so they seat fully in the fittings.

Assembly

1. Build the base. Join the two 24-inch pieces and two of the 12-inch pieces into a rectangle, with a three-way fitting at each corner. Turn the fittings so the third outlet on each points straight up.
2. Add the posts and elbows. Push a 12-inch post into each upward outlet, then cap each post with a 45-degree fitting, angled toward the center of the frame.
3. Fit the rafters. Insert a 10-inch piece into each 45-degree fitting. At each end, the two rafters lean in until they meet. Join each pair at the top with one of the two remaining three-way fittings, turned so its spare outlet faces along the length of the frame.
4. Set the ridge. Slide the last 24-inch piece between the two peak fittings. The frame is now complete and should stand square without glue.
5. Cover it. Drape the plastic over the ridge and down both long sides. Cut it a little oversize and tuck the extra under the clamps rather than trimming it flush; a larger sheet is more useful later if the plastic gets reused. Press snap clamps over the plastic along the ridge and base to hold it.
6. Close the ends. Cut pieces for the two gable ends, fold the edges around the posts and rafters, and clamp them in place. A length of paracord tied between the two peaks under the plastic keeps the sheet from sagging onto the plants.

In a windy spot, pin the base to the ground with two or three tent stakes.

Using the frame

Cold frames overheat quickly, and on a sunny day the air inside can get hot enough to scorch seedlings even when it is cool outside. Iowa State University Extension advises venting once the inside passes 85°F, warns that the temperature can change rapidly, and says to close up again before sunset to hold the day’s heat (ISU Extension). With a frame this light, venting means lifting one side or taking it off entirely for the day. For many plants, covering them only overnight is enough.

In spring the cover warms the soil sooner, shields seedlings from light frost and wind, holds humidity in, and keeps some animals away from tender leaves. In summer, cool-season crops such as lettuce do better if the plastic is swapped for shade cloth, a woven fabric that passes water and part of the light. It is sold in several densities for different amounts of shade.

At the end of the season, pull the frame apart, bundle the pipe, and store the fittings and clamps in a bag. The same pieces can be reassembled next year as a longer tunnel, a taller frame, or a different project altogether.