How to Build a Cold Frame

Protect your plants from frost with this inexpensive, easy-to-make cold frame.

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Protect Your Plants With a Cold Frame

Get an early start on spring planting or extend your gardening season with this DIY cold frame. A type of “mini-greenhouse,” a cold frame built with a glass or plastic lid allows plants access to sunlight and traps heat to prevent cold weather from putting an early end to your new crops. Follow these simple steps to construct your own cold frame using just a few tools, lumber, and a reclaimed window frame. The addition of a temperature-sensitive foundation vent will keep air flowing without risking plant loss due to unexpected cold snaps.

Tools and Materials

You will need: reclaimed 24-inch x 48-inch window (check window replacement shops, thrift and antique stores) / 2-inch x 8-inch x 16-foot rough cedar plank / one foundation vent / twelve 4 1/2-inch lag screws / two 3 1/2-inch galvanized T hinges / 8 hinge screws / two 4-inch eye hook latches / tape measure / drill with pilot bit and screwdriver tip / circular saw / straightedge / c clamps / jigsaw

Measure and Cut Lumber

From cedar planks, measure and cut three 48-inch lengths, two 18-inch lengths and one 20-inch length. These will be used to construct a simple raised bed and a second-tier angled cap to which the hinged-lid window frame will be attached. This project uses a 24-inch x 48-inch reclaimed window, but may be adjusted to accommodate windows as large as a storm door by matching the box size to window size. A larger cold frame may benefit from the addition of a second vent.

Measure and Cut Angled Tier

Cut the 20-inch cedar plank diagonally from corner to corner. C clamps and a long straightedge can be used as a saw guide, but are not required. These two pieces are the sides and will support the lid at a sloping angle to maximize sunlight.

Assemble Box

Measuring 1 inch from the long side of the two 48-inch planks, drill a hole at each marked point 3/4 inches from the short edge using a pilot drill bit. Using eight 4 1/2-inch lag screws (two at each corner), assemble bed by driving screws through prepared holes into two 18-inch sides to form a rectangular box. Make sure all corners are square and don't overlap.

Assemble Window Mount

Mark a spot 1 inch from the top and bottom on the back of the second tier at each end, then drill a hole at each of the four marks 3/4 inches from the end using pilot bit. Attach the angled sides to the back of the second tier using 4 1/2-inch lag screws driven through the pilot holes you have made.

Measure Placement for Foundation Vent

On the back of the cold frame, use a straightedge to mark a horizontal 8-inch x 15-inch rectangle with sides centered horizontally and two inches from the top edge for the foundation vent. The addition of a foundation vent will allow air to circulate during warm period and trap heat when temperatures drop below 40 degrees F.

Cut Foundation Vent Opening

Use a jigsaw to cut the hole for the foundation vent. Foundation vents are designed with spring-loaded mounts, allowing the vent to be inserted without the need for additional screws once the frame is complete.

Attach Hood and Eye Latches

At the midway point, drill pilot holes to attach 4-inch hooks and eye latches inside the frame at each end. The latches will hold the top and bottom tiers together when in use, but allows for easy disassembly. Window latches may also be used instead, if desired. With the lid removed, the bottom tier may be used as a raised bed during summer months and the lid stored until cold weather returns.

Attach Hinges to Window Frame

Using hinge screws, attach a hinge 6 inches from the corners of the window frame along one long edge.

Attach Lid to Cold Frame

Place the window frame on top of the angled tier so the hinged edge rests evenly with the back. Use remaining hinge screws to attach hinges to the cold frame.

Insert Foundation Vent Into Frame

Push the foundation vent into the prepared hole until it snaps into place. The passive vent will automatically open during warm weather and close when temperatures drop below 40 degrees F. A 30-inch dowel can be used to prop the lid open to take full advantage of warm weather and maximize air circulation, but this vent is an invaluable fail-safe.

Your Cold Frame is Ready

This attractive cold frame is simple to build and easy to disassemble for warm-weather storage. Adding a cold frame to your gardening routine can extend the growing season by weeks and reduces the risk of plant loss when germinating seeds before the last frost date.

A Permanent Solution

This sturdy cold frame is easy to disassemble and store during warmer months, but may be anchored in place using stakes driven into the ground at each inside corner for permanent placement.

A Variety of Uses

Your cold frame may be filled with an appropriate soil mix like any raised bed or used to house potted plants or germination trays to protect established plants from harsh weather or to get an early start on spring planting.