Build A Cold Frame

An inexpensive foray into greenhouse gardening, a cold frame is practical for starting plants six to eight weeks earlier in the growingseason and for hardening off seedlings. Basically, a cold frame is abox set on the ground and topped with glass or plastic. Althoughmechanized models with thermostatically controlled atmospheresand sash that automatically open and close are available, you can easily build a basic cold frame yourself from materials you probably already have around the house.

The back of the frame should be about twice as tall as the front so the lid slopes to a favorable angle for capturing sunrays. Build the frame tall enough to accommodate the maximum height of the plants before they are removed. The frame can be made of brick, block, plastic, wood, or just about any material you have on hand. It should be built to keep drafts out and soil in.

If the frame is permanently sited, position it facing south to receive maximum light during winter and spring and to offer protection from wind. Partially burying it takes advantage of the insulation from the earth, but it also can cause water to collect and the direct soil contact will shorten the lifespan of the wood frame parts. Locating your frame near a wall, rock, or building adds additional insulation and protection from the elements.

TIP: The ideal temperature inside is 65 to 75 degrees Fahrenheit during the day and 55 to 65 degrees at night. Keep an inexpensive thermometer in a shaded spot inside the frame for quick reference. A bright spring day can heat a cold frame to as warm as 100 degrees, so prop up or remove the cover as necessary to prevent overheating. And remember, the more you vent, the more you should water. On cold nights, especially when frost is predicted, cover the box with burlap, old quilts, or leaves to keep it warm inside.

Cold Frame Parts Illustration

Cutting List

Key Part No. Size Material
A Side 2 3/4 x 16/28 x 36 inches Ext. Plywood
B Front 1 3/4 x 16 x 36 inches Ext. Plywood
C Back 1 3/4 x 28 x 36 inches Ext. Plywood
D Lid frame 2 3/4 x 4 x 31 inches Ext. Plywood
E Lid frame 2 3/4 x 4 x 38 inches Ext. Plywood
F Cover 1 1/8 x 37 x 38 inches Plexiglass
Tools and Materials

(2) 3 x 3 inch butt hinges (ext.)

(2) 4-inch utility handles

(4) Corner L-brackets (3/4 x 2-1/2 inch)

(1) 3/4 inch x 4 x 8 foot plywood (ext.)

1/8 x 37 x 38 inch plexiglass

Exterior caulk/adhesive

Exterior wood glue

Exterior paint

2 inch deck screws

No. 8 x 3/4 inch wood screws

Circular saw

Drill/driver

Pipe clamps

Straightedge cutting guide

1. Cut the parts. This project, as dimensioned, is designed to be made entirely from a single 4-by-8 sheet of plywood. Start by cutting the plywood lengthwise to make a 36-inch-wide piece. TIP:Remove material in 4-inch-wide strips and use the strips to make the lid frame parts and any other trim you may want to add.
2. Trim the parts to size with a circular saw or jigsaw and cutting guide. Mark the cutting lines first.

3.Assemble the front, back, and side panels into a square box. Glue the joints and clamp them together with pipe or bar clamps. Adjust until the corners are square.

4. Reinforce the joints with 2-inch deck screws driven through countersunk pilot holes. Drive a screw every 4 to 6 inches along each joint.

5. Make the lid frame. Cut the 4-inch-wide strips of 3⁄4-inch plywood reserved from step 1 into frame parts. Assemble the frame parts into a square 38-by-39-inch frame. There are many ways to join the parts so they create a flat frame. Because the Plexiglas cover will give the lid some rigidity, simply gluing the joints and reinforcing with an L-bracket at each inside corner is adequate structurally.

6. Paint the box and the frame with exterior paint, preferably in an enamel finish. A darker color will hold more solar heat.

7. Lay thick beads of clear exterior adhesive/caulk onto the tops of the frames and then seat the Plexiglas cover into the adhesive. Clean up squeeze-out right away. Once the adhesive has set, attach the lid with butt hinges and attach the handles to the sides.

8. Move the cold frame to the site. Clear and level the ground where it will set. Some gardeners like to excavate the site slightly.