Build a cold frame

An afternoon · Some tools · 7 steps

A bottomless box with a clear lid: start greens weeks early in spring and keep harvesting into winter.

A cold frame with the lid open, facing south
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Fig. A cold frame with the lid open, facing south.

  1. Clear lid: an old window or polycarbonate
  2. Hinges on the tall back edge
  3. Automatic vent opener lifts the lid as it warms
  4. Thermometer: open the lid above 75°F
  5. Soil banked against the sides blocks drafts
  6. No bottom: greens grow right in the ground

What you'll need

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You'll also need

  • An old storm window, or a sheet of clear polycarbonate on a wood frame, for the lid
  • Boards for the sides: 2×10 or 2×12, cut to fit the lid
  • Two hinges and exterior screws
  • A saw and a drill

Steps

  1. Measure the lid first, and build the box to fit it. A 3 by 5 foot lid is easy to lift and covers plenty of greens.
  2. Make the back about 18 inches tall and the front about 12, so the lid slopes toward the sun. Cut the two side boards on an angle to match.
  3. Screw the box together at the corners. Leave it bottomless: plants grow right in the ground.
  4. Set it on level ground facing south, and bank soil or mulch against the outside to block drafts.
  5. Hinge the lid to the back board, so it opens toward you.
  6. Fit the automatic vent opener, or prop the lid open with a notched stick on sunny days.
  7. Plant spinach, lettuce, kale, or scallions inside, and keep the thermometer in there to see how warm it gets.

Tips

  • The biggest risk is heat, not cold: on a sunny day it can pass 90°F inside even when it's freezing out. Open it once it tops 75°F.
  • Old windows made before 1978 may have lead paint. Use polycarbonate, or a newer window, for food crops.
  • On very cold nights, throw an old blanket over the lid.

Use it in Planting Year