Adding Hinged 1/4-Inch Hardware Cloth Vent Screens and a Vole-Proof Floor to a 3-by-6 Cold Frame

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Hang two 2x2 screen frames covered in 1/4-inch galvanized hardware cloth inside the cold frame on cleats, with their tops flush with the rim, so they sit right under the clear lids. Then you can prop or pull the lids on a warm day while squirrels, birds, and cats stay off the seedlings. Lay the same mesh flat on the ground inside the box, turn it 6 inches up every wall, and fill 6 inches of soil over it, so voles and gophers can't tunnel in from below.

Why Venting Leaves the Bed Open

A closed cold frame overheats fast, so the lid has to come up. South Dakota State University Extension's cold frame guide says closed frames have limited air circulation, so lids should be vented or removed on warm, sunny days and closed again in the late afternoon before air temperatures drop. It also puts the lid slope at about 10 to 15 degrees, with the back wall higher than the front.

University of Missouri Extension's hotbed and cold frame guide says the sash may be opened wide or removed entirely on warm, sunny days, with a thermometer as the best guide. It also says to raise the sash on the side away from the wind, so tender plants don't get wind burn and gusts can't lift the sash.

Every hour that lid is up, the bed is an open seed tray. A screen under the lid keeps that open-vent time from costing you a flat of lettuce to a squirrel.

Venting Targets

University of Missouri Extension gives these daytime and night air temperatures for plants in a frame. Hang a thermometer at plant height in the shade of a board, not in direct sun.

Crop group Day air temp Night air temp Screen position
Cool-season (lettuce, cabbage, onions, pansies) 65-70°F 55-60°F Closed all season; lids propped once inside air passes the day target
Warm-season (tomato, pepper, eggplant, squash) 65-75°F No lower than 60°F Closed; lids propped or pulled on sunny afternoons
Hardening off (any transplant) Outdoor temps Outdoor temps Closed; lids off all day, screen keeps birds and squirrels out

Frame and Screen Spec

Part Spec Why
Box 3 × 6 ft outside, cedar or ACQ pressure-treated lumber Two 3 × 3 ft bays, each with a lid and a screen
Front wall (south) One 2x12 (actual 11-1/4 inches) Low edge faces the sun
Back wall (north) Two stacked 2x10s (actual 18-1/2 inches) Gives about an 11-degree lid slope over 3 ft
Sides Two stacked 2x10s, cut on a taper from 11-1/4 to 18-1/2 inches Lids seat flat on the slope
Center rail 2x2, front to back at the 36-inch mark Carries the lids and screens where they meet
Screen frames Two 2x2 frames, 33-1/4 × 32-1/2 inches Drop inside each bay onto the cleats with a little play
Screen mesh 1/4-inch galvanized hardware cloth, 23 gauge Stops squirrels and birds; passes light, rain, and air
Screen cleats 1x2, top edge 1-1/2 inches below the rim on the front, sides, and center rail Screen tops sit flush with the rim, under the lids
Floor mesh 1/4-inch hardware cloth, 45 × 81 inches, flat at grade, turned 6 inches up each wall, under 6 inches of soil Stops voles and gophers tunneling in; deep enough that a trowel doesn't snag it
Lids Two 3 × 3 ft frames with 8 mm twin-wall polycarbonate Light enough to prop with one hand
Fasteners T50 1/2-inch staples every 3 inches, 1x2 battens screwed every 6 inches Keeps the mesh tight when a squirrel jumps on it

Skip creosote- and penta-treated wood for the box. Fumes from those preservatives build up inside a closed frame and can damage plants.

Materials and Tools

  • Amagabeli 1/4-inch 23-gauge galvanized hardware cloth, 48 in × 50 ft, for the floor liner and both vent screens; one 48-inch width covers the floor with the wall turn-ups
  • One 2x12 × 6 ft and four 2x10 × 6 ft
  • About 24 ft of 2x2 for the center rail, screen frames, and corner posts
  • About 40 ft of 1x2 for cleats and battens
  • One 4 × 8 ft sheet of 8 mm twin-wall polycarbonate, or two old 3 × 3 ft window sashes
  • Four 3-inch galvanized strap hinges for the lids and four 2-inch butt hinges for the screens
  • Two hook-and-eye latches and two 12-inch notched prop sticks
  • 2-1/2-inch and 1-1/4-inch exterior screws, T50 staple gun and 1/2-inch galvanized staples
  • Aviation snips, circular saw, drill/driver, speed square, shovel, tape measure, leather gloves, safety glasses

Build Sequence

  1. Site it. Set the frame on level ground with the 2x12 front wall facing due south and a building, fence, or straw bales blocking the north wind.
  2. Level the footprint. Rake the 3 × 6 ft footprint level and pull out roots and stones.
  3. Build the box. Screw the walls to 2x2 corner posts with 2-1/2-inch screws, then screw the center rail across the top at the 36-inch mark.
  4. Lay the floor mesh. Wear gloves and glasses, and cut a 45 × 81 inch panel on a cross wire. Set the box in place, lay the mesh flat on the ground inside it, and fold 6 inches up every inside wall.
  5. Fasten the turn-up. Staple the turned-up mesh to the inside of each wall every 3 inches, and screw a 1x2 batten over the top edge. The same staple-and-batten floor detail is covered in more depth in lining raised garden beds with hardware cloth.
  6. Fill the bed. Spread 6 inches of topsoil and compost over the mesh, and water it in to settle. That leaves about 3-3/4 inches of headroom under the screen at the front and 11 inches at the back, so set taller transplants toward the north wall.
  7. Screw the cleats. Fasten 1x2 cleats inside the front wall, both side walls, and both faces of the center rail, with their top edge 1-1/2 inches below the rim.
  8. Build the screens. Screw two 33-1/4 × 32-1/2 inch 2x2 frames, square them on the diagonals, and staple 1/4-inch mesh to the bottom face every 3 inches. Screw 1x2 battens over the edges every 6 inches, so the top face stays flat for the lid to close on.
  9. Hang the screens. Set each screen on its cleats and hinge its back edge to the inside face of the back wall with two 2-inch butt hinges, so it swings up for watering and harvest.
  10. Hang the lids. Build two 3 × 3 ft lid frames, screw on the polycarbonate, and hinge each to the top of the back wall with two 3-inch strap hinges. Add a hook-and-eye at each front corner so wind can't flip a closed lid.

Daily Venting Routine

  1. Check by mid-morning. Read the thermometer once the sun is on the frame, even on a cold, clear day.
  2. Prop or pull the lids. Prop the lids on the notched stick once the air inside passes the day target, or pull them off on warm days. The screens stay down.
  3. Open the screen only to work. Lift a screen for watering, thinning, or harvest, then drop it back on its cleats.
  4. Close by late afternoon. Drop and latch the lids in the late afternoon to hold the day's heat overnight.

Common Cold Frame Screen Mistakes

  • Propping the lid with no screen and losing the seedlings to squirrels by noon
  • Setting the screen right on the plants instead of on cleats above them
  • Skipping the floor mesh, so voles tunnel in under the walls
  • Covering the floor mesh with less than 6 inches of soil, where a trowel or hoe snags it
  • Leaving lids propped through a cold night
  • Using chicken wire, which birds and squirrels reach through
  • Building the box from creosote- or penta-treated wood

FAQ

Will the screen shade my seedlings?

Very little. 1/4-inch galvanized cloth casts a light grid shadow that moves with the sun, and the plants grow normally under it.

Can I leave the screens down all winter?

Yes. Brush heavy snow off the lids so light gets through. The screens sit under the lids and carry no snow load.

Does 1/2-inch mesh work for the screens?

For squirrels and cats, yes. Small birds can reach seedlings through 1/2-inch mesh, so 1/4-inch is the safer choice over seed flats.

Quick Cold Frame Screen Checklist

  • 3 × 6 ft box, front facing south, back about 7 inches higher
  • Floor mesh flat at grade, turned 6 inches up every wall, under 6 inches of soil
  • Two 1/4-inch hardware cloth screens on cleats, tops flush with the rim
  • Staples every 3 inches, battens screwed every 6 inches
  • Lids on strap hinges with hook-and-eye latches
  • Thermometer at plant height, out of direct sun
  • Lids propped or pulled once the day target is passed; closed by late afternoon
  • No creosote- or penta-treated wood

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