Build a Cold Frame Greenhouse: Extend Your Growing Season

A cold frame acts as a mini-greenhouse, protecting seedlings and extending harvests into winter. This 4'×3.5' design uses affordable plywood, furring strips, and plastic sheeting—buildable in a weekend with basic tools.

Materials

• 1 sheet ½" plywood (4'×4')
• Two 2×2×8' furring strips
• 1×8×4' lumber
• 1½" and 2½" screws
• 5'×5' plastic sheeting (2+ mil)
• Two hinges

Box Construction

1. Cut plywood per diagram: two angled side panels (16"→8" slope), one 16"×4' back panel.
2. Cut furring strips: four corner posts (two 16", two 8"), plus top reinforcements.
3. Assemble box: screw panels to corner posts. Add horizontal braces at top of back and sides.
4. Install 45° angled front braces for rigidity.

Lid Construction

1. Build frame from 2×2 or 2×3: two 4' pieces, two 39" pieces.
2. Screw corners. Attach hinges equally spaced.
3. Stretch plastic sheeting over frame. Sandwich between double wood layers for tear resistance and water shedding.
4. Attach lid to box with hinges.

Usage Tips

• Face south for maximum sun
• Vent on warm days to prevent overheating
• Insulate with blankets during hard freezes
• Ideal for hardening off seedlings, winter greens, early starts

This design adapts easily—use reclaimed windows for glazing, resize for space. A cold frame adds weeks to both ends of your growing season for minimal investment.