Extending your growing season doesn't require expensive kits. By building a simple cold frame, you can protect tender seedlings from late spring frosts or keep hardy greens like Spinacia oleracea growing well into the winter months in most USDA zones. This project uses reclaimed lumber to create a miniature greenhouse that traps solar heat and shields plants from drying winds.
Gather Your Materials
To build a standard 2-foot by 4-foot frame, you need a few basic supplies:
- Reclaimed cedar or rot-resistant pine boards (roughly 12 inches wide).
- An old window sash or a sheet of clear polycarbonate plastic.
- Exterior-grade screws and a drill.
- A heavy-duty staple gun or construction adhesive.
Construction Steps
Building the frame is straightforward, even for beginners. Focus on creating a slope to help shed rain and maximize sunlight capture.
- Cut your wood to create a rectangular box, ensuring the back wall is roughly 6 inches taller than the front wall.
- Screw the boards together at the corners, reinforcing them with small blocks of wood if the reclaimed lumber is splintered.
- Attach your chosen top (the window sash or polycarbonate) to the back wall using hinges to allow for easy access and ventilation.
Always position your cold frame facing true south to ensure it collects the maximum amount of direct sunlight during the short days of winter.
Proper Ventilation
Temperature control is critical to your success. On sunny days when temperatures exceed 50°F, trapped heat can easily scorch your plants. Simply prop the lid open an inch or two during the middle of the day to allow for air circulation. If you expect a hard freeze, cover the entire frame with a thick blanket or old burlap sack to provide extra insulation through the night.
Planting Strategies
Select cold-hardy varieties for best results. Crops like Brassica rapa or Valerianella locusta thrive in the protected environment of a cold frame, even when ground temperatures drop. Start these seeds 4 to 6 weeks earlier than your typical outdoor planting dates to see a significant difference in your total harvest.
Keep a small thermometer inside your cold frame to monitor conditions and ensure your plants remain in their ideal growing range.