Building a Simple Cold Frame for December Winter Greens

A cold frame is an invaluable tool for stretching your gardening season into winter and starting earlier in spring. In December, a cold frame keeps hardy salad leaves, Asian greens, and young seedlings protected from frost, stormy weather, and hungry pests—providing you with crisp, homegrown greens when the outdoor beds are dormant. Here’s a clear, step-by-step guide to building a simple cold frame perfect for December use.

What is a Cold Frame?

A cold frame is a low, bottomless box with a clear lid that lets in light but shields plants from cold, wind, and rain. It acts as a mini-greenhouse, providing extra warmth and a microclimate for winter crops.

Best Materials for a December Cold Frame

• Old window sashes, shower doors, or clear plastic/perspex sheets for the lid
• Untreated timber, bricks, or cinder blocks for the sides (avoid pressure-treated wood for edibles)
• Hinges or a simple propping stick for easy lid access

Step-By-Step: Building Your Cold Frame

1. Choose a Sunny, Sheltered Spot
   • Face the frame south or southeast for maximum winter sun.
2. Prepare the Base
   • Level the ground and lay bricks, boards, or cinder blocks in a rectangle.
   • Size: Anything from 1m x 60cm (3ft x 2ft) up to 1.5m (5ft) long.
3. Assemble the Frame
   • Stack sides to about 30–45cm (12–18in) high at the back, and angle the front lower for drainage and sunlight.
   • Securely fit your old window or clear panel as a hinged or lift-off lid.
4. Ventilation Is Essential
   • Prop the lid open on mild days to keep air fresh and prevent overheating.
   • Close before dusk to trap daytime warmth.
5. Fill with Quality Compost or Position Over a Bed
   • You can fill the frame with a 15–20cm (6–8in) layer of compost for direct sowing, or place it over soil beds or trays of seedlings.

Using Your December Cold Frame

• Sow or transplant winter greens: Lettuce, mizuna, rocket, pak choi, mustards, and spinach thrive inside a cold frame.
• Space plants slightly wider than usual to improve airflow and prevent mold.
• Water sparingly: Overwatering encourages rot in cold conditions.
• Harvest regularly: Pick the biggest leaves to allow light and space for the rest.

Tips for Success

• Add extra insulation (bubble wrap, old blankets, straw) around the outside for severe frost.
• If snow is forecast, brush it off the lid to keep light coming in.
• Use pots or modules for easy crop changes and quick replanting.

With an easy DIY cold frame built in December, you’ll keep the salads and healthy leaves coming—proving gardening doesn’t need to stop for winter!