A cold frame extends your growing season by four to six weeks in both spring and autumn. It is essentially a bottomless box with a transparent lid — simple enough to build in a few hours from materials you may already have. Using reclaimed lumber reduces the cost to almost nothing and keeps timber out of landfill.
What Is a Cold Frame?
A cold frame is a low, unheated structure that traps solar heat and shields plants from frost and wind. The lid is tilted to shed rain and angled toward the sun for maximum light transmission. It can harden off seedlings started indoors, protect overwintering greens, or give early crops a head start before the last frost date.
Sourcing Reclaimed Lumber
Old fence boards, pallet wood, scaffold boards, and dismantled decking all work well. Look for pieces at least 25mm thick. Avoid wood treated with creosote or older pressure-treated timber near food crops. Cedar, oak, and Douglas fir from old structures are ideal choices.
Sizing and Building the Box
A frame 1200mm wide by 900mm deep is a practical size. Height at the back should be 300-400mm; the front can be as low as 200mm. The slope helps rain run off and angles the lid toward winter sun. Cut your four wall pieces to length. The back piece is taller than the front; the two side pieces are cut at an angle to create the slope. Join the corners with 80mm screws. Treat the interior faces with linseed oil if using softwood.
Making the Lid
A reclaimed window sash is the classic lid. Attach two hinges at the back so the lid opens upward and add a prop to keep it open for ventilation. If you cannot source a window, polycarbonate twin-wall sheet is an excellent alternative — lighter than glass, virtually unbreakable, and retaining around 80% of available light.
FAQ
Can I use pallet wood for a cold frame?
Yes, with caution. Check for the IPPC stamp — HT (heat treated) is safe, MB (methyl bromide) is not. Avoid any pallet with dark staining or a chemical smell.
How cold can a cold frame get?
A standard cold frame protects to about -5C (23F). Below that, add bubble wrap or horticultural fleece over the lid on the coldest nights.
Do I need to water plants in a cold frame?
Less often than outdoor plants, but yes. The lid prevents rain from reaching the soil, so check moisture every few days in winter and more frequently in spring.
Can I use it in summer?
Yes — remove the lid entirely and use the box as a bottomless raised bed for heat-loving crops like basil and aubergines.
How long will a reclaimed lumber cold frame last?
With linseed oil treatment and good drainage, a softwood cold frame typically lasts five to ten years. Hardwood reclaimed timber can last twenty or more.
See also: How to Protect Outdoor Wood Naturally and How to Build a Raised Bed That Lasts.