A cold frame is one of the most useful structures in any garden — and one of the cheapest to build. It’s essentially a bottomless box with a transparent lid that creates a protected microclimate for starting seeds, hardening off transplants, and extending your growing season by weeks in both spring and autumn. Build one from salvaged materials and it costs almost nothing.
What a Cold Frame Does
A cold frame works by trapping solar heat during the day and insulating plants from frost, wind, and heavy rain at night. The temperature inside a cold frame can be 5–10°C warmer than outside on a sunny day, which makes a huge difference for tender seedlings. It’s not a greenhouse — you can’t heat it, and it won’t protect against prolonged hard frosts — but it bridges the gap between indoor sowing and outdoor planting.
Benefits
- Start seeds earlier — sow 2–4 weeks before the last frost date
- Harden off transplants — gradually acclimatise indoor-raised seedlings to outdoor conditions
- Extend the autumn harvest — protect salads, herbs, and tender crops from early frosts
- Overwinter hardy crops — lettuce, spinach, and spring onions grow slowly through winter in a cold frame
- Protect cuttings — a humid, sheltered environment is ideal for rooting cuttings
Design Options
| Design | Materials | Cost | Durability | Notes |
|---|---|---|---|---|
| Old window on a timber frame | Salvaged window + timber | £0–£20 | Good (5–10 years) | Most common DIY approach |
| Polycarbonate on timber frame | Polycarbonate sheet + timber | £20–£40 | Very good | Lighter than glass, shatter-proof |
| Straw bale frame | Straw bales + old window | £10–£15 | 1–2 seasons | Excellent insulation, temporary |
| Brick/block base | Bricks/blocks + glazed lid | £30–£60 | Excellent (decades) | Permanent, heavy, retains heat well |
How to Build a Cold Frame from an Old Window
What You’ll Need
- An old window frame with glass or polycarbonate (double-glazed units work brilliantly)
- Treated timber — 150mm × 25mm boards for the sides
- 50mm × 50mm timber for corner posts
- Exterior screws (50mm and 75mm)
- 2× hinges
- A prop stick or stay (to hold the lid open)
- Wood preserver
Step 1: Size the Frame to the Window
Measure your window and build the frame to match. The window becomes the lid, so the frame dimensions should match the window’s outer dimensions exactly. Most salvaged windows are around 600×900mm to 900×1200mm — all perfectly sized for a cold frame.
Step 2: Build the Base Frame
Cut the side boards to create a rectangular frame. The key design feature is a sloped top — the back wall should be 50–75mm taller than the front. This slope serves two purposes: it angles the glass toward the sun (maximising heat gain) and allows rainwater to run off the lid. Cut the side boards at an angle to create this slope.
- Cut the back board to 300mm height
- Cut the front board to 225mm height
- Cut the side boards with a diagonal from 300mm at the back to 225mm at the front
- Screw 50×50mm corner posts inside each corner
- Screw the side, front, and back boards to the corner posts
Step 3: Attach the Lid
Hinge the window to the back (tallest) edge of the frame using two sturdy exterior hinges. The lid should open from the front and rest against a prop stick at whatever angle you need. A simple notched stick works, or buy a proper greenhouse vent stay (£3–£5) for adjustable ventilation.
Step 4: Treat the Wood
Apply exterior wood preserver to all timber surfaces. The inside of the frame is permanently damp from condensation and soil contact, so thorough treatment is essential. Consider lining the inside with pond liner or heavy polythene for extra moisture protection.
Step 5: Position the Cold Frame
Place the cold frame:
- Facing south — maximum sun exposure. The sloped lid should face south to catch the most light
- Against a south-facing wall — the wall absorbs heat during the day and radiates it at night, providing additional frost protection
- On level, well-drained ground — waterlogged soil inside the cold frame defeats the purpose
- Sheltered from north winds — wind strips heat from the frame quickly
The RHS cold frame guide covers positioning and usage in detail.
Ventilation Is Critical
On sunny days — even in spring — the temperature inside a closed cold frame can soar above 30°C, which will cook your seedlings. Check the temperature daily and prop the lid open when it’s warm:
- Cool/cloudy days — keep closed
- Mild sunny days (10–15°C) — prop open 10–15cm
- Warm sunny days (15°C+) — prop wide open or remove the lid entirely
- Nights — close the lid to trap warmth
An automatic vent opener (the same type used in greenhouses, about £15–£20) removes the guesswork — it opens the lid when the temperature rises and closes it when it drops, even when you’re not home. Harrod Horticultural sells reliable auto-vent openers.
Seasonal Growing Calendar
| Month | What to Grow in the Cold Frame |
|---|---|
| January–February | Start early peas, broad beans, lettuce, spinach |
| March–April | Sow tomatoes, peppers, courgettes, squash for transplanting later |
| April–May | Harden off indoor-raised seedlings before planting out |
| June–August | Root cuttings from herbs and perennials |
| September–October | Sow overwintering salads, spring onions, hardy lettuce |
| November–December | Protect winter salads, overwinter tender herbs |
Extra Tips
- Place dark-coloured containers of water inside the frame — they absorb heat during the day and release it at night, moderating temperature swings
- Line the inside back wall with aluminium foil or reflective material to bounce more light onto plants
- On extremely cold nights, drape an old blanket or carpet over the lid for extra insulation
- If using a glass window, consider replacing with polycarbonate — it’s safer (no shatter risk), lighter, and provides similar insulation with less heat loss
