How to build a cold frame from old windows
A cold frame is a simple, low-cost growing box that uses sunlight and a transparent lid to create a warmer, more sheltered space for plants. It can extend the growing season, protect seedlings from wind and heavy rain, and give tender herbs a better chance in changeable weather. Reusing old windows makes the project especially appealing because the glass already provides light and insulation.
For Australian gardeners, the design needs to suit local conditions rather than copy a cold-climate greenhouse. A frame in Melbourne may need protection from winter chills, while one in Brisbane must shed heat quickly on sunny days. In Perth, Adelaide, Sydney or Canberra, careful placement, ventilation and watering will determine whether young plants thrive or overheat.
Choose suitable windows and a good location
Start with one or two old windows that are structurally sound. Timber-framed windows are easy to attach to a wooden box, while aluminium frames can also work if you use suitable screws and avoid cracking the frame. Hinged windows are convenient because they can become a ready-made opening lid, although a fixed window can be attached with hinges during construction.
Avoid glass with deep cracks, loose panes or badly rotted timber. Small imperfections may be manageable, but a cracked pane can fail under pressure, wind or sudden temperature changes. Measure the windows before buying timber so the growing box is built around their actual dimensions rather than a guessed size.
Choose a position that receives good winter sun, ideally facing north in southern parts of Australia. A north-facing location helps capture low winter sunlight, while a little afternoon shade may be useful in Brisbane or northern New South Wales. Keep the cold frame close to the house or garden shed if possible, where it is easier to check, water and ventilate every day.
The ground should be level and reasonably well drained. A slightly raised position is helpful in areas that receive winter rain, including parts of Tasmania and Victoria. Avoid a low spot where cold air and water collect, and do not place the frame beneath a large tree where falling branches, roots and heavy shade could cause problems.
Gather materials and plan the dimensions
A basic cold frame requires an old window, treated outdoor timber, exterior screws, two or three hinges, and a handle or small lifting strap. You may also need corner brackets, weatherproof wood glue, silicone sealant, gravel, weed mat, and a prop or automatic vent opener. Reclaimed bricks or concrete blocks can replace some of the timber if you prefer a masonry base.
A sloping lid is more effective than a flat one. It allows rain to run away, improves exposure to winter sunlight and gives taller seedlings extra headroom. The back wall should be higher than the front wall, with the difference depending on the size of the window. Even a modest slope is useful, but a steep angle can make the lid awkward and more vulnerable to strong wind.
Keep the frame shallow enough to reach all plants from the front. A depth of around 600 to 900 millimetres is practical for most home gardens. The length can match the window, but a smaller frame is easier to ventilate and move. A compact box is often more useful than an oversized structure that becomes difficult to manage during heatwaves.
Before cutting timber, place the window on the ground and sketch the four sides around it. Allow the lid to overlap the outside edges by a few millimetres, giving rain less opportunity to enter. If the window is heavy, add a central support or use stronger hinges so the frame remains stable when opened.
Make reclaimed materials safe to use
Old windows can carry lead-based paint, particularly those made before the 1970s. Do not sand, burn or aggressively strip unknown paint in the garden. Wear gloves, keep children and pets away, and use a damp cleaning method that limits dust. If the paint is flaking heavily or the frame is in poor condition, replace it with a newer salvaged window or a clear polycarbonate panel.
Glass edges need careful inspection. Wear safety glasses and sturdy gloves when moving the window, and never lift a large pane alone. Remove loose putty and repair unstable glazing before fitting the window to the box. If a pane is cracked, a glazier can replace it, or you can use horticultural polycarbonate as a lighter and safer alternative.
Treat cut timber ends with an outdoor preservative, but allow the product to dry fully before bringing it near edible crops. Use galvanised or stainless-steel fasteners because ordinary steel screws rust quickly in damp soil. Avoid using old painted timber, railway sleepers or chemically treated scraps where they may contact compost or vegetables.
Line the inside of the timber walls with a durable barrier if you want to reduce contact between soil and treated wood. A pond liner or heavy plastic sheet can work, but leave drainage paths open at the base. The frame should protect plants from weather without becoming a sealed container that holds water.
Build the timber box and attach the lid
Cut the front, back and side boards to size, then assemble them into a rectangular box with the taller back wall. Pre-drill every screw hole near the ends of the boards to reduce splitting. Check the corners with a square and measure diagonally from corner to corner; equal diagonal measurements indicate that the box is properly aligned.
Add a second layer of timber around the top if the window needs a wider ledge. This creates a more stable seating area and helps block draughts. You can fill small gaps with exterior-grade sealant, but do not seal the lid permanently because the frame must open for watering, harvesting and ventilation.
Fix two or three strong hinges along the back edge of the window. Position them so the screws sit securely in the timber rather than only in thin aluminium. Add a handle to the front edge and test the lid several times. It should open smoothly without scraping the box or placing pressure on the glass.
A simple prop made from timber is essential. Cut it with several notches so the lid can be held open at different angles. An automatic vent opener is a useful upgrade for gardeners who are away during the day; these devices respond to heat and gradually lift the lid, reducing the risk of cooked seedlings.
| Feature | Reclaimed glass window | Polycarbonate sheet |
|---|---|---|
| Light transmission | Excellent and clear | Very good, with diffused options |
| Weight | Heavy | Light |
| Heat retention | Strong | Strong, especially twin-wall panels |
| Safety | Can break into sharp pieces | More resistant to impact |
| Installation | Often needs stronger hinges | Easy to cut and attach |
| Best use | Small, permanent frames | Lightweight or mobile frames |
Spread a 50 to 75 millimetre layer of gravel beneath the box if the soil drains poorly. Fill the frame with quality compost blended with garden soil, or use seed-raising mix for young plants. The growing medium should sit below the top edge so water does not wash soil onto the glass or timber.
Control temperature, moisture and ventilation
A cold frame warms quickly because sunlight passes through the window and becomes trapped inside. On a clear Australian day, the internal temperature can rise far above the outside temperature, even in winter. Check the frame during the morning and open the lid before plants begin to wilt. In summer, the lid may need to remain fully open or the frame may need temporary shade cloth.
Ventilation is especially important in Sydney, Perth and Brisbane, where bright sun can create intense heat. Open the lid a few centimetres on mild days and wider during warm weather. Close it in the late afternoon when temperatures fall, but leave a small gap if humidity is high and condensation is persistent.
Water in the morning so foliage dries before evening. Touch the surface of the growing medium before watering rather than following a fixed schedule. Glass reduces direct rainfall, but it does not prevent evaporation, and seedlings in shallow trays can dry quickly in hot weather.
Use a thermometer placed at plant level to understand the frame’s conditions. A maximum-minimum thermometer is inexpensive and provides useful information about overnight cold and daytime heat. In Canberra or inland areas, frost cloth can be laid over seedlings on very cold nights, but it should be removed once the morning sun strengthens.
Strong wind can lift an open window or damage a hinged lid. Fit a chain, stay or adjustable support to prevent it swinging too far. During storms, close and secure the lid, and consider adding a latch if the frame is exposed. A heavy base and sheltered position will make the structure more reliable.
Grow seasonal plants in a sheltered space
Use a cold frame for hardening off seedlings before they move into the garden. Begin by placing trays inside for a few hours each day, gradually increasing exposure over a week or two. This helps young plants adjust to outdoor light, wind and changing temperatures without suffering sudden transplant shock.
In southern Australia, winter and early spring crops may include lettuce, spinach, silverbeet, parsley, coriander, spring onions and compact brassicas. Small seedlings of broccoli, cabbage and kale can benefit from protection while they establish. In warmer regions, the frame can support herbs and leafy greens during cooler months, but heat-sensitive crops may need shade once spring arrives.
A cold frame is also useful for cuttings, newly divided herbs and plants that need protection from heavy rain. Keep different crops in separate pots or trays so watering can be adjusted. Good air circulation matters even in cool weather, as crowded seedlings are more likely to develop fungal problems.
Do not treat the frame as a permanent home for every plant. Once seedlings have strong roots and several true leaves, gradually move them outside if that is where they will grow. The frame works best as a transition area, propagation station and seasonal shelter rather than a replacement for a full greenhouse.
Practical ways to make the frame work harder
- Add a layer of bubble wrap or horticultural fleece on the inside during cold nights, keeping it away from wet foliage.
- Paint the exterior timber with a low-toxicity outdoor finish, while leaving drainage points clear.
- Place dark water containers inside to absorb warmth during the day and release it slowly overnight.
- Use removable shade cloth in hot weather instead of permanently blocking valuable winter light.
- Label sowing dates and plant varieties so you can compare growth across seasons.
- Check hinges, sealant, timber and glass at the start of each winter.
- Keep a small hand brush nearby to remove leaves, dust and standing water from the lid.
Maintain the frame through every season
Regular maintenance keeps a homemade cold frame safe and productive. Wipe the window occasionally to remove dust, bird marks and mineral deposits that reduce light. Clean the glass with plain water or a mild detergent, then inspect the frame for loose screws, swollen timber and damaged hinges.
Refresh the growing medium between crops by removing old roots and adding mature compost. If seedlings have suffered from disease, discard affected material rather than adding it to a household compost heap. Rotate the type of plants grown in the frame when possible, especially if the box is used for vegetables year after year.
At the end of summer, check whether the position is still suitable. Nearby plants may have grown tall and cast shade, while winter storms may expose the frame to stronger winds. Trim surrounding vegetation, reinforce the base and replace any broken glazing before the cooler season begins.
A salvaged window cold frame can last for many years when the lid is supported properly and water is kept away from vulnerable timber joints. It also adds a practical feature to a productive garden without requiring a large budget or specialist building skills. For Australian conditions, the most important habits are checking the temperature, opening the lid early and adapting the amount of shade to the season.
Gather a sound old window, select a sunny but manageable location, and build the box to suit the materials you already have. With safe preparation and daily attention, this small growing shelter can help raise stronger seedlings, protect herbs and provide fresh garden produce through changing weather.