A cold frame, also called a garden frame, is a low, bottomless growing box with a sloping, transparent lid of glass or polycarbonate. You set it straight onto the soil, so your plants stand in the open ground (source: Tuinadvies). The lid keeps wind and cold out and traps the sun’s warmth, which makes it a few degrees warmer inside than in the rest of your garden. That lets you sow weeks earlier in spring and protect winter vegetables in autumn that you keep harvesting well into the cold months.
In short: a cold frame stretches your growing season at both ends. Below you’ll read exactly how to build one yourself, with dimensions, materials, a sowing calendar and the mistakes you’re better off skipping.
Cold frame or hotbed: what is the difference?
This is where a lot of people get muddled. There are two versions.
The cold frame (unheated). It runs on solar warmth alone. During the day the air and the soil under the glass warm up, and at night that warmth hangs around for a while. Simple, maintenance free and by far the most commonly built version.
The hotbed (with fresh manure underneath). Below it sits a layer of fresh horse manure or straw. That starts to compost and gives off heat as it does, from below. So you get a heated bed with no plug and no gas. In the old days cucumbers were even grown this way early in the season (source: Wikipedia).
A third term that comes up a lot is the greenhouse or polytunnel. The big difference: in a greenhouse or tunnel you walk upright and the plants stand at full height. A cold frame is low (knee height) and meant for young plants, seedlings and low-growing winter vegetables. So a cold frame isn’t a mini greenhouse, it’s a sheltered growing spot close to the ground.
Why build a cold frame?
The gain comes down to four things:
- Sowing earlier in spring. Under glass you can sow lettuce, radish, spinach and carrot from February onwards, while the open garden is still too cold.
- Harvesting longer in autumn and winter. Lamb’s lettuce, winter purslane, winter lettuce, spinach and endive keep growing under the frame when it’s already freezing outside.
- Hardening off young plants. Seedlings you’ve started indoors or in the greenhouse get used to outdoor air in the cold frame before they go into open ground.
- Shelter from rain and frost. Frost-tender or damp-sensitive plants stay dry and protected.
Most leafy greens need around 10 degrees to grow (source: Tuiniertijd). Under glass you simply reach that threshold earlier in the year and hold it longer in autumn than you do in the open garden.
Where do you put the cold frame?
The spot largely decides whether your frame works well.
- Face it south. Position the frame south-facing so the sloping lid catches as much low winter sun as possible (source: Tuinadvies).
- In shelter, out of the north wind. Against a wall, shed or fence is ideal. The wall breaks the wind and holds a bit of warmth itself.
- Lid sloping to the south. Make the back higher than the front. That way the glass sits at an angle to the sun (more light) and rain and condensation run off towards the front on their own.
- Easy to reach. You need to get at it daily to ventilate and water. A path alongside saves a lot of hassle.
Dimensions and slope
Don’t make the frame wider than you can comfortably reach across, because you have to get at every plant from the side. In practice that comes out at roughly 80 to 100 cm front to back (source: Tuiniertijd).
A proven and cheap approach is to let the size of your lid lead. If you’re using an old window or a pallet, measure that first and build the frame around it. A standard 100 x 120 cm pallet as the base with an old window of about 115 x 130 cm as the lid is a classic combination (source: Gardeners’ World / Tuiniertijd).
For the slope there’s a simple rule of thumb: make the back about 10 to 20 cm higher than the front. On a frame 100 cm deep that already gives you a pleasant slope of roughly 6 to 11 degrees. Enough to shed the water and catch the sun better, without your lid acting like a sail in the wind.
A workable set of internal dimensions:
| Part | Guide size |
|---|---|
| Depth (front to back) | 80 to 100 cm |
| Width (per window) | 100 to 130 cm |
| Height at the back | 40 to 50 cm |
| Height at the front | 25 to 35 cm |
| Soil layer inside the frame | at least 15 cm of garden soil (source: Tuinadvies) |
Which materials do you use?
For the sides:
- Timber is the most popular: easy to work with, and it insulates better than metal or stone. Choose a durable species or treated timber that stands up to damp and ground contact (source: Tuinadvies).
- Avoid old railway sleepers and creosote-treated timber. They’ve been treated with creosote, a tar product that can release harmful substances. In a vegetable garden you don’t want that next to your food. On top of that, selling and passing on creosote-treated timber to private individuals is banned in the UK and the EU (source: REACH restrictions). So don’t use it for your cold frame.
- Bricks or concrete blocks work too. Heavier and more permanent, and stone stores warmth during the day that comes back out at night.
For the lid:
- Reusing old windows or frames is the cheapest and most sustainable option. Make sure the glass is intact and that you can hinge it safely.
- Polycarbonate (twin-wall) is lighter, doesn’t break and insulates better. Twin-wall polycarbonate sheets insulate about 40 per cent better than single glazing (source: Tuinadvies).
- Single glazing catches plenty of light but insulates the least. Fine for spring, less so in a hard winter.
For the filling: at least 15 cm of garden soil, mixed with a bit of well-rotted compost if you like (source: Tuinadvies).
Building a cold frame: step by step
- Work out the size of your lid. Measure the window or the sheet and mark the frame out around it. The lid leads.
- Cut the four sides. Back panel tallest, front panel lowest, and the two side panels sloping down from back to front. That gives you the slope automatically.
- Assemble the frame. Screw the sides together at the corners, with angle brackets or sturdy timber blocks on the inside for extra rigidity if you want.
- Set the frame in its final spot. Put it level on firm ground. A cold frame has no bottom: the plants will root in the open soil.
- Fit the lid on hinges at the tall back edge. That way you can lift it to ventilate and harvest. Make sure the lid closes properly against the sides.
- Make a prop for ventilating. A simple wooden block or a stick that lets you hold the lid ajar at different heights is essential (see below).
- Fill the frame with at least 15 cm of good garden soil and work some compost through the top layer. Ready to sow.
The hotbed: heating with horse manure
If you want to start even earlier in the year, you build a hotbed with fresh manure underneath. The heat comes from composting fresh horse manure.
Here’s how you go about it:
- Dig a trench 40 to 80 cm deep and at least 50 cm wide. The deeper and wider it is, the more heat you get (source: Wikipedia).
- Fill the trench with fresh horse manure and tamp it down lightly.
- Put 20 to 25 cm of good garden soil over the top (source: Wikipedia). You sow and plant in that soil layer, never in the manure itself.
- Never plant straight into the manure. Fresh manure is too strong and burns the roots. If you do want to plant into a fresh manure heap, first make a hollow of around 30 cm and fill it with soil (source: Moestuin Mania).
The temperature curve is predictable: after a few weeks the soil temperature climbs to around 30 degrees, sometimes higher, then drops slowly over roughly eight weeks to about 20 degrees (source: Wikipedia). So you get several weeks of serious bottom heat, ideal for an early start.
No horse manure? Straw works as a heating material too, but then you have to add nitrogen (calcium cyanamide, for instance) plus enough moisture to get the composting going. Straw starts cooler, around 15 degrees, and rises over eight weeks to about 18 degrees (source: Wikipedia). Less warm than manure, but odourless and easier to get hold of.
Ventilating: the most important daily job
This is the mistake that finishes off most cold frames. On a sunny day, even in March, the temperature under closed glass shoots up fast. Your plants cook, scorch or bolt.
So: ventilate on every sunny day. Prop the lid open as soon as the sun is on it, and close it again before evening.
The classic solution is a wooden block that lets you fix the lid at three different heights (source: Wikipedia). Simple and free.
If you’d rather not have to remember, go for an automatic vent opener with a thermal cylinder. It works without electricity: an oil-filled cylinder expands in the warmth and pushes the lid open, then pulls it shut again as things cool down. Most models start opening between 15 and 23 degrees and are fully open at around 35 degrees (source: Tuinkassenwinkel/kitchen garden sources). Reckon on a lifting force of 7 to 30 kg depending on the model, so check that against the weight of your lid.
Watch two things with an automatic opener:
- Price: a basic vent opener costs roughly 25 to 50 euros (source: bol.com/Tuinadvies).
- Winter: take the cylinder out in hard frost. If the fluid freezes, the cylinder can be damaged.
What do you sow and harvest, and when?
A cold frame earns its keep all year round. This calendar gets you started (source: Tuiniertijd, Groei & Bloei).
| Crop | Sowing | Harvesting |
|---|---|---|
| Spinach | February-March | April-May |
| Radish | February-April | March-May |
| Loose-leaf lettuce | February-May | April-July |
| Rocket | February-April | March-June |
| Winter spinach | October | from February (mild winter) |
| Lamb’s lettuce | September-October | October-December and on |
| Winter purslane | September-October | right through winter, repeatedly |
| Winter lettuce (e.g. ‘Arctic King’) | early October | from March |
| Overwintering onions (sets) | mid October | spring onions from February |
The broad picture: in early spring (February-March) you start the fast spring vegetables, and in autumn (September-October) you sow the hardy leafy greens you’ll keep picking through the cold months.
Maintenance and winter insulation
- Insulate in hard frost. At night, lay bubble wrap, a reed mat, old blankets or a layer of straw over the lid. During the day you take it off again for the light.
- Keep condensation and mould at bay. Ventilate regularly, including a quick airing on cool days. Moisture that lingers gives you grey mould on your leafy greens.
- Water at the root, not over the leaves (source: Tuiniertijd). Wet foliage in a closed frame is asking for mould.
- Keep the glass clean. Dirty glass lets through noticeably less light in the dark months. Give it a wipe now and then.
- Check the timber every year for rot, especially where it meets the ground.
Common mistakes
- Not ventilating on sunny days. The number one killer. Your plants scorch under closed glass.
- Making the frame too wide. If you can’t reach the plants at the back, maintenance becomes a punishment.
- Laying the lid flat. Without a slope, water sits on it and the glass catches less sun.
- Planting into fresh manure. Burnt roots. Always a layer of soil in between.
- Using creosote-treated timber or old sleepers. Harmful in a vegetable garden, and banned for reuse besides.
- Forgetting to ventilate in winter. Moisture builds up then too, and you get mould.
- Leaving the vent opener’s cylinder in through a frost. Freezing means broken.
What does it cost to build a cold frame yourself?
Costs vary a lot, depending on whether you reuse or buy new.
| Version | Indicative cost |
|---|---|
| Entirely from reclaimed material (old pallet + old window + screws) | roughly 0 to 40 euros |
| Self-built with new timber and an old or second-hand window | roughly 40 to 100 euros |
| New timber plus a new twin-wall polycarbonate lid | roughly 100 to 200 euros |
| Optional: automatic vent opener | 25 to 50 euros extra |
These are ranges; the exact price depends on the timber species, the dimensions and what you already have lying in the shed. The nice part is that the reclaimed version can be almost free if you get your hands on an old window.
Frequently asked questions
What is the difference between a cold frame and a hotbed?
In practice both terms get used interchangeably for the same growing box. Strictly speaking a cold frame is unheated (it runs on solar warmth), while a true hotbed has a base layer of composting horse manure or straw that gives off bottom heat (source: Wikipedia).
How warm does it get in a cold frame?
That depends on the sun, but on a sunny day the temperature climbs fast, in winter too. That’s why you always have to ventilate on sunny days. Most leafy greens grow from around 10 degrees upwards (source: Tuiniertijd).
When should I ventilate the cold frame?
On every sunny day, as soon as the sun is on the glass. Prop the lid open and close it before evening. An automatic vent opener takes this job off your hands and usually opens between 15 and 23 degrees (source: kitchen garden sources).
Which vegetables can I harvest from a cold frame in winter?
Lamb’s lettuce, winter purslane, winter spinach, winter lettuce and endive are the classics. Sow them in September or October and you’ll be harvesting from December into early spring (source: Groei & Bloei).
Can I just use old windows as a lid?
Yes, that’s exactly the cheapest and most sustainable option. Make sure the glass is intact, that you can hinge it safely, and that the window’s dimensions set the size of the frame.
Which timber is best?
A durable or treated species that stands up to damp and ground contact (source: Tuinadvies). Never use old railway sleepers or creosote-treated timber: it releases harmful substances and is banned for reuse besides.
Get started
A cold frame is one of the cheapest ways to keep your vegetable garden productive for months longer, at both the front and the back end of the season. Build it around an old window, put it south-facing against a wall, and never forget to ventilate.
Want to get straight to work with a fully worked-out plan and a complete materials list cut to size? Have a look at Fred’s DIY Plans.