Cold frames, greenhouses and season extension

The growing season for most gardeners is shorter than it needs to be. A modest investment — or no investment at all — in season-extending structures opens up weeks of extra growing time at both ends of the year, and protects plants from weather that would otherwise do real damage.

Why extend the season

In most temperate climates, the window for growing tender crops outside — tomatoes, peppers, cucumbers, courgettes, basil — is defined by frost risk. In practice this means that without any protection, you may have only four or five frost-free months in which to grow the crops that want warmth. That is enough for reliable harvests, but leaving significant potential unused.

Starting seeds under cover several weeks before conditions outside are safe is one of the most effective things any food grower can do. A tray of tomatoes germinated in a warm cold frame in early spring will be sturdy transplants ready to go outside weeks before anything started in the open ground. At the other end of the season, a simple cloche over a pepper plant or a fleece jacket thrown over salad leaves can mean another few weeks of picking before hard frost arrives.

Protection also matters for overwintering. Tender perennials — pelargoniums, dahlias in cold areas, half-hardy herbs — that would die in an unheated garden can be kept alive through winter in an unheated cold frame or greenhouse. This saves the cost of buying replacements each spring. Climate and season length vary enormously by region, altitude and local microclimate — the specific dates that matter to you will be different from those that apply elsewhere, and local knowledge (or a local gardening group) is more useful than any general guide on that detail.

Options from cheap to bigger

Season extension exists on a spectrum from a single sheet of fleece to a full heated greenhouse. Here is an honest overview of each option, roughly in order of cost and scale:

• Horticultural fleece: the simplest and cheapest option. A sheet of lightweight, permeable fabric draped over plants or supported on hoops provides several degrees of frost protection, keeps off wind, and lets through rain and reasonable light. It is ideal for protecting rows of salad leaves, brassica seedlings and tender plants on cold nights. Reusable for multiple seasons. Not a substitute for a more substantial structure, but surprisingly effective for its cost.
• Cloches: rigid or semi-rigid covers over individual plants or short rows. Traditional glass bell jars are beautiful but fragile and expensive; modern plastic versions are cheaper and effective. Cut plastic bottles make free individual plant covers. Wire-frame row cloches with fitted polythene or fleece are versatile and easy to move around the garden. Good for warming soil before planting and protecting small seedlings.
• Cold frames: a low box with a clear or translucent lid (called the light) that traps warmth. Unlike cloches, cold frames have enough headroom for pots of seedlings, tray sowings, and young plants being hardened off. They can be left in position year-round and used for multiple purposes across the seasons. A well-built cold frame from second-hand materials costs almost nothing — see the step-by-step below.
• Mini polytunnels: low metal or plastic hoops pushed into the ground over a bed, covered with polythene sheeting. Larger than cloches, they can cover a whole raised bed or significant row length. They are useful in the vegetable garden for overwintering crops and getting early sowings going. Polythene needs replacing after a few years but the hoops last much longer.
• Walk-in polytunnels: a larger-scale structure — basically a tunnel of metal hoops covered with UV-stabilised polythene. More affordable than a glass greenhouse and highly productive. Polythene has a shorter lifespan than glass but is less expensive to replace. Polytunnels retain heat well, produce excellent results for tomatoes, cucumbers and peppers, and can be sourced secondhand. They need a reasonable amount of space and some assembly effort.
• Greenhouses: the most versatile, durable and expensive option. Glass or polycarbonate panels over an aluminium or timber frame. They can last decades, provide excellent light, and allow the full range of season-extending and propagation activities. Buying secondhand — dismantled from someone else's garden — is significantly cheaper than new and just as effective. The main running costs are cleaning, ventilation hardware, and (if you choose to heat) energy.

How each structure works

All of these structures share the same basic principle: they trap solar radiation as heat. Sunlight passes through glass, polycarbonate or polythene and warms the air and surfaces inside. The glazing material prevents that warmed air from escaping as quickly as it would in open air — the same principle as a car left in the sun, on a much more benign scale.

The result is that the temperature inside is typically several degrees higher than outside, with the difference being most pronounced on clear, sunny days. On cold, clear nights with no cloud cover to act as an insulating blanket, heat loss can be significant — an unheated cold frame or greenhouse can drop close to outside temperature on a hard frost night, though the lack of wind still protects from wind chill damage. Adding thermal mass — large containers of water, bricks or stone flooring — absorbs heat during the day and releases it slowly at night, improving overnight temperature.

Polythene and polycarbonate are better insulators than single-pane glass; double-walled polycarbonate significantly so. If you live somewhere with cold winters, the material choice matters more than in mild climates.

Using them through the year

Getting the most from a cold frame or greenhouse means using it in every season, not just for spring seedlings:

• Late winter and early spring: start seeds of slow-maturing crops like tomatoes, peppers and aubergines in gentle warmth. A cold frame on a south-facing wall can gain enough solar heat to germinate many seeds without any additional heating. For seeds needing consistent warmth, a small heated propagator inside a cold frame or greenhouse is more reliable than ambient temperature alone.
• Spring: harden off seedlings raised indoors or in a heated propagator. The cold frame acts as an intermediate environment — warmer than fully outside, cooler than indoors. Open the lid for a few hours each day and increase ventilation progressively over one to two weeks before transplanting. See our plant propagation guide for more on hardening off rooted cuttings and seedlings.
• Summer: grow tomatoes, cucumbers and peppers in a greenhouse or polytunnel for better yields and lower disease risk than outside. A cold frame can be used for hardening off summer bedding, growing on melons in a warm climate, or simply left open on warm days as a sheltered environment.
• Autumn: extend salad production well into the cool season. Lettuces, rocket, spinach, mizuna and winter purslane all tolerate cool conditions but benefit from cover that keeps off the worst of the wet and cold. A cold frame full of salad leaves can produce pickings into early winter in many climates.
• Winter: overwinter tender perennials. Pelargoniums, dahlias (in pots), half-hardy herbs such as lemon verbena, and young plants of biennial flowers all survive in an unheated cold frame in mild-winter climates. In colder areas, fleece inside the cold frame adds an extra layer of protection. Store dormant bulbs, overwinter alpine plants that dislike wet rather than cold, and keep slug and snail populations in check by clearing plant debris.

Ventilation and overheating

Overheating is the most common cause of damage to plants in enclosed structures, and it happens faster than most people expect. On a bright spring day, the temperature inside a closed greenhouse or cold frame can rise to a level that wilts or kills plants within an hour or two of the sun coming out — even when the outside temperature is mild.

Practical ventilation principles:

• Open vents at the top and doors or lower openings simultaneously — this creates a through-draught that flushes hot air out efficiently. Hot air rises, so roof vents are more effective than side ones alone.
• In summer, leave vents open all day and consider leaving the door ajar at night in warm climates — the risk of frost is past, and overnight ventilation reduces disease risk.
• Apply shading paint to the outside of glazing in late spring (early enough to matter, but after the last hard frosts) and remove it in early autumn. Purpose-made shading paint washes off easily with water when you want to remove it.
• Damping down — wetting the hard floor and staging surfaces on hot days — raises humidity and cools the air through evaporation. It also deters red spider mite, which thrives in hot, dry conditions.

Watering and hygiene

Plants under cover cannot receive rain, so watering is entirely your responsibility — and in summer, that may mean daily attention. Pots and growing bags dry out faster in a warm greenhouse than outside. Touch the compost surface to judge when watering is needed rather than watering on a fixed schedule, which ignores what the plant and weather are actually doing. See our water-wise gardening guide for more on efficient watering.

Hygiene matters more in enclosed structures than in the open garden because pests and diseases can build up rapidly without the natural checks provided by rain, wind and predators. Practical habits:

• Remove dead leaves, fallen fruit and spent plants promptly. Do not leave plant debris on the floor or staging.
• Clean the structure inside and out once a year — ideally in autumn when most tender crops are finished. Use warm soapy water and a scrubbing brush to clean glass or polycarbonate, removing the algal film that builds up and reduces light transmission.
• Wash pots and trays in hot soapy water before reuse. Disease organisms and vine weevil eggs overwinter in old compost in reused pots.
• Encourage natural predators — a well-placed pot of nectar-rich flowers outside the door attracts hoverflies and other beneficial insects. See our guide to beneficial insects for more on creating habitat for garden helpers.

Siting for best light

Light is the limiting factor for plants under cover in winter and early spring. Any shading from nearby trees, fences or buildings will reduce growth significantly during these lower-light months. When positioning a cold frame or greenhouse:

• Choose the sunniest position available, prioritising winter and early-spring sun angles (lower in the sky than summer) as well as summer.
• Face the main glazed side towards the equator — south in the Northern Hemisphere, north in the Southern Hemisphere. For a cold frame, the sloping lid should face towards the sun for maximum light capture.
• Avoid positions directly under or beside large deciduous trees — even bare branches cast shade, and autumn leaf fall on glazing is a maintenance nuisance.
• Some shelter from prevailing winds is beneficial for heat retention, but not so much enclosure that air circulation is severely limited.

Build a simple cold frame from reclaimed materials

A basic cold frame is one of the most useful garden structures and costs almost nothing to make from salvaged materials. Old window frames with glass in them are often available free from house renovations. Timber from dismantled pallets, scaffold boards or any untreated wood can form the sides.

1. Find a window frame to use as the lid. This is the starting point — the lid (the light) determines the size of the cold frame. Solid wooden window frames with original glass are ideal. Check salvage yards, local online exchange groups, and building skips. Alternatively, a sheet of twin-wall polycarbonate cut to size works very well and is lighter to lift.
2. Cut or source timber for the four sides. The back (north-facing side in the Northern Hemisphere) should be taller than the front — this creates a slope on the lid that catches more sun and sheds rain. A difference of 15–20 cm between front and back height works well. Old scaffold boards, reclaimed floorboards or fence boards are all suitable. Avoid treated or painted timber in contact with food crops.
3. Assemble the box. Screw or nail the four sides together at the corners. For a small cold frame, sturdy corner joints are sufficient without any other structure. For a larger frame, add corner posts for rigidity. Check it is level and square.
4. Position the frame in its final location. Set it directly on the ground in your chosen sunny spot. You do not need to fix it down for a simple cold frame — its own weight is sufficient, and you may want to move it between seasons.
5. Lay the lid on top. The window frame or polycarbonate sheet rests on the angled top of the box. If you have hinges, attach the lid to the back of the frame so it can be propped open with a stick. If not, simply lift it off to access the interior — a piece of rope tied to the lid makes this easier.
6. Add insulation for colder nights (optional). Old carpet offcuts, bubble wrap or thick cardboard laid over the lid on cold nights provides extra frost protection at no cost. Remove it in the morning to let light in.

• Sunniest available position identified; south-facing (north in Southern Hemisphere) aspect confirmed.
• Structure type chosen to match space, budget and purpose.
• Cold frame or cloche materials sourced second-hand where possible.
• Automatic vent opener fitted or manual ventilation routine established.
• Shading paint applied in late spring and removed in early autumn.
• Watering checked daily in warm weather; schedule adjusted by touch rather than calendar.
• Annual clean of structure and pots completed.
• Plan for each season — propagation, hardening off, summer crops, autumn salads, winter overwintering — in place.