Is a Polytunnel or Greenhouse Worth It At All?

A polytunnel or greenhouse is worth it in Scotland if you want to grow tomatoes, chillies, peppers or other heat-lovers, or if your plot has a short season (upland or north-east). It is hard to justify if you mostly grow kale, leeks, beans and roots, or have very little space; a cold frame, clochesA small, portable clear cover placed over an individual plant or short row to trap warmth and keep off wind, rain and pests. Full definition → and fleeceA lightweight, breathable fabric laid or draped over plants to trap a little warmth and keep off frost, wind and some flying pests. Full definition → will do most of the job for less. Most advice about what you can grow in Scotland is really advice about time. A tomato doesn't fail here because gardeners are worse at growing it. It limps along with a handful of green fruit in October because it runs out of warm weeks. A structure buys some of that time back, starting before the ground and air outside are safe and keeping tender plants cropping after the first outdoor frost. Even unheated, using solar gain and the shelter of the structure alone, it stretches the season at both ends.

How short is your season?

What a structure stretches is the outdoor season you already have. The table gives the safe planting-out date and first autumn frost for representative stations, from Met Office records (1991-2020). The regional frost dates page covers every station, including upland and lowland splits.

The spread is the story: about 10 weeks between the mildest lowland station (Inverness, on the Moray Firth) and the harshest upland one (Braemar). The Met Office's summary for eastern Scotland describes a frost-free season "often as little as 3 months" on higher ground. That is the difference between growing a decent outdoor tomato crop most years and racing the clock on almost everything.

Stornoway has the longest, mildest frost record in the table, thanks to its maritime exposure. Islands still need protection, but for wind, not frost: see securing a greenhouse or polytunnel against wind. Wind anchoring and cover tensioning matter on plenty of exposed mainland sites too, such as an open Central Belt plot, an upland garden or a stretch of the south-west coast, so read it before you buy if your plot is at all exposed.

What an unheated structure buys you

No single week-count applies everywhere, because it depends on the structure, its siting and aspect, and how much frost protection you add. The reliable description is "several weeks at each end": sowing and transplanting before it is safe outside, and cropping for a meaningful stretch after frost has finished the outdoor plants. For an Edinburgh or Dumfries gardener with a 24-week outdoor season that is a real but modest gain. For a Braemar or upland Central Belt gardener with 14 to 16 weeks, the same few weeks could be the difference between "can't reliably finish a tomato crop" and "can." That is the clearest Scottish case for a structure: the shorter your season, the more a fixed extension is worth. Lowland gardeners still gain, just proportionally less.

Which crops need the cover

Crops that need cover in Scotland

Chillies. Chillies usually crop best in a greenhouse, polytunnel or conservatory, and fruiting may be reduced outdoors, especially in cooler summers. Outdoors they need night temperatures reliably at least 12°C, and they do better kept above 15°C. A Scottish summer's nights rarely stay that warm for long enough.

Tomatoes. Plants shouldn't go outside until temperatures are reliably above 16°C, a bar much of Scotland struggles to clear consistently even in midsummer. That is a warmth threshold, not strictly a latitude one: a mild coastal station like Inverness can clear 16°C nights about as reliably as Edinburgh, while an upland site much further south can lag behind both, so "marginal outdoors" tracks altitude and coastal exposure more than how far north you are. Outdoor tomatoes also need a really warm, sunny, sheltered site and are susceptible to blight outdoors but rarely in a greenhouse. Blight thrives in cool, damp, humid weather, which is Scotland's baseline, so that pushes tomatoes under cover too.

Peppers. Sweet peppers are usually grown under cover in the UK, in a greenhouse, polytunnel, conservatory or cold frame, and outdoor growing is only viable in milder regions or exceptional summers. They grow best above 15°C, though temperatures over 30°C reduce fruiting, so a cool Scottish summer holds them back and a hot greenhouse needs ventilating and shading.

Aubergines, melons and soft fruit such as strawberries and grapes are in the same broad category: they need sustained warmth or shelter from wet weather to crop and ripen well. No Scotland-specific figure as precise as the tomato and chilli guidance exists for them, so treat this as the general version of the claim.

Crops that don't

Kale, leeks, broad beans, most brassicasThe cabbage family: cabbage, kale, broccoli, cauliflower, Brussels sprouts, kohlrabi, swede, turnip and more, sharing the same pests, diseases and rotation rules. Full definition → and root vegetables are adapted to Scotland's cool, damp climate and grow well outdoors without protection. A structure lets you start them a little earlier, but the outdoor crop was never marginal. If your garden is mostly these, the classic allotment plot of brassicas, beans and roots, a structure is solving a problem you don't have.

A structure's worth tracks how many heat-loving, marginal crops you want to grow. A garden built around tomatoes, chillies and peppers gets a transformative benefit; one built around kale, leeks and broad beans gets a marginal one. A Braemar garden growing mostly kale may not need one, and an Edinburgh garden set on chillies and tomatoes might, even with a 24-week season.

Running costs and chores

Ventilation and grey mould. An unheated structure closed up in a damp Scottish summer traps humid air quickly, which is what botrytis (grey mould) needs, and tomatoes, peppers and soft fruit under cover are all vulnerable if ventilation is poor. Open the vents and doors regularly on mild, still days. In a greenhouse, roof vents are the most useful, ideally covering 15 to 20% of the floor area. It isn't a reason to avoid a structure, but allow for the extra checking time.

Winter use. An unheated structure can keep hardy salad leaves, winter lettuce and other cool-tolerant crops going into or through winter in milder parts of Scotland, and it is useful for overwinteringKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → tender potted plants. At Scottish latitudes weak, low-angle winter light limits growth as much as cold does, so a structure sited in shade or facing away from the sun gives up much of this benefit however well it holds heat.

Pests and heating. Damp Scottish gardens are slug country, and a structure's warm, humid shelter can suit pests such as whitefly and red spider mite as well as it suits your crops, so budget some pest-watching time. Heating is a further optional cost. The figures here assume an unheated structure on solar gain alone. Electric or paraffin heating, or insulation such as bubble wrap, adds a running cost that most gardeners don't need in order to gain weeks, though some choose it for an earlier, more reliable start.

Cost, space and commitment

A modest 6x8ft polytunnel commonly runs to the low-to-mid hundreds of pounds, a similar glass greenhouse several hundred more; polytunnel vs greenhouse covers the full cost picture, including cover and glazing replacement. Cost is only half of "worth it". Signs a structure isn't for you:

• Very little space. Even a 6x8ft structure is a meaningful chunk of a small urban garden or patio. If fitting one means giving up most of your growing space, the trade is often not worth making.
• A handful of plants. For a few pots of herbs and a couple of tomatoes, the fixed cost and space are disproportionate. A couple of large pots on a sheltered, south-facing patio, or a growbag against a warm wall, can carry two or three tomato plants without a structure, imperfectly but adequately.

• You're likely to move soon. Polytunnels can be dismantled and moved with real effort; glass greenhouses on a base effectively can't. If you're renting or may relocate within a couple of years, the payback may not arrive.
• You mostly grow hardy crops. As above, the crop case is weak however much space or time you have.

Two checks before you commit: a larger structure may need checking against your council's permitted development rules (see polytunnel planning permission in Scotland), and an upland or inland site with heavy winter snow needs a plan for clearing roof snow, since either structure can be damaged by a heavy, wet load. If two or three of these apply to you, look at the alternatives below first.

Cheaper alternatives

You're not choosing between a polytunnel and nothing. Each of these captures some of the same season-extension benefit at a fraction of the space and cost.

Cold frames. A low box with a hinged glazed lid, sitting on soil, gravel or paving with no foundation. They are essential for raising seedlings and hardening offGradually getting young plants raised indoors used to outdoor conditions over 7–14 days before planting out, so cold, wind and direct sun don't shock or damage them. Full definition → tender plants (acclimatising them gradually to outdoor conditions), and they extend a small bed's season by weeks at either end, including ripening tomatoes, aubergines and chillies in summer and protecting plants from autumn frost. Choose a lid that allows varying ventilation, and prop it on mild days. For a small garden with a modest number of plants, a cold frame can deliver most of the practical benefit of a full structure.

Cloches and tunnel-cloches. Single-plant covers, or a row-length mini-tunnel over a bed: the cheapest, most portable protection. Good for getting an early sowing of a hardy or semi-hardy crop in weeks before it would be safe, or protecting a late crop from the first frosts. Young chillies outdoors can be covered with cloches, ideally until the end of June.

Horticultural fleece. Lightweight, breathable fabric laid over plants or supported on hoops. It comes in weights suited to different jobs; garden fleece grades and timing covers which to use when.

A sunny windowsill or porch. For starting tomatoes, chillies and peppers indoors, a bright windowsill, porch or unheated spare room can replace a greenhouse's propagation role, as long as you harden the seedlings off gradually before they go outside or into a cold frame. Chillies need about 21°C to germinateA seed sprouting: breaking dormancy and putting out its first root and shoot, given the right warmth and moisture. Full definition → and a long season, so start them from late winter. See hardening off seedlings.

These aren't consolation prizes. For a small garden, or one growing mostly hardy crops, a cold frame plus fleece plus a windowsill start can capture most of the realistic benefit, and the marginal gain from a full structure may be small.

The verdict

Weigh the factors together: it is the combination of a marginal crop list and a tight or exposed site, not either alone, that makes a structure worth it. If that points to no for now, a cold frame, some cloches, fleece and a bright windowsill do a useful amount of the same job for a fraction of the outlay, and you can upgrade later. If it points to yes, the next decision, polytunnel or greenhouse, has its own trade-offs in polytunnel vs greenhouse.

Common questions

Sources