In much of Scotland a polytunnel is not a luxury. It is what makes the warmth-loving crops possible at all. Outdoor tomatoes are marginal north of the Central Belt, and basil is not hardy anywhere in the UK. A tunnel or greenhouse is the difference between a reliable crop and a gamble. It buys season length at both ends and keeps the worst of the wind and rain off.

The four problems a tunnel concentrates

Here is the striking part. Four of Scotland's common growing problems want completely different conditions: wet air, dry leaves, warmth, a gap in the cover. Yet the same structure encourages all of them at once.

Grey mould (botrytis)wants wet air

High humidity and poor ventilation drive it, and it infects without any wound. A closed tunnel is exactly the still, damp air it wants.

Powdery mildewwants damp air, dry leaves

It favours dry conditions on the leaf but needs humid air to make spores. That describes a tunnel precisely: rain never wets the foliage, but the air stays damp. Cover is mildew's ideal.

Tomato blightwants a breached tunnel

Under cover, plants are less likely to be hit because wind-blown spores struggle to reach them. If spores do get in through doors or vents, the high humidity inside usually leads to very rapid development.

Glasshouse whiteflywants warmth

It thrives in warm conditions, which is why it is rarely a problem on outdoor plants, and why it is active all year round under cover.

One lever moves all four: ventilation

In practice that means: open doors and vents whenever the weather allows, even on cool days; do not cram plants, so air can move between them; water at the base early in the day rather than over the leaves late; and check the tunnel for gaps, because a torn panel is how blight gets in. Damping down for heat and ventilating for humidity pull in opposite directions. On a warm, muggy Scottish afternoon, ventilation wins.

A polytunnel with its end doors fully open for ventilation, its plastic skin plain and unbroken, rows of plants spaced well apart inside
Doors open at both ends for a through-draft, plants spaced apart rather than crammed. The master control that answers all four problems at once.
The crop most Scots need cover forOur tomato guide covers varieties, timing and blight for Scottish conditions.
Growing tomatoes →

What a tunnel is worth

None of this is an argument against growing under cover. In Scotland it is often the only way to succeed with the crops that most reward it. It is an argument for going in with clear eyes: a tunnel changes which problems you will face, not whether you will face any. Manage the air inside it and the trade comes out firmly in your favour.

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