Building a low-cost polytunnel for a smallholding

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24 Sep 2026

Building a low-cost polytunnel for a smallholding

Why a polytunnel earns its keep

A polytunnel is the most useful thing you can put up on a smallholding for the money. For a few hundred pounds and a couple of weekends, you gain six to eight weeks at the start of the season and another four or five at the back end — enough to ripen tomatoes, sweet peppers and aubergines that would otherwise sulk outdoors, and to keep salads, spinach and winter herbs cropping right through until spring.

It earns its keep in other ways too. A tunnel gives you a dry, sheltered spot for potting on, a place to harden off plants before they go out, somewhere to dry onions and garlic, and a refuge to work in on a wet February afternoon when the rest of the plot is uninviting. It doesn't need to be big to be transformative. A well-built three by six metre tunnel will feed a household generously.

Siting and sizing before you spend a penny

Get these decisions right and the build becomes straightforward. Get them wrong and you will fight the structure for years.

Level, free-draining ground. A slight slope is manageable; a hollow is not. Cold air pooling in a dip will undo much of the benefit of the cover.
All-day sun, especially in winter. Run the ridge east to west so the long southern side catches the low winter light. Avoid siting under or beside large trees — you will lose light and gain a carpet of leaves and dripping branches.
Face the gales sensibly. Point one end into the prevailing wind rather than presenting a whole side to it. A shelter belt or hedge upwind makes an enormous difference to how long your cover lasts.
Water and access. A tap nearby saves a hundred journeys with a watering can. Leave room for a barrow and a good path around at least one long side.

On size, be realistic but not timid. A 3m x 6m tunnel is the classic smallholding size: enough room for four decent beds plus a path down the middle, and tall enough at the ridge for cordon tomatoes and climbing cucumbers. Insist on a door height of at least 1.8m — you will thank yourself every single day.

Building a sturdy frame for less

The frame is where home-built tunnels either shine or fail. The cheapest reliable approach uses 32mm blue MDPE water pipe for the hoops, bought as a coil of 50m or 100m, which costs far less per metre than cut lengths. For something more permanent, salvaged scaffold tube is superb, though heavier to handle.

Hoops at 1.2m to 1.5m centres. Any wider and the cover will drum and flap in a gust.
Anchors driven deep. Push or hammer a length of scaffold tube or a stout timber stake 60cm into the ground at each hoop position, then slide the pipe hoop over it.
A timber base rail. Bolt a tanalised 100 x 50mm rail along both long sides to the anchor posts. This ties the whole structure together and gives you something to bury the skirt against.
A ridge pole and crop bars. A length of pipe running along the apex, plus two or three cross bars, transforms rigidity and doubles as a support for strings and hanging crops.
Diagonal bracing at both ends. Corner braces stop the tunnel racking sideways in a storm.

Set the anchors with a spirit level and a string line. Ten minutes of care here saves a season of wonky hoops and a cover that never sits right.

Putting the cover on properly

Buy UV-stabilised polythene of at least 600 gauge, ideally with an anti-condensation layer, and choose the widest sheet you can so the skirt reaches the ground on both sides. Fit it on a warm, still day — polythene is far easier to handle and tension when it is soft.

Tape every point where the cover touches the frame with anti-hotspot tape, because chafing is what kills covers prematurely. Lift the sheet over rather than dragging it across the ground, pull it evenly from both ends, then dig a 30cm trench around the whole perimeter, tuck the skirt in and backfill firmly. Finish the ends by battening the cover to the door frames. Do not overtighten in cold weather: the plastic will relax as it warms, and a cover pulled drum-tight in January will sag by June.

Ventilation, watering and winter care

Managing the climate inside is where most beginners come unstuck. Overheating and humidity cause more problems than cold.

Ventilate early. Open doors at both ends on any mild day, even in March. Damping off, botrytis and blight all thrive in still, humid air.
Water at the roots, in the morning. A drip line or soaker hose on a timer is the single best investment you can make. Keep foliage dry, especially on tomatoes.
Damp down paths on scorching days to raise humidity without soaking the beds.
Watch the snow. Clear heavy snow off the roof promptly or you will be rebuilding hoops in the spring.

Feed the soil, not just the plants. Load beds with well-rotted compost each autumn, keep off the ground where you can, and the tunnel will reward you with crops that simply will not grow outside.

Getting the most from the space

Plan the tunnel like a garden in miniature. Put the heat-lovers — peppers, aubergines, cucumbers — in the warmest central beds, use the shadier end for salads and herbs, and grow anything tall upwards on strings rather than outwards. Follow early potatoes with winter salads, and sow quick crops between slower ones. Keep a simple notebook of what went where and when, because a tunnel is a long-term investment and each year's records make the next one better.

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5 Comments

Robert Sonny says Jul 21, 2024 at 10:00am

I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
Reply
- John Abraham says Jul 21, 2024 at 10:00am
  
  I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
  Reply
  - Robert Sonnysays Jul 21, 2024 at 10:00am
    
    I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
    Reply
Robert Sonny says Jul 21, 2024 at 10:00am

I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
Reply

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