Why Coastal Polytunnels Fail First

On an inland plot, a polytunnel is a forgiving structure. It shrugs off a blow, the cover slackens a little, and you tighten it in spring. On a coastal holding, the same tunnel meets salt-laden air at forty miles an hour, sand that scours like sandpaper, and gusts that arrive with nowhere to hide. The result is predictable: covers that balloon and shred, hoops that walk out of the ground, end walls sucked inward, and fixings that rust through in a single winter.

The good news is that almost every coastal polytunnel failure is a failure of siting, anchoring or bracing — the cheap parts of the job. Spend a little more time and money below the cover line, and a tunnel here will give you tomatoes in October and salad right through the worst of the weather.

Pick Your Site Before You Dig Anything

Wind is the design brief. Walk your land on a blustery day before you commit, and watch where the gusts funnel between buildings, hedges and gateways. You are looking for the calmest spot that still gets full sun.

• Run the tunnel broadside to the prevailing wind. Let the gale strike the long side, so it flows up and over the curved roof. A flat end wall facing square into the weather is a sail, and the first thing to go.
• Put the doors at the sheltered end. You will be opening and closing them for years; make that a pleasant job, not a wrestling match.
• Stay clear of solid windbreaks. A wall or dense hedge creates violent turbulence on its leeward side, sometimes for ten metres or more. Leave a generous gap and let the airflow settle.
• Avoid the frost pocket and the exposed crown. Mid-slope, facing south, is the ideal. Cold air drains downhill, so the flat bottom of a coastal field will hold frost long after the rest has thawed.

If you can plant a shelter belt of hardy coastal species — sea buckthorn, escallonia, tamarisk — two or three years before the tunnel goes up, do it. That hedge will do more for your growing season than any piece of hardware.

Ground Anchors: The Bit Everybody Skimps On

Off-the-shelf kits come with a handful of anchors intended for a sheltered garden. Ignore that and anchor every single hoop. On a gale-prone site, the hoops themselves become the weak point if they can move independently.

You have two dependable options. Screw-in ground anchors of around 750mm, fitted with a plate and driven at an angle away from the tunnel, work well in firm ground and can be checked and tightened each year. In sand or soft coastal soil, concrete is safer: a hole of 600–900mm depth with a galvanised anchor bar set into it, poured in one go and left to cure fully before any frame goes on.

Bolt a tanalised timber base rail of roughly 150 x 50mm along both sides, fixed down through the anchor plates every metre or so. This rail ties all the hoops together at ground level, spreading the load of a gust across the whole footprint rather than one hoop. Use stainless or hot-dip galvanised fixings throughout — coastal salt destroys cheap zinc plating within a season or two.

Bracing the Frame Against Racking

Racking is the sideways collapse that happens when wind pushes the frame into a parallelogram. It starts with a slight lean and finishes with a wrecked tunnel, and it is entirely preventable.

• Close up the hoop spacing. A sheltered tunnel may manage 1.5m between hoops; on the coast, 1.2m is wiser.
• Fit diagonal braces in the end bays, running from the base corner up to the first or second hoop, on both sides and both ends.
• Add crop bars every third hoop — horizontal tubes across the width at waist height. They stiffen the frame and give you somewhere to tie strings and support canes.
• Add storm braces if your kit allows, particularly over the winter months, then remove them in spring if they obstruct your beds.

Buy the heaviest gauge frame you can justify. A thicker-walled hoop of 35mm or more, properly braced, will outlive two or three cheaper tunnels on the same site.

Fitting the Cover So It Doesn't Flap

Flapping, not wind pressure, destroys covers. Every slap of loose polythene against a hoop wears a hole, and a hole becomes a tear within a week. Fit the cover on a still, mild day, and get it drum-tight.

Run anti-hotspot tape over every hoop, crop bar and brace where the cover will touch it. This is the single most neglected job on coastal tunnels, and it is cheap. Replace the tape whenever you replace the cover.

Choose a UV-stabilised woven cover of at least 720g/m² with a stated multi-year warranty. Bury the skirt in a trench roughly 300mm deep and 400mm wide, backfilling with soil and stone, or use a clamping base rail if your frame has one. On wet coastal ground, put a little drainage stone in the bottom of the trench first so it does not become a moat. Pull the cover tight as you work along, and finish by tensioning the end panels properly — a loose end wall is the first thing to fail.

A Winter Rhythm of Checks

After every named storm, walk the tunnel before you do anything else. Check that the anchors are still firm, look for chafing where the cover meets the frame, and press the polythene with your palm — if it gives more than a centimetre, it wants tightening. Keep a roll of repair tape, spare patches and a spare end panel in the shed, because a small repair made in November saves a whole cover in January.

Rinse salt off exposed fixings and door runners when you think of it, and re-tension everything in late spring once the material has relaxed. Do that, and your tunnel becomes what it should be: a bit of Cornwall, or Norfolk, or the Hebrides that behaves like a Kentish garden.