A polytunnel can turn a short British growing season into months of useful planting time, but only when it suits the garden, the crops and the weather it will face. This ultimate guide to polytunnels: from selection to replacement explains what to look for before buying, how to put one up properly, and when repairing the cover stops being the sensible option.
Start with what you want to grow
The right polytunnel is rarely just the biggest one that will fit. A compact tunnel can be ideal for starting veg plants, protecting salad crops and growing a few tomato plants, while a longer walk-in model gives a household room for beds, shelving and a clear path down the middle.
Think about how you will use it across the year. If your main aim is early potatoes, courgettes and tomatoes, a simple seasonal setup may be enough. If you want to grow through autumn and winter, raise seedlings in spring and keep tender plants protected, it is worth choosing a taller, stronger frame with good ventilation from the outset.
Allow space around the outside as well. You need to tension the cover, secure the base and reach the sides for cleaning or repairs. Placing a tunnel tight against a fence may look efficient on paper, but it makes maintenance much harder.
Choosing the right polytunnel size and shape
Width, length and height all affect how practical a tunnel feels. A narrow model is easier to position in a smaller garden, but can become crowded once growing bags, staging and mature plants are inside. Wider tunnels offer more bed space and better airflow, although they need a more open site and stronger anchoring.
Height matters particularly for tomatoes, cucumbers, beans and gardeners who do not want to bend each time they step inside. A taller apex also gives warm air somewhere to rise, helping to reduce condensation around foliage. That said, extra height can catch more wind, so a tall tunnel needs a well-secured frame and cover.
For many home gardens, a walk-in polytunnel with enough room for a central path and one bed either side is the most useful starting point. If space and budget allow, choose slightly larger than your immediate plan. Gardeners often find that seed trays, pots and summer crops fill a tunnel far sooner than expected.
Frame material and strength
Most domestic polytunnels use galvanised steel hoops. Galvanising helps resist rust in damp garden conditions, and a steel frame is generally the practical choice for value, strength and longevity. Check the diameter of the tubing, the number of hoops and whether there are crossbars or bracing bars. These details make a real difference in exposed gardens.
A lightweight frame can be suitable for sheltered plots and occasional use, but it may need more care in strong winds. For an open garden, a heavier frame with proper ground rails and bracing is usually better value than replacing a flimsy structure after the first difficult winter.
Doors deserve attention too. A zipped door is straightforward and common on smaller tunnels. Larger models may have roll-up doors, which are convenient for moving compost, tools and larger plants in and out. Whatever the design, it should close securely without pulling hard on the cover.
Selecting a cover that suits UK weather
The cover creates the growing environment, so do not treat it as an afterthought. Polythene covers vary in thickness, UV treatment and reinforcement. A thicker, UV-stabilised cover normally costs more but should cope better with sunlight, rain, frost and regular handling over several seasons.
Clear covers let in maximum light, which is useful in lower-light months. Diffused covers spread light more evenly and can reduce harsh hot spots in summer. Neither is automatically better: clear can suit a shaded garden, while diffused film may be useful where plants are exposed to strong midday sun.
Reinforced covers include a visible scrim pattern and can offer extra tear resistance. They are a sensible choice where children, pets, branches or windy conditions make accidental damage more likely. The trade-off is that they may let through slightly less light than a plain clear cover.
Avoid judging a cover by thickness alone. The quality of the UV protection, the fit over the frame and the way it is secured will have as much influence on its life. A good cover fitted loosely will flap, stretch and wear out quickly.
Site preparation and installation
A level, well-drained site makes installation easier and gives your crops a better start. Avoid a low point where water sits after rain, as this increases humidity and makes paths muddy. If the only suitable area slopes, level it as much as possible before assembling the frame.
Position the tunnel where it receives good daylight but is not directly exposed to the strongest prevailing wind. Nearby hedging can offer useful shelter, although overhanging branches can rub or puncture the cover. Keep enough distance to prevent shade and allow air to move around the structure.
Anchoring is not optional in the UK. Ground anchors, base rails, buried cover edges or a combination of methods help prevent lifting in gusty weather. Follow the supplied fitting instructions carefully, especially for fixing the cover and tightening ratchets or straps. A cover should be taut enough not to flap, but not stretched so hard that it strains at corners and doors.
It helps to install the cover on a mild, still day. Cold film is less flexible, while wind turns a manageable job into a frustrating one. Ask another person to help with larger models so the cover can be pulled evenly across the frame.
Set it up for healthy plants, not just more plants
A polytunnel gets hot surprisingly quickly, even when the outdoor temperature feels modest. Open doors on warm days and use side ventilation where your model provides it. Good airflow reduces excessive heat, condensation and fungal problems such as botrytis.
Arrange planting with access in mind. Beds along each side and a firm central path are easy to manage. Avoid planting right against the cover, where leaves can be damaged by cold condensation or hot plastic. Tall crops work best near the centre under the highest point, while salads and herbs can make good use of the lower edges.
Watering needs are different under cover because rain cannot reach the soil. Check moisture regularly rather than relying on the weather forecast. Drip irrigation, a watering can and mulching can all work well, depending on the size of the tunnel and how often you are at home.
On hot days, shade netting can protect sensitive plants. In colder periods, horticultural fleece inside the tunnel adds another layer of protection. These are useful tools, but ventilation still matters. A sealed tunnel full of damp air is not a healthy growing space.
Ongoing maintenance that extends cover life
A quick monthly check can prevent small faults becoming expensive replacements. Look for loose straps, movement at the anchor points, rubbing at joins in the frame and early signs of tears. Remove leaves, algae and dirt from the cover using lukewarm water and a soft cloth. Abrasive cleaners can scratch the plastic and shorten its useful life.
After high winds, inspect the structure before assuming everything is fine. Tighten fixings only where needed and make sure the frame has not shifted. Do not leave snow building up on the roof in winter, as its weight can distort hoops or split a weakened cover.
Small punctures are often repairable with purpose-made polytunnel repair tape, provided the cover is dry and clean. Apply a patch with rounded corners and leave a generous overlap around the tear. A patch is a sensible, low-cost fix for isolated damage, but repeated tears are a warning that the film is reaching the end of its working life.
When to repair and when to replace
Replacement covers are often the best way to refresh a sound frame. Consider replacing the cover when it has become brittle, cloudy, heavily stretched or difficult to keep tight. Widespread cracking, multiple failed patches and torn fixing points are also signs that repairs are no longer likely to last.
If the frame is bent, corroded through or unstable at the joints, assess the whole structure rather than ordering a new cover straight away. A replacement cover needs a smooth, secure frame beneath it. Any sharp edges should be repaired or protected first, otherwise they can damage new film.
Before ordering, measure the frame rather than relying only on the original listed size. Measure the width, length and height, and check the cover style needed for the door and side fixing method. A correctly sized replacement is easier to fit, stays tauter and gives better protection for your plants.
For gardeners comparing practical options, AnyDayDirect's polytunnels and greenhouses range can be a useful place to weigh up walk-in space, frame design and cover style before choosing.
Frequently asked questions
Can a polytunnel stay up all year?
Yes, a well-anchored polytunnel with a UV-stabilised cover is designed for year-round use. Remove heavy snow, keep the cover taut and inspect it after storms to give it the best chance of lasting.
Do I need a base for a polytunnel?
Not always. Many models are installed directly into soil with ground anchors. A level site and secure anchoring matter more than a solid base, although firm paths inside make everyday use more comfortable.
Is a polytunnel warmer than a greenhouse?
Both can create warmer growing conditions than the open garden. A greenhouse usually retains heat more effectively overnight because of its rigid glazing, while a polytunnel often gives more growing area for the money. Your choice depends on available space, budget and the crops you want to grow.
A well-chosen polytunnel does not need to be complicated. Give it a stable site, a cover suited to the weather and regular attention, and it can become the most productive part of the garden from the first spring sowing to the last winter salad.