Polytunnels, advice on buying

BUYING A POLYTUNNEL

Some tips to help you make a good choice

Polytunnels are wonderful structures and good value compared to greenhouses. Once erected they will be there for several years, so check out my tips before buying.

I have used polytunnels since 1983 when I bought 3 of 18×55 feet (5.5x17m) and they served me well except that their 1in tubes were a little too thin for my exposed site; in a gale of March 1988, one of them collapsed after the wind ripped out its polythene on the far side, see below for how that works.

How big?

I would buy the largest you can fit into your area, because the space is useful for storage and drying washing as well as for growing! and one always finds more to plant in a tunnel. Hoops are usually 5 feet (1.5m) apart so length is multiples of that. I reckon that 20-30 feet is a good length for even ventilation, whereas tunnels of say 60 feet length can have pockets of ‘dead’ unventilated air in the middle.

Orientation

  • North to south is ideal so that tall crops in summer cause equal amounts of shade to either side, also to reduce overheating in summer.
  • An east-west structure is face on to the midday sun and in a hot climate this is not ideal during summer. It also means some shading of crops in the north bed when summer crops grow tall in the middle.

However any orientation is possible and I have tried many, with success. Other important factors to consider are aligning a tunnel up-and-down any slope (beds across slope are harder to water and mulch), and having the doors at convenient points for access and watering

Ventilation

Plants like fresh air and its important they have enough, rather than striving for maximum heat. Air in a polytunnel circulates from one end to the other, and if possible I recommend leaving a gap between the top of doors and the frame above them, say 6in (15cm), so that a small amount of air can always flow through and increase carbon dioxide levels, without a draught at ground level. I find that winter salads stay healthy, without mildew problems, yet without any need to open and close doors for much of the winter. One job saved!

The option of side vents (instead of the polythene going to ground level) is in my opinion not necessary or suitable for vegetables, because it reduces temperature too much in windy weather, and the doors offer enough possibility to admit fresh air. Its cheaper, simpler and more effective on other ways to cover the hoops with polythene only, to ground level and below. Keep side vents for long tunnels only, above a length of 40-50 feet (13-16m).

Polythene attached at ground level, or buried?

I recommend burying the polythene all around in trenches of one spade’s depth, in order to have a polythene barrier in the ground. This protects against

  • pests entering, such as rabbits, slugs, cats and dogs
  • weeds creeping in, including couch grass, buttercup, bindweed
  • too much draught and cold air around plants’ leaves.
Filling a trench for new polythene at Homeacres, January 2013
Filling a trench for new polythene at Homeacres, January 2013

It is sometimes claimed to be easier and quicker to attach polythene at ground level to battens or rails. But having tried this, I do not agree. because before doing that, you need to dig holes for the foundation tubes rather than simply knocking them in, also you may need some concrete in there for windy sites. All the fixings you need involve extra expense and are only worthwhile if there is simply no room to dig a trench.

Foundation tubes

When burying polythene in a trench, tubes simply need to be hammered into the ground about 12-18in (30-45cm), where their role is to give the structure some rigidity, that is all. When polythene is bured, the tubes are not anchoring the structure in place, but keeping it steady in wind.

In contrast to this, when polythene is attached to a rail and not buried, the foundation tubes have more importance as they are what holds the structure in the ground (rather than the buried polythene) and so you need more expensive tubes with metal brackets on their ends, and each one requires a dug hole, sometimes with concrete.

While we were looking at this on one of my courses, a participant lamented how her new tunnel had blown away within a month – the polythene was attached to rails, rather than buried.

Exceptions to the above are sheltered gardens, and sites where you cannot dig a trench e.g. too much gravel, concrete etc. But then it is difficult to dig holes for foundation tubes too.

Crop support bars

Although an “extra” I would always buy these, so you have support for climbing plants from tomatoes and cucumbers to beans and melons. They span across from side to side at about or just over 6ft (2m) high, so you can walk underneath, yet also reach to tie strings or stakes.

How many years before re-cladding?

Polythene needs replacing every 5-8 years, depending how windy your site is and how tight you can stretch it. Polythene is easier to make tight when the cover is put on in warm sunshine, if possible. I have often done it in January and managed alright, but the skin is definitely slacker and more prone to damage than when clad in warm sunshine.

What kind of polythene?

More types become available and choosing is then difficult. From tunnels I have seen with the more expensive thermic or anti-condensation polythene, I doubt that their extra cost is worthwhile. I recommend the standard, light-diffusing polythene which admits around 89% of daylight. Always use horticultural grade polythene, which has a uv stabiliser, without which it would be brittle and break within a year.

A friend recently noticed the great clarity of my standard, light diffusing polythene which is now 3 years old. I had recently washed it with a wet sheet and brush to remove lichen and moss.

Doors

You use these a lot so suffice to say, they are worth spending money on to have the ones you like. Home-made is certainly possible, note my tips on ventilation. Its the area where you can be most creative, especially if you enjoy carpentry. For door frames, 4×2.

17 thoughts on “Polytunnels, advice on buying

  1. Thank you so much for this wonderful article!

    I am a beginner gardener and my allotment is not close to the house. Should I buy a tunnel or a small greenhouse (my garden as well as all the alotments around me have a rat problem that becomes more difficult in the winter and rat poison or traps are not 100% efficient)? The greenhouse would have glass walls, aluminum supports and a brick and concrete foundation on top of which we would put soil and compost in oder to keep rats out.

    Sincerely,
    Alexandra

  2. I’ve never done polytunnel gardening. I live in South Carolina, USA on the coast. What would you suggest? What brand? Thankyou

    1. Hi Barbara, afraid I do not know the USA brands but you call them hoop houses. Look for one with sturdy metal hoops say 2in diameter in case of high wind, I suggest 14ft minimum width and any length, make sure you have water available.

  3. What would we do without Charles
    I am completely addicted after meeting a colleague who told me about him and no dig while on a course in Coventry University in Social and Therapeutic Horticulture
    He has changed my life for the better ….
    It really works better than all the other methods I m sure after trying things myself as he recommends…
    Proof is in the pudding
    Joyce Fitzpatrick
    Ireland

    1. Hello Joyce, and thanks so much for writing, and I am happy to help.
      I’m pretty sure that your contact was Lara, who also is special, and that course sounds so worthwhile.

  4. My allotment field only allows tunnels up to a max of 3.5 by 2.5 meters, do you think that this size is worth having. Took on a second allotment recently and used no dig with cardboard and leaf mold and now have a weed free set of beds although we had teething problems as the local foxes and cats kept digging it up and letting the weeds through.

  5. Hello Charles, I live on high ground 8 miles from the sea and I have 3 20×10 polytunnels but have not seen put them up here because one flew away at a more protected site. I might get a profession polytunnel company to put up a new one. What is your advice please. Also in answer to your previous correspondent, i had a small polytunnel and it was marvellous, you can grow a lot of especially if the beds are reasonably deep.

    1. Nice to hear Mel and on your windy site, it’s probably worth paying professionals to erect the tunnels.
      First Tunnels did a fantastic job on mine, apart from a small mistake on the irrigation, but that’s a detail 🙂

  6. Hi Charles, this is such a helpful article, thank you so much. You say orientation North to South. Would you think a slight north facing slope would impact heavily? Alternative is to go across the slope east to west. With a south-west prevailing wind, how would this affect orientation? Also do you know about need for planning permission for polytunnels? What is the max size before needing planning? Many thanks, Olivia

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