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How to clean a polytunnel

Over winter, damp conditions will have coated your tunnel with green algae. It’s vital that the polytunnel cover is cleaned to let in as much light as possible, especially in early spring when growth is struggling to get going. You may only see the algae coating on one side of the tunnel, but don’t be fooled – it’s everywhere, and it’s substantially reducing the light transmission of your tunnel film. Think it’s not going to be significant? Think again!

A polytunnel (high tunnel) with algae, image

To clean an ordinary (single-span) tunnel, you’ll need the following; a willing helper, an old sheet, two short lengths of rope (or clothesline or similar), a bottle of extremely cold beer, two tennis balls, a soft car-washing brush, a garden hose, and hand spray bottle and a bucket of soapy water.

how to clean polytunnel, image

‘Flossing’ the polytunnel

Prepare the cleaning rope

First, tie a length of rope securely each end of the sheet. Don’t make any holes in the sheet, as it will only tear. Instead, place a tennis ball near the end of the sheet and wrap the sheet end back around it, so that the ball is in a pouch of sheeting; then tie the rope around the neck of the pouch, so that the ball acts as an anchor to secure the rope. Then dip the whole thing in the soapy water.

Clean the top

Using the hose, thoroughly wet the tunnel and throw the sheeting over the top of it so that it hangs down on either side, as if it were an extra hoop. You and your helper now have to take turns to pull the line so that it “flosses” the tunnel from side to side, as you might dry your back with a towel, and work your way from one end to the other and back. You can see us doing this in the promotional video for The Polytunnel Handbook on Youtube, below.

Once this is done, repeat the process without the back-and-forth movement; this is because unless your film is absolutely taut the back-and-forth movement tends to miss any natural creases that form under pressure. Depending on the width of the tunnel you may find that your sheet is not long enough to clean the whole upper surface at one go, in which case adjust the line so that you concentrate on one side at a time. As with washing a car, hose off the dirty water before it has a chance to dry on again. If the day is warm, it may be best to clean the tunnel in sections.

Andy cartoon, image

‘This process might sound like a lot of work, but it doesn’t take long. With one helper I can clean the outside of my 14’x24′ tunnel in just over twenty minutes.’

Clean the sides

Now that the top of the tunnel is done, use the carwash brush to clean the sides. Wet them first with the hose, squirt on a little detergent solution using a hand spray bottle, brush thoroughly but gently and then hose off the muck.

Clean the inside

The inside of the tunnel film should also be sponged down with fresh citrox solution, as should any staging and so on, and this should be done in the morning to give the film a chance to dry before temperatures fall. As well as increasing light transmission, this reduces the chance of fungal infections from last season overwintering successfully. Note that lacking rainfall, the inside of the tunnel is highly susceptible to toxin buildup – which is why it is so important to use an organic, plantsafe detergent like Citrox.

Well done – now drink the beer!

Postscript: don’t, whatever you do, try to use a pressure washer to clean a polytunnel. If there is the slightest flaw in the skin the water will find it, and the results are catastrophic.

I’m also aware that some people recommend Algon as a leave-on treatment to save all the work. I don’t recommend it, partly because getting it right up onto the top of the tunnel is very difficult, but mostly because it only works really well for the first couple of years of the cover’s life. On older covers, it still helps – but there’s just no substitute for a proper clean.

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13 Responses to “How to clean a polytunnel”

  1. NP says:

    great information fantastic..

  2. Catherine says:

    Great info, but can you tell me if it would be safe to put pinholes in the “pleats” at the gables where the plastic has been gathered, to allow the collected condensation/water to drip out.I find that the collection goes green and all “dead bodies” get trapped in the pleats.
    Thanks.

    • Andy McKee says:

      Sadly it doesn’t actually work as small holes quickly get blocked with debris, so don’t bother; learn to love your green soup and simply ignore it. In the Polytunnel Handbook Mark and I recommended doing your pleats so that the upward-facing ‘pockets’ are on the outside, leaving downward-facing ones inside the tunnel. Bear it in mind when you’re re-covering your tunnel, which will hopefully be a good long time away!

  3. Mark Gatter says:

    Hi Catherine – and Andy – I figured out a way to do this. Pinholes don’t work, as Andy says – but if you use a hole punch, making a hole about 3mm across, it works. I used an old leather punch on mine. As the pleats aren’t load-bearing the holes don’t stretch and tear, and they’re big enough to allow the pleat to drain. Woohoo!

  4. Andy McKee says:

    If you want to know the way… ask a buddhist! You could probably use a stationery holepunch then. just be careful…

  5. hopefarm says:

    ok so I have a 96×30 inflated two layer greenhouse with roll up sides (I bit off more than I could chew… but it has lots of potential…lol!) anyway, there is chicken wire on the inside so the rolled down flaps are next to impossible to access. I am so tempted to pressure wash it, but I know there are tears hear and there from the last 4 years of use. No I have never cleaned it. But it is turning green this year! So, how to clean with the chicken wire in the way… anything that I can spray on (organic) that will loosen it so I can just hose it off? Help! Thanks

    • Andy McKee says:

      Yep, there’s an organic product in the UK called Algon that will loosen the green algae and you can hose it down. Don’t be tempted to pressure wash, as the potential for damage is impressive to say the least!

  6. Nicki says:

    Hi, great video have just bought your book too. In the video it shows you collecting rainwater on a curved polytunnel….where can I get these kits from please I can’t find them anywhere!
    Thank you!
    A new polytunnel owner :)

  7. the information you give is perfect as a novice i have to start from scratch thank you

  8. Phoebe says:

    My polytunnel is on a plot with no water mains supply so no hose pipe. Can’t afford fancy specialist cleaners. Any ideas please?

    • Andy McKee says:

      Certainly – but you won’t like it! If you follow the advice above when it’s raining – reasonably hard – the rain will carry all the loose dirt off the plastic for you. Just remember to unhook your runoff collection system from your water butts as you don’t want all the crap to end up in there.

  9. Thomas says:

    Greetings.
    I am in the island of Dominica. For years I have laboured to clean the top of my polytunnels. Using a step ladder but never being to thoroughly clean the top. And here you give me the simple solution.
    Thanks.
    Live well.

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