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!
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.
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.
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.