How To Clean A Polytunnel/Hoophouse

It's that time of year. The days are lengthening at last, and although air temperatures may still be low, inside the polytunnel (a.k.a. hoophouse) things are already warming up. Overwintering plants are growing again, or at least they should be - unless damp conditions have coated your tunnel film with algae. Chances are you'll be able to see a greenish 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 quart of beer, two tennis balls, a soft car-washing brush, a garden hose, and hand spray bottle and a suitable detergent.

1. 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.

2. Dip the sheet into your detergent solution. To prevent damage to the tunnel film a mild detergent is needed, and I recommend Citrox (aka Aussan). This is an organic cleanser which inactivates many fungal spores, retards algal growth, and more importantly is harmless to plants, breaking down harmlessly in the soil.

3. Using the hose, thoroughly wet the tunnel and throw the sheeting over 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. 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.

4. 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.

5. 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.

6. The inside of the tunnel film should also be washed, 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.

7. Well done - now drink the beer!