Cladding a polytunnel
Cladding a polytunnel
A polytunnel cannot be covered or ‘clad’ on just any day. The weather has to be still – any wind will make the plastic sheet act like a parachute. In addition, the sun has to be shining – the warmth from the sun’s rays trapped in the polytunnel make the plastic more flexible and easier to handle and stretch. If there is no sun the plastic is prone to tearing.
If the weather is not right do not attempt to clad a polytunnel – even if a team of people have arrived to help or you have taken a day off work. Any attempt to clad a polytunnel in inappropriate weather will, at best, result, in a loose cover that will flap in the wind and may shorten the life of the plastic by years. At worst bad weather could result in a complete failure.
In the days before you intend to clad your tunnel there are certain things that must be done:
1. A ditch must be dug around the tunnel frame. 2. Hot spot tape must be stuck on the metal hoops so that the plastic sheet does not rest on metal in any place.
3. All rough edges or sharpe points must be covered with thick tape so that it will not catch on the plastic.
For details see the article "How to erect a polytunnel".
To clad a polytunnel you need at least three people committed to working until the job is done. It always helps, though is not necessary, to have a few extra people when lifting the plastic sheet over the frame.
Lifting the plastic sheet over:
Photo 1. Before starting it really helps to make sure all the necessary tools and equipment are available. In particular, a knife is needed for cutting the plastic, spades for filling the ditch, and hammer, nails and wood for attaching the plastic to the doorframe.
Photo 2. The plastic sheet needs to be unrolled along one side of the polytunnel frame. Once unrolled the sheet will still be folded several times along its length. The different layers tend to stick together, so before lifting over the polytunnel frame the layers should be separated.
Photo 3. The sheet should then be pulled over the polytunnel frame. At least two people, preferably three or more, are needed for this job. The bigger the polytunnel the more people are needed. Holding both ends of the plastic sheet it should be gently lifted onto the frame and eased up.
Photo 4. The plastic sheet should never be forced, just gently manoeuvred to the apex of the frame. There may be times it seems impossible, but it will eventually get there. The only problem that might occur is if there is a gust of wind. The plastic sheet will act like a parachute. If it is too windy, give up. If there are occasional gusts of wind it might be exciting but it is not really a problem.
Photo 5. As soon as the sheet is over the polytunnel frame it should be weighted down with soil. This must be done quickly if there is any risk of there being more gusts of wind.
Photo 6. The plastic sheet must lie evenly over the frame, with equal amounts of spare plastic on each side and each end. Checking that the creases run parallel to the ground is a useful way of ensuring that the plastic is not skew. To temporarily secure the sheet to the frame the spare plastic at the ends can be tucked in through the doorframe. If the breeze is a little strong and the sheet is at risk of being blown a little soil can be put on the plastic in the ditches – but not too much as it will have to be moved.
It is worth taking a little break at this point to allow the air inside the tunnel to heat up. The warmer the plastic is, the more flexible it becomes, making the job of attaching the sheet to the frame much easier.
Attaching the plastic sheet to the polytunnel frame:
Photo 7. The plastic is first attached to the top of the doorframe at both ends. The plastic above one doorframe should be smoothed out, and a cut made diagonally, down and towards the centre from both corners, creating a flap of plastic that comes to a point 2–3 feet below the top of the doorframe.
Photo 8. The flap should be smoothed out, but not pulled. It should be folded inside and attached to the top of the doorframe on the inside of the polytunnel.
Photo 9.The door at the other end is attached in the same way, but before being attached the plastic sheet must be pulled as hard as possible. If the sun is shining the air inside the tunnel will quickly heat up, warming the plastic as well. It is worth letting this happen – take a break, have a cup of tea – as warm plastic stretches better.
Photo 10. If the plastic is warm it becomes supple and can be pulled extremely hard without any risk of damage. If the sun is not shining, particularly if the air temperature is cold as well, the plastic is brittle and will easily tear. Even if the plastic is warm, though, it must be held correctly for pulling. The best way is to bunch the plastic together and hold that. Clutching a single layer of the sheet when pulling is risky as fingers easily make a hole in the plastic.
Photo 11. The plastic can be attached to the doorframe by sandwiching it with wood strapping. Use several strips, and many nails.
Photo 12. Alternatively, in exposed sites where the polytunnel may experience strong gales in the winter, the plastic sheet can be attached to the wooden frame by cutting it into strips and rolling it around 1x2 inch pieces of wood, which are then nailed onto the frame. This method gives an extremely strong attachment and will not allow any slippage or tearing of the plastic.
Photo 13. After the plastic has been attached to the top of the doorframe at both ends, it must be buried in the ditch on the sides. This is done in a very particular order. Bury the plastic at the centre hoop on one side, do not pull the plastic down hard.
Photo 14. Then bury the plastic on the other side, at the exact opposite position. From then onwards, the plastic must be buried, hoop by hoop, first one side then the equivalent opposite side, moving gradually from the centre to the ends.
Photo 15. When burying the plastic the first time (see photo 13) it should be smoothed out but not pulled. From then on before the plastic is buried it should be stretched as hard as possible over the hoops. To do this put soil on the plastic sheet; lift the soil up by holding the loose plastic on the outside; and then stand on the soil (still holding the plastic). Your body weight will slide the bulky soil down into the bottom of the ditch, thus stretching the plastic. The warmer and sunnier the day the more pliable the plastic is, and the better the results.
Photo 16. Work from the centre to the ends of the polytunnel. To save time at this stage it is only necessary to wedge soil into the ditch where the hoops are situated. The ditch between the hoops can be filled in later. Only bury the plastic on the sides, the ends will be done later.
Photo 17. If the site is particularly vulnerable to strong winds, the plastic can be buried in the ditch even more securely by folding the spare plastic over, like an “S”, and adding more soil.
Photo 18. With the ditch done the next step is to attach the plastic to the sides of the doorframes. Starting at the top the plastic is pulled around the last hoop and the doorframe and nailed onto the frame on the inside of the tunnel. The job really needs two people, one to keep a tight pull on the plastic, while the other does the hammering.
Photo 19. The plastic must be pulled as hard as possible around the corner of the last hoop.
Photo 20. Towards the bottom of the doors there will be a lot of spare plastic to deal with. This is quite natural and just has to be folded out of the way. Folds must be introduced to accommodate it all.
Photo 21. The plastic at ground level at the ends can now be buried in the ditch. The soil should be tightly packed in, and the extra plastic sticking up form the ditch should be cut just at the soil surface.
Photo 22. When all the ditches are filled in and the doorframes done the tunnel sides should look neat, and there should be no loose plastic.
Making the door
There are as many designs for tunnel doors as there are tunnel owners. They can range from very sophisticated methods using hinges and proper door latches to just hanging a sheet of spare plastic from the doorframe. Generally, the simple methods are the easiest to maintain.
Photo 23. A simple but effective door is a sheet of plastic wrapped around a piece of wood and nailed onto the top of the doorframe. The plastic should be larger than the doorframe.
Photo 24. Two planks of wood sandwiching the plastic at the end and at a couple of places in the middle will give the plastic ‘door’ extra weight and strength.
Photo 25. The door is opened by rolling the plastic up and hooking some string around it.
Photo 26. In situations where strong gales can be a problem it is important that the door is able to prevent the wind entering the tunnel. In these cases a slightly stronger door is necessary. One method is to make a wooden frame that is slightly larger than the doorframe. This can then be covered with plastic and kept on with string crossed over the door.
Photo 27. A more sophisticated system involves hinges and doors that fit accurately within the doorframe. These systems are easy to manage day-to-day, but do require more maintenance over the years.
Preparing the inside of the polytunnel
Photo 28. Many polytunnel crops, such as tomatoes and cucumbers, need some sort of support when growing. It is much easier to put the supporting wire up before any crops are planted. The wire should be laid out along the length of the tunnel.
Photo 29. The wire should be attached to the end hoops with attachments slipped onto the end hoops when the polytunnel was erected.
Photo 30. With the wire firmly secured at both ends of the polytunnel, it should then be attached onto each internal hoop. A short piece of wire wrapped around the hoop and wire does the job nicely.
Photo 31. When the four lengths of wire are secured to each hoop they are unlikely to get in the way of anyone working in the polytunnel. The polytunnel is now ready for planting.
The finished job... and a few weeks later
For details about how to grow crops in a polytunnel click here.
© Joy Michaud