Erecting a polytunnel

Polytunnels at Sea Spring Farm

From Pasture to Polytunnel

Though it might look daunting, erecting a polytunnel is not difficult. It is simply a step-by-step process.

Preparing the site

1. The first step is to find a suitable site. There are certain requisites; firstly the land must be relatively flat or at least only gently sloping in one direction. The site must also not be shaded and have a water supply for irrigation. Shelter from the wind is desirable, as is soil that is not too stony.

2. Cutting the ryegrass pasture. If there is plenty of time a sheet of black plastic (silage plastic bought from agricultural suppliers) spread over the proposed site will also kill the grass.

3. It is easier to cultivate the soil before erecting the polytunnel.

Pounding the posts

Tunnel frames are quite simple structures, composed of posts pounded into the ground in two parallel lines, and hoops that slot into the posts, forming a line of arches. The first job in erecting a polytunnel is to pound the corner posts firmly into the ground. If they are placed in exactly the correct position the polytunnel will be easy to erect and will stand firm for many years.

Mark out where the corner posts go. If they have been measured out correctly, the two diagonals will be the same length. A simple equation called ‘Pythagoras’ theorem’ can be used to calculate the correct length of the diagonals.

If your tunnel is measured in feet (a lot still are) convert the lengths into metres before calculating the length of the diagonals; doing Pythagoras’ theorem in feet and inches is very difficult.

The Pythagoras equation is the length squared + width squared = diagonal²

The tunnel in the photos is 14 x 40 feet. This is 4.2 x 12.2 metres

So:

> 4.22 + 12.22 = diagonal²

> 17.64 + 148.84 = diagonal²

> 166.48 = diagonal²

Therefore the diagonal for the tunnel is = √166.48 = 12.903 metres.

It is a wise precaution to check and double check your measurements, and remember to measure the diagonals both ways – they must be identical lengths. It is easy to make a mistake and any discrepancies at this stage will affect the quality of the rest of the job.

4. Once the exact position of the corner posts has been determined they are pounded into the ground. A sledge hammer is good for the job, but it should not be allowed to hit the metal posts directly as this will damage the rims and the hoops won’t be able to slot in. In the photo a rubber plug that fits over the post is being used. However, an old plank of wood held on top of the post does just as well.

The posts should be vertical – use a spirit level to ensure this.

If a post stops going into the ground when it is hit there may be a stone in the way. To remove the obstacle the post has to be pulled out (wobble it from side to side to loosen it). In most cases a stone can be dealt with by pounding a crowbar into the hole until the stone has broken up. Occasionally, this does not work and the only way to remove the stone is to dig it out. The hole must then be filled in and packed down as hard as possible with soil, then the post hammered in again.

5. Once the corner posts have been hammered into the ground, string should be attached to them marking out the perimeter of the tunnel.

6. The string must be attached to the top of the posts and pulled tight so there is no slack. It should be level throughout – use a spirit level, do not do it by eye. This means if there is a slope some posts might have to be knocked deeper into the ground than others.

It is possible to erect a tunnel on a slope provided it is even. In this case the corner posts should be hammered in all to the same depth, so that when the perimeter string is tied around it is not level, but rather runs parallel to the ground.

7. Once the string is in place lay the other posts along each side.

8. Using a measuring tape running the length of the tunnel, the posts can be pushed into the soil in exactly the correct place.

9. The posts are then pounded into the ground.

10. It is essential that the posts are all hammered in so that their rim is the same height as the string.

Erecting the hoops

This is the easiest stage, and is a bit like putting up a meccano structure. Provided the posts have been put in the right place and are vertical the whole structure will be assembled simply.

11. Depending on the model of the tunnel the hoops come in two or three parts. They are put together by slotting them into the posts. A nail should be placed through each hoop and bent over to prevent the hoops from sliding down into the post too far.

12. Hoops are placed in every post.

13. Then the cross pieces at the apex of the hoop have to be added. To reduce friction with the plastic make sure that the cross piece runs under the hoops rather than above them.

14 …. and the last piece!

Many crops in a tunnel, such as tomatoes and cucumbers, need to be supported. The easiest way to provide that support is to run wire along the length of the tunnel, and this is the stage to put the attachments in.

Also, to provide structural strength polytunnels need a side bar running between the last and penultimate hoop on both ends, and that has to be put in now.

15. Lift the hoops out of the base post and slide in the attachment for the wire and the T-connection for the supporting bar.

16. The attachment for the wire. Ideally there will be two attachments on both sides of both end hoops.

17. Put the supporting bar between the last and penultimate hoop on both sides and at both ends.

18. The support bar adds extra strength.

Preparing the doorframes

Tunnel doors are made out of a wooden frame. Tunnels should have a door on both ends as this improves ventilation and ultimately the quality of the crops that grow in it.

19. Tunnel doorframes can be made beforehand or put together in place. The frame should be high enough to allow an adult to walk through and as wide as possible given the width of the tunnel.

20. To make the doorframe, metal plates, which can be bought from any building supplier, are very useful.

21. To be sure that the door is placed in the centre of the tunnel a plumb line should be tied from the top of the centre of the end hoop.

22. String should be tied between the end posts, and the point that the plumb line touches is where the door will go.

23. Dig a hole for the doorframe legs to be buried….

24. and drop it in.

25. Fit doorframe so that it fits snuggly under the metal hoop.

26. Attach doorframe to the polytunnel hoop with metal strapping.

27. Then fill in the holes, firming the doorframe into position.

Preparing for the plastic

Before the plastic can be fitted over the polytunnel frame there are still a couple of jobs to do:

• Applying the insulating tape.

28. The metal hoops in a polytunnel tend to get very hot and over time will make the plastic on a polytunnel become very brittle. To prevent this happening an insulating tape should be stuck on the top of the hoops so the plastic and the metal hoops never touch.

29. Sticking hot spot tape to the tunnel to ensure the plastic never touches the metal hoops.

30. Up and over – every part of the tunnel hoops that might touch the plastic should have hot spot tape.

• Digging the ditch

A ditch has to be dug right around the tunnel frame. This is for burying the plastic sheet, thereby securing it firmly into the ground.

31. The ditch should run a few inches away from the posts. A 2x4inch plank of wood placed flush against the posts is ideal for acting as a guide. Using a flat bladed cutting spade the soil can then be cut along the edge of the wood.

32. The ditch should be about 8 inches wide. The spade should be used to cut the outer side of the ditch.

33. With the sides of the ditch already cut the soil can simply be cut out. The ditch should be about 8 inches deep.

34. Move the wooden plank as the ditch progresses. A neat, carefully dug ditch makes the burying process easy and helps in achieving a well-covered tunnel.

35. The ditch goes all around the polytunnel frame except where the doorframes are.

Cladding the polytunnel

• Lifting the plastic sheet over:

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 supple 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.

36. 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.

37. 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.

38. 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.

39. 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.

40. 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.

41. 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 secure the sheet further the spare plastic at the ends can be tucked in through the doorframe.

• Attaching the plastic sheet to the polytunnel frame:

42. 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.

43. 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.

44.The door at the other end is attached in the same way, but before being attached it 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.

45. 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.

46. The plastic can be attached to the doorframe by sandwiching it with wood strapping. Use several strips, and many nails.

47. 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 will not allow any slippage or tearing of the plastic.

48. 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.

49. 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.

50. When burying the plastic the first time 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.

51. 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.

52. 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.

53. 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.

54. The plastic must be pulled as hard as possible around the corner of the last hoop.

55. 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.

56. 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.

57. 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.

58. 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.

59. 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.

60. The door is opened by rolling the plastic up and hooking some string around it.

61. 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.

62. 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

63. 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.

64. The wire should be attached to the end hoops with attachments slipped onto the end hoops when the polytunnel was erected.

65. 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.

66. 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

© Joy Michaud