Burtown Kitchen Garden Diary – How to Build a Polytunnel
Given the relatively cool climate in the British Isles the only answer to Dermot's dilemma is to build a polytunnel but luckily he is an expert having built and grown in numerous tunnels over the years. Dermot helped me build my own tunnel which turned out to be a pretty simple process when you know how but a bit daunting if you don't. I asked Dermot a few questions on your behalf on how to build and lay out a tunnel which should explain the dark art pretty well.
I guess the first question is how do I know where to place a polytunnel? If you think about the types of growing you want to do in a tunnel it will be obvious that you want to place it in the sunniest spot possible. Tunnels allow us to grow sun loving crops like peppers, cucumbers and tomatoes in cooler climates and also allow us to grow earlier and later in the season, both activities need as much light as possible.
If you look at any old Victorian glasshouse you will always find it is built along the North-South axis. The idea is that both sides of the glasshouse will receive equal amounts of light so you don't have problems with plants shading each other. If you have to build your tunnel on an East-West axis the South side of the tunnel will get more sun than the North. You can help avoid shade problems by staggering you planting in a zig zag pattern rather than using rows. You can also plant taller crops on the North side of the tunnel and lower growing ones on the South which will also help.
Some low shelter around a tunnel will be an advantage but avoid a site near trees both for the shade they cast and they fact their roots will seek out the fertile soil of the tunnel and steal nutrients from your plants.
If you have an exposed site or a garden that funnels wind into your tunnel avoid facing the doors into the prevailing wind (especially if you only have a door on one end). If strong gusts are forced into a tunnel with nowhere to go it may take the plastic with them!
How do you mark out the tunnel site so you know it is square? A good method for finding a right angle is to use the 3, 4, 5 method, this will allow you to make sure your hoops are lined up correctly and the finished tunnel is nice and square. To use this method first mark your North-South axis (or whatever orientation you are using) with a string line and pegs, this line marks the side of the tunnel. Place a peg on the line 3 foot from the corner and place another peg 4 foot from the corner at right angles to your North-South axis.
Now put your measuring tape on peg 1 and measure the distance of the diagonal line to peg 3. Adjust peg 3 until your tape shows a distance of 5ft. If the sides of the triangle you have marked measure 3, 4 and 5ft you have a perfect right angle. You can then continue around your tunnel site using the same method to get all corners square.
How far about do you place the hoops and how are fixed in the ground? I wouldn't place the hoops any more than 2 meters apart (6'6") for strength, especially on an exposed site. There are different ways of fixing the poles in the ground but usually a short tube is set into the ground which the hoops either slot into or fit over. Foundation tubes are usually 2-4 foot long depending on the size of the tunnel. I would advise concreting the corner tubes in if you have a large tunnel, soft or boggy ground or if you are not burying the plastic to tension it. Once the ground tubes are in it is a relatively easy job to slot the hoops in and build the structure according the manufacturers instructions.
I would definitely include crop bars in the tunnel which are usually sold as an optional extra. These are the bars running from one side of the tunnel to the other which you see in the photo.
Crop bars add strength the tunnel but are also essential for supporting climber and cordon type plants. I run wires the length of the tunnel which are attached to the crop bars; I can then drop twine from my wires for my tomatoes and cucumbers to grow up. You can see the crop bars being used to create a temporary staging for chili plants above.
How do you attach the plastic to the tunnel? There are 2 different methods for fixing the plastic along the sides of the tunnel. You can fix the plastic to a rail running along the sides of the tunnel or the polythene can be buried in the ground. The burying method is a little more involved but I feel is worth it in the long run as it makes the finished tunnel more stable in strong winds and is also, conveniently, cheaper. The reason buried plastic gives better stability is because the tunnel is fixed not only by its poles but also all around the edge by the plastic which makes it impossible to flip over. To bury the plastic you will need 3 people; two to hold the plastic and one to stand in it. The method for covering the polytunnel is as follows:
Unwrap the polythene and lay out on the ground (avoiding any sharp objects) if it is a warm day. Putting the poly on warm is thought to give a tighter tunnel as it will stretch more; this takes into account expansion and contraction on hot and cold days. You are best to choose a calm day to cover your tunnel as even light winds can make handling a large sheet of polythene difficult.
Dig a trench 35cm deep and a spades depth around the tunnel. Leave at least a 10cm gap between the hoops and the trench to avoid them splaying outwards into the hole. Drape the polythene over the tunnel being careful not to pierce or crease it as you put it up; it is easier to pull the plastic up over the side of the tunnel rather than from gable to gable. Stretch the plastic along the spine of the tunnel by nailing to one end to the lintel and pulling tight on the other. If you roll a timber 2x1 timber batten into the end you are tightening it gives an easier grip, you can then nail the batten to the door frame once you have the cover tight.
Starting in the center of the tunnel and working towards the gables pull the plastic tight into the trench by having one team member stand in it while the others back fill the trench.
Once one side is secure you can get very good tension by repeating the process on the other side but this time have 2 team members holding the plastic while the other stands in it using his weight to pull it down into the trench. The weight of the team member will pull the polythene very tight over the hoops. This is difficult to explain without a diagram or video, we'll have to do one some day!
Once the sides of the tunnel are on tight the plastic is pulled around the gable and nailed to the door frame using timber battens, the excess is buried in the trench or cut off around the door.
What is the best way to layout the beds in the tunnel? The number one mistake I see is paths that are too wide. The space in the tunnel is prime real estate and probably the most expensive area in your garden; you need to use as much of the space as you can for growing. I like to say a polytunnel should be designed around plants, not people so keep your paths narrow, 1 foot is plenty. I hear a lot of people say the path should be wide enough for a wheelbarrow but I feel this is wasted growing space. I rarely bring a wheelbarrow into the tunnel anyway and if I do the skids are fine resting on the edges of the beds, you only really need enough space for the front wheel. I like to see the tunnel looking like a jungle chock full of plants, you should need a machete to hack your way through!
I make my beds 42 inches (just over a meter) wide for both inside and outside growing as they are easy to reach from both sides and they suit my row method. I use a row marker (which I will go into more detail on if Andrew allows me in a later post) to divide the beds into rows either 6, 12, 18, 24 or 48 inches apart.
You can see me using the adjustable row marker in Andrew's garden opposite, I can change the row widths by swapping the pegs in the holes and quickly mark up the beds. Andrew uses a wider bed than I normally would because he uses 'Growgrid' planting mats, my beds are made the same width as the row marker.
Do you add much to the soil when starting a fresh tunnel? Yes, I do tend to feed the soil when the tunnel is finished as tunnel crops like tomatoes, squash and cucumber are demanding plants and need high soil fertility.
The Burtown tunnel is 12 meters long by 6 meters wide so quite large and to get started I will add 2 tonne bags of Envirogrind compost to improve fertility and soil structure. I like Envirogrind in the tunnel as it doesn't bring in any weed seed and I have had consistently good results from it over the years. I would always add large amounts of compost like Envirogrind to add organic matter to the soil first as any other liquid or pelleted feed nourishes the plant but ads very little to the soil.
I also like to scatter seaweed meal as it adds micro nutrients and trace elements which help avoid any deficiencies in the crops later on. For this size tunnel I will be adding about 15kg of ground seaweed meal spread on the surface and raked in before planting.
I presume you will be using an irrigation system on a large tunnel like that? Yes, I find irrigation is very important in a tunnel as hand watering is very time consuming so rarely gets done properly. I don't like light frequent watering as it doesn't penetrate very deeply and can encourage problems with fungal disease.
I also prefer soil based dripper irrigation rather than sprinkler systems for the same reason as I want to avoid watering the plant leaves. Sprinkler systems also need a high water pressure to work properly while most garden taps are relatively low, worth remembering if you are torn between the two. They are also much more expensive and fiddly to get right so I would champion the dripper tubing which I call a 'low pressure, low maintenance and low cost' solution!
We have spoken before about irrigation before and you know I believe less is often better than more from the seedling stage through till harvest time. I like to make plants work for their water as it makes them grow more extensive root systems in the search for moisture. In peak season in a tunnel I would only turn the irrigation on twice a week but leave it running for 6-8 hours. This long watering period is crucial as it deep waters the soil and provides a reservoir of moisture to keep the plants irrigated for the days ahead.
I use the 'Fad Drip' brand piping which has mini valves set inside the pipe every 12 inches which seep water at 2 litres per hour and can work at a very low pressure. Pipes need to be placed a minimum of 2ft apart running up and down your beds. In the Burtown tunnel I have 6 lines in total, 2 in each bed. I would also recommend you put a tap on each line so you can water beds separately, this is good for accurate irrigation but is also handy if you have low water pressure in your tap as you only need to water one section at a time.
Thanks Dermot, we have some very comprehensive information there, I look forward to seeing the tunnel in full bloom! No problem, will keep you posted. If any readers need any advice on tunnel growing I am more than happy for you to pass on any queries.