Growing Crops in Polytunnels: The Pros, Cons & Other Considerations
Growing Crops in Polytunnels: Tunnels Are Lower Cost Than Greenhouses & Have Many Advantages Including Many Crops, Such as Salad Crops Love Them.
In this post I’m briefly looking at the advantages and disadvantages of tunnels over other growing methods and locations, what you should consider before ordering or buying a polytunnel, what crops are best grown in them and how growing in them differs from in greenhouses and outside. It’s a huge topic so please post questions to me and I’ll endeavour to add extra information as required.
I spent nine years in my earlier life owning a market garden where we had a dozen or more tunnels ranging from 120 ft X 14ft walk-in tunnels to the much bigger 120ft X18ft drive in tunnels. The difference being that the 14 ft tunnels were rotovated by hand whilst the 18 ft tunnels, being much taller, were cultivated with a tractor.
All my tunnels had overhead watering systems and wide access doors.
Today I’ve a small garden and use mini tunnels like the one in the image
Crops Suitable for Polytunnels
Polytunnels often have lots of headroom so are ideal for cordon crops such as tomatoes and cucumbers. Peppers and aubergines can also be grown up strings if you wish, there is a tradeoff between space requirement and extra work.
Plastic also raises humidity so is ideal for crops like celery which loves it.
However, most crops can be grown in tunnels. But I’d advise thinking carefully about space considerations eg potatoes and sprouts etc.need a lot of space and can be grown very well outside. Plus crops that need insect pollination are more challenging due to the problem of insects getting access. Remember some crops are self-fertile and only need movement to pollinate. eg tomatoes and peppers.
What to Consider When Buying or Building a Polytunnel
Firstly think about the size of the tunnel you want. The standard 14 or 18 ft is ideal in many situations and can be as long or as short as you wish. Normally, due to the construction method, the length will be in 6 ft multiples.
However, don’t forget that on a smaller scale there is no reason why low tunnels can’t be used. I’ve recently built one in my garden. It’s 9ft by 4ft. And being 4 ft high I have to lift it to get “inside”. I do this as if it were hinged at one end and just prop the other end up. It works perfectly well and is ideal for low growing crops such as celery, lettuce, aubergine, peppers etc. See the image above.
Ideally, a tunnel is built on level ground and can either run to catch the maximum light. But if you land slopes then orientating it up and down the hill makes working it much easier AND provides natural ventilation.
Polytunnels Access and Doorways
Access doorways in smaller tunnels are usually at the ends, though it’s possible to have side access doors in larger commercial tunnels. The size depends on your preference, though personally I can see no point in making them too small and go for full-width doorways. In most cases the doors are hinged to a wooden frame and open outwards (therefore not taking up growing space in the doorways). However, commercially I used to use a sheet on plastic on battens that fitted the end of the tunnel like a blind. It was held in place by hooks and pins (six-inch nails dropped into a hole in the frame), and just removed when I wanted access on foot or with a tractor.
Irrigation in Polytunnels
In small tunnels a watering can is fine. but as the area covered by tunnels expands you might want to think about easier solutions. Overhead irrigation is easy to fit and use. It usually consists of PVC pipes that can be glued into long lengths and have a series of spaced nozzles facing upwards (to prevent dripping when turned off). Connect to a water source, open the tap and a fine mist of water can provide all the water needed. It’s ideal for lettuce, celery love it (they are a marsh plant), cucumbers love the humidity it creates in a tunnel but we need to be more careful with crops such as tomatoes.
Tomatoes are prone to blight (they are solanum, the same species as potatoes and can get blight which is spread by water splashes and loves high humidity and rain). So its best here to use drip irrigation. The simplest are perforated rolls of plastic tubing or seep hose but I find they are ineffective over any distance as the pressure drops very quickly and one end of the row tends to get too much water whilst the other end gets none.
I prefer the nozzle type drip hose. These drip a measured amount of water an hour out of each nozzle, drop by drop. The ones I use drip at a rate of 4 pints an hour so you know exactly what quantity of water is being added. Beware putting too much water on tomatoes as they just grow upwards and yield decreases .. but underwater and they lose yield as well – I’ll write more about this in another post.
Polythene Type Choices for Polytunnels
When I grew commercially the choices were few. Tunnel “liners” didn’t last long, three seasons was regarded as good before the wind ripped them. The problem was that they degraded in the sun. today we have more inhibitors included in the film and they last many years. Though they cost a bit more its well worth it. So select based on weight, I prefer 600micron but other weights are available. Also consider the inhibitors available. There are many specialist Polytunnel companies online and all stock various plastic sheeting.
Snow Loading on Tunnels
Snow can be a problem on tunnels. I’ve seen tunnels collapse under a heavy snow loading (though fortunately never suffered this myself). Snow is good in small quantities as it insulates the tunnel and the crops are better for it. Provided it doesn’t cut out the light for many weeks. But a lack of light for days, or even a few weeks, does little harm. After all, if the grass on your lawn survives it why can’t other crops?
To remove snow you can use a broom to gently shake the plastic from the inside. Unless it is frozen solid it will slide off the plastic quite easily.
Polytunnel Tube Type
Tunnels are usually constructed of metal tubes bent into an arch. These can be sectional with joiners at the top or, in the case of smaller tunnels, can be a single piece of tube that spans the full width. My first amateur tunnel was homemade from 19mm water pipe bent over a framework to bend it. The problem with this is that the curve is rarely perfect, it’s not a problem on a small scale but not ideal on a larger scale.
Aluminium is the normal material used in commercial and larger amateur tunnel construction. It’s relatively light and strong. Alternatively, for smaller, cloche like tunnels its possible to use alkathene piping. It doesn’t need pre-bending, you just bend it into shape by hand as its needed and it springs back when released after use.
Tunnel hoops are secured in place with ground anchors. Ie steel tubes are driven into the ground into which the hoops sit. To prevent them slipping too far, and to tension the liner when in place, a series of holes are drilled through which pins (more large nails) are used to secure it. Erecting a Polytunnel
Growing Crops in Polytunnels: How to Erect a Polytunnel
- Prepare the site by removing weeds, rubbish etc. You could erect on a lawned area and follow the advice on converting from lawn to growing area contained in our previous post
- Mark out size ensuring it is square/rectangular and not trapezoidal
- Install ground anchors
- Install framework and door frames
- Pull sheet over the frame (preferably on a warm still day), pull tight and batten to the frame or dig in around edges
- Install doors
- Install irrigation, if being used
Greenhouse or Polytunnel: Which is Best? Advantages and Disadvantages of Glass v Polytunnels
Compared with glass tunnels have the following advantages
- Polytunnels are lower cost per square metre
- Polytunnels are quicker to erect than glasshouses
- They are relatively easy to construct
- Polytunnels are very flexible when it comes to size
- Polytunnels provide good light levels
- Diffused light due to nature of polythene
- Door sizes are flexible
- Overheads and layflat irrigation can be easily added
- Good protection from wind, snow, rain etc.
- Side and end ventilation is easy to fit
- Doors can be sealed or mesh added relatively easily
- Transportable, tunnels are relatively easy to move to a new site
- Fewer draughts .. (but need to ventilate more carefully because of this)
- Wide span is possible. Up to 30ft wide tunnels are available for purchase. Most glasshouses have narrower bays due to weight loadings of glass
Disadvantages of polytunnels compared with glasshouses
- Tunnels are often colder inside than outside during a frost
- Harder to shade than glass eg with spray-on shade treatments
- Are more storm-prone in extreme weather .. especially the plastic blowing away
- Additional periodic cost and labour of plastic replacement
Growing Crops in Polytunnels: Your Questions
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