“Polytunnels have a key role to play in the grow-your-own revolution, and a comprehensive guide such as this is long overdue.” —Simon McEwan, Country Smallholding magazine
The Polytunnel Handbook: Planning/Siting/Erecting/Using/Maintainingby Mark Gatter, Andy McKee
A polytunnel can be used as an affordable, low-carbon aid to growing your own food all year round, and this manual looks at all aspects of using a polytunnel, from planning your purchase to harvesting the rewards. It includes a step-by-step guide detailing how polytunnels are constructed and maintained. There are chapters on developing healthy soil and preventing
A polytunnel can be used as an affordable, low-carbon aid to growing your own food all year round, and this manual looks at all aspects of using a polytunnel, from planning your purchase to harvesting the rewards. It includes a step-by-step guide detailing how polytunnels are constructed and maintained. There are chapters on developing healthy soil and preventing pests, and a jargon-free guide to the range of often mystifying accessories that many tunnel retailers offer. For the DIY enthusiast, there is a full set of instructions for building a polytunnel from scratch, and the authors explain how to keep your polytunnel productive in every season.
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The Polytunnel Handbook
By Andy McKee, Mark Gatter
Green Books LtdCopyright © 2010 Andy McKee and Mark Gatter
All rights reserved.
Planning your Purchase
A step-by-step guide from first thoughts to ordering
Having made the decision that polytunnel ownership is for you, the temptation is to rush off and order one straight away. Catering for customers who do exactly that, there are some friendly-looking sellers around who keep everything looking very simple and offer you a choice of only a few different kits. Don't be fooled; this simplicity is for their benefit, not for yours. At best you will miss out on some of the choices that make polytunnels such a flexible asset: at worst you will end up with a completely unsuitable product.
On the other hand, approaching a polytunnel manufacturer directly can leave you bewildered by the range of products and accessories on offer: different films, different ventilation strategies and a choice of door options, to name but a few. In reality these things are only confusing if you are not clear about exactly what you need from your polytunnel, and that's why there are a few things to consider before you're completely ready to go ahead.
The very first thing you need to think about is whether anything is likely to prevent you from putting a tunnel where you would like to put it. You should contact your local council's planning department to find out if you need planning permission. Despite controversy over larger commercial applications, at the time of writing single-span tunnels for domestic use don't need permission in most areas, but the position varies depending on where you live; in Andy's stamping ground in West Dorset, for example, you don't need permission unless the structure is concreted into place or connected to mains services. We would advise anyone who is told that they need permission to ask for a copy of the relevant guidance that the planning officers follow, since there is often confusion between the rules for domestic use and those for commercial growers.
There is also a layer of more local bureaucracy to consider before you go any further. If the polytunnel is on your own property, this just means checking to see whether there are any restrictions written into your deeds. This is unlikely to be a hindrance, but if you are considering erecting the tunnel on an allotment, then there will probably be specific rules that you will have to follow. While you need to be aware of what these rules say, don't speak to the allotments manager until you have a better idea of just what you would like to do. Nothing is more likely to produce a negative reaction than vagueness on your part.
Decisions on use
Now it's time to ask yourself a very simple question, but one that often goes unasked; what exactly do you want to use your tunnel for? This question will strike at the very heart of your installation. All subsequent questions – where the tunnel will be sited, what size it should be, whether water will be needed – hinge on what you intend your polytunnel to do. For example, if you intend to use it to bring on hanging baskets of flowers, then you will need easy access to water, a decent potting bench, and perhaps some extra insulation and/or heat for very cold conditions. On the other hand, if you just want to shelter chickens in winter, then you need excellent ventilation and room outside one end to give the birds a netted run. Incidentally, polythene-covered tunnels are not suitable for livestock in summer; for that, you need a shade house (see p.14).
Make a list of all the uses you have in mind for your tunnel, and have a stab at listing the characteristics you think will be needed for each. At this point it is essential to read some of the later chapters first, particularly the sections on irrigation (pp.53-6) and ventilation (pp.58-9), and then come back to your list again. You might be surprised at what you find, and save yourself a lot of heartache into the bargain.
How big, and where?
Moving from the general to the highly specific, size and site questions have to be decided on together. There's no point in opting for a 48'-long tunnel if it means running it diagonally across the back garden, and likewise there's no mileage in identifying a nice spot hidden behind a hedge if it means getting a tunnel that's too narrow for the big beds you've got planned. So, with size and site in mind, here are a few things to think about.
Unheated polytunnels stay warm inside by trapping solar energy (the aptlynamed 'greenhouse effect'), so it's absolutely essential that the tunnel receives good light. This is particularly important in the morning to get the interior temperature up quickly after the cool of the night, so ensure that there is no dense shading directly to the east of the proposed site. If you want to make the most of the sun, then orient the ridge-pole of the tunnel as close to eastwest as you can, whereas if you don't want things to get too hot in summer you might swing more towards north-south, or place your tunnel a little way to the north of a deciduous tree which can provide some shade in summer only. However, don't site your tunnel beneath overhanging branches, or you run the risk of leaves and other debris falling on to the film.
Take a good look at any polytunnels in your area; if they all face in the same direction then it may be to provide some protection from strong prevailing winds. Tunnels will resist wind better if it strikes them broadside on, even though the profile offered to the wind is larger, since the most vulnerable part of the structure is around the doors. Polytunnels are immensely strong if they are installed correctly, but never underestimate the power of a winter storm. Try to make sure that your site is sheltered from the prevailing winds, and if that means planting a windbreak, you might wish to consider this now, while you are still at the planning stage.
Although polytunnels can be mounted on gentle slopes, there are drawbacks to doing this. A gradient running end-to-end can be coped with, although the doors will have to open onto the downhill side unless you are prepared to do some digging to accommodate them. Slopes running from side to side, however, cause more serious problems since each pair of ground tubes which anchor the structure to the ground have to be exactly level. For best results and least work, the site should be as level as possible. Don't underestimate the work involved in levelling even a fairly small area by hand; if possible, work with what you've got.
"My tunnel is on a slight end-to-end slope and it's not a problem – in fact, given the heavy clay soil here, it's an advantage. If the tunnel was on level ground, there'd be nowhere for runoff to go."
Another factor to consider is the fertility of the site's soil (although raised beds are a possibility if the soil is poor) and how well it drains, since the runoff to either side in a downpour is very considerable. Hollows and patches that are already damp are a poor idea. Finally, beware of putting the tunnel in a frost pocket – any spot in which frost lingers on a cold morning is to be avoided.
When you are considering what size the 'footprint' of the tunnel is, allow at least 1m (3') spare on all sides. In this way, a 4.25m x 5.5m (14' x 18') tunnel ideally needs a footprint of 6m x 7.25m (20' x 24'). Not only will you need this space when you put the film on the frame, but you will need to keep it clear of tall growth if you are to avoid damage and low-level shading.
At the very least, it's essential to have a standpipe or substantial water storage near your polytunnel (see 'Water in the Tunnel' on pp.52-8). Even with the best water management, the bigger the tunnel the more water it will use – so make things easy on yourself. Remember that if this means laying a water-pipe to supply a new tap you will need to bury it deeply enough to avoid frost damage. Your water company will be able to offer you advice on how deep this should be, but this advice may not be terribly practical. As an alternative, ask some local gardeners, or else drain the system for the winter. The closer you can site your tunnel to an existing water supply the better.
By the same token, running an electricity supply to your tunnel does not come cheap. However, relatively few things in the tunnel are likely to need power, and most needs can be met by using battery or solar power instead. Adding mains power may also require planning permission, so think carefully about whether you really need electricity.
Thanks to some well-publicised opposition to large-scale agricultural operations, the appearance of polytunnels is sometimes the source of contention. The visual impact can often be softened with screening plants, but take the time to consider where the tunnel could be placed to minimize its impact. Do remember that your neighbours may be looking at the structure too, and might well appreciate your taking the time to look at the plot from their side of the fence. Another visibility factor to consider is security; although it's not something anyone likes to think about, an incon-spicuous tunnel is less likely to draw attention from thieves and vandals.
In permaculture terms, a polytunnel falls into Zone One. In simple terms, this means that it needs frequent visits during the day; even if you do nothing else, you will need to open the doors in the morning, close them at night, and pop in once or twice to check on plants or do a spot of watering. There are other options for watering if your polytunnel is not on the same site as your home, but it should normally be placed as close to the house as other factors allow. At the very least there should be a good path to get you there and back without any spills on soggy winter mornings.
Convenient access to your tunnel is a big plus – especially when you're in the middle of cooking dinner and remember that you should have picked some basil earlier. For more information on zoning, see Permaculture Two by Bill Mollison (detailed in Appendix 2: Further Reading).
The tunnel's own effects
Do not forget that the tunnel itself will affect the microclimate around it. To the north side there will be some shading, particularly in winter; to the south there will be some extra reflected light which may benefit sun-loving plants. The tunnel will act as a partial windbreak on the leeward side, but putting it near other structures may well result in a funnelling effect as the wind is forced between them – and this is something to ignore at your
Probably the most significant effect, however, will be that the ground to both sides of the tunnel will become damper than before due to run-off. Depending on the slope and soil type of your site this may become a problem during wet weather, in which case you may need to take steps to improve drainage, such as laying a section of land drain to carry the water away. As an alternative, use the run-off to your advantage by attaching flexible gutters to the cover so that some of the rainfall can be stored for future use (this is detailed on p.24).
Before going on to make a final decision about the size of your tunnel, you need to consider what sort of structure to buy.
A basic polytunnel is a simple single-span structure made from hoops supporting a polythene film. This creates a bubble of still air which is quickly warmed by solar energy, creating a warm micro-climate; basically, it is a walk-in cloche. Compared with a greenhouse it is more difficult to keep frost-free in winter and needs additional insulation if it is to be heated. Due to diffusion of light by the film, propagation is slower than in a greenhouse in the spring but by the same token leaf scorch is less of a problem in summer.
Polytunnels come in a wide range of widths and styles, and there are many additional options to choose from, making them extremely versatile for general use. Older models have curved sides, leading to a certain amount of wasted space, and unless your tunnel is very wide it is well worth paying a little extra for straighter side sections. Polytunnel covers typically last five years before becoming brittle, with light transmission falling a little each year. They are easy to replace and recycle, however, and cost only a fraction of the original tunnel price.
By using a separate sheet of polythene for each gable end, flat-ended tunnels such as the 'Harlow' avoid the need to pleat the cover around the door frame and are fitted with sliding doors, rather than hinged ones. Gable ends of this type are only available in widths up to 3m, and while they can be covered in tinted polythene and provide an overall appearance which is neater than that of a regular tunnel, they are more expensive and confer few practical advantages.
Shade houses (also known as airflow houses) are a variation of the polytunnel where the polythene cover is replaced by shade netting. This cuts down sunlight by around 50% and allows gentle airflow through the tunnel at all times. Sometimes a high side rail is used to allow a section of polythene to act as a 'roof' on top, giving some shelter in wet conditions. Shade houses are used for some specific applications including hardening off championship vegetables, providing shelter for livestock, and for growing shade-loving or temperature-sensitive plants such as chrysanthemums or orchids. Since only the covers are different, costs are similar to those of polytunnels.
A more sophisticated version of the polytunnel is the solar tunnel, which in terms of price and performance falls in the middle between standard polytunnels and the much more expensive Keder house. Solar tunnels have covers made from PVC with an embedded green mesh, giving a softer appearance and a longer life than a polytunnel cover – typically seven years to the polytunnel's five. The covers are supplied in sections with welded loops through which the tunnel-frame sections are passed, making them fit tightly and giving an easier installation than a standard tunnel.
This modular design confers one big advantage, which is that an additional section can be added at a later date without re-covering the whole tunnel. Solar tunnels are held down by screw-in anchor bolts, so it is possible to unscrew them from the ground and carry the whole thing to another site. However, on windy sites it is advisable to concrete the bolts into the ground, in which case the advantage of the screw-in anchor bolts is lost. The covers are easy to repair should a rip happen, and insulate slightly better than a plain PVC film. Solar tunnels are straight-sided and available in widths of 3m (9' 10") and 4.2m (13' 9"). There are also wider models with curved sides.
Keder houses are the Rolls-Royce of polytunnels, and are usually professionally assembled. Fabricated from a sandwich of bubbles between rigid plastic sheets, the structures will support several feet of snow without cracking (they are said to be strong enough to jump up and down on). They are guaranteed for ten years, and typically last about fifteen. Keder houses combine the advantages of both greenhouse and polytunnel, being straight-sided, very well insulated and practically airtight when closed – a real boon on freezing winter nights. They are available in widths of 2m (6' 6"), 3m (9' 10") and 4m (13'), the last size being a more serious piece of work with guttering and sidewall ventilation as standard. The most serious drawback is price of course, as they cost around five times as much as a typical polytunnel.
The biggest single advantage that the more sophisticated structures offer you is improved insulation, but this comes at a price. It is, however, possible to further insulate and heat areas within a standard polytunnel, so unless insulation comes high on your list of priorities or unless cost is not a major concern, it may well be that an ordinary polytunnel will be your best option. This is a decision you should take now because it affects the range of widths available to you, which in turn determines your internal layout. Once again, it is worth remembering that unless you choose a straight-sided option (offered by all manufacturers but not by all retailers) there will be a certain amount of space at the sides that is less useful because of the lack of clearance, the curvature of the cover leaving room for only quite low-growing plants.
Excerpted from The Polytunnel Handbook by Andy McKee, Mark Gatter. Copyright © 2010 Andy McKee and Mark Gatter. Excerpted by permission of Green Books Ltd.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Meet the Author
Mark Gatter began growing vegetables while homesteading in north California in the early 1980s, and has been a gardener ever since. He relies on a polytunnel to keep fresh food on the table through the winter. Andy McKee uses his polytunnel to keep his family entirely self-sufficient in vegetables. They are the authors of How to Grow Food in Your Polytunnel.
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