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Growing Under Cover: an introduction

Being able to grow under cover is crucial in any plot – but it is far more important for homesteaders than for hobby gardeners, who are mainly concerned with being able to grow warmer climate crops in summer. Of course it’s nice to be able to produce reliable crops of melons and tomatoes, but if you depend on your own fruit and vegetables all year round there are more important advantages; a greater variety of fresh produce through the winter, less need to rely on stored produce, and a longer growing season. Under cover you can sow a full month earlier in spring than you could outside, and crop four to six weeks later in autumn.

peppers in polytunnel, imageWalking into a polytunnel on a sunny day in March is like walking into a bubble of summer. Quiet and warm on even the coldest and rainiest day, it makes the perfect spot to retreat to with a gardening book when the drizzle gets you down. The plants outside may be battered by squalls and wilted by frost, but inside the tunnel the growing season is well under way. The new potatoes, planted in February, are already up – and there are carrots, radishes, lettuce, spinach, rocket, mizuna and many others just waiting to be picked.”

—(The Polytunnel Handbook, A McKee & M Gatter, 2009)

Polytunnels vs greenhouses

To a certain extent you can use cloches (low tunnels) and fleece (floating row cover), but for the full benefit of the under cover environment you need a larger structure – a greenhouse or a polytunnel (high tunnel). So which to choose? The deciding factor is usually cost. Tiny mass-produced greenhouses may be very reasonably priced, but larger ones soon become cripplingly expensive. Polytunnels, on the other hand, get less expensive per unit area as they get larger – and once you begin to realise the true benefits of growing under cover, you’re going to want all the space you can get.

Glass structures offer around 10% better light transmission than polytunnel film. This matters most when it comes to bringing on seedlings in early spring, and so polytunnel owners often have cold frames or a small greenhouse to take full advantage of the light until the sun strengthens a couple of weeks later. However, in the summer direct sunlight can cause plants inside greenhouses to scorch and shrivel, making it necessary to provide shade for much of the day using shade netting or deciduous planting. In the polytunnel, however, the cover acts to diffuse incoming light so that plants grow more evenly and do not suffer scorch.

Glass structures do insulate somewhat better than polytunnels, but it is important to note that because polytunnels are apt to be larger, they take longer to cool down than a similarly-priced greenhouse – particularly if you choose a cover option that retains infra-red radiation better. The superior insulation properties of glass are only important if you intend to heat your greenhouse. Ventilation is often said to be easier with glasshouses since they can have roof vents, but modern polytunnels offer a range of size-appropriate ventilation options; provided you buy from a reputable manufacturer, ventilation should not be a problem.

The final consideration over your choice of structure is its environmental impact. For many people glass is an instinctive choice because of its longevity – whereas with a polytunnel the plastic cover has to be replaced periodically (typically every six years or so). Glass, however, requires a large amount of energy to manufacture – at least ten times that of the same area of polythene. Unless you realistically expect a modern greenhouse to last for sixty years, polythene is the greener option. Polythene covers are also easily recycled at their end of life, being made of the same material as plastic bags.

Our books

the polytunnel handbook and how to grow vegetables in your polytunnel, image

If you’d like to find out more about polytunnels, The Polytunnel Handbook gives a comprehensive guide to planning, erecting, using and maintaining them. If you already have a polytunnel, or are just about to launch into your first growing season with a new tunnel, How to Grow Food in Your Polytunnel will tell you all about what to plant, when and how to plant it, and when you can expect to begin your harvest – plus loads of hints, tips and resources. Both books are distributed in the USA by Chelsea Green and are also available on Kindle. An Italian language version is currently in production.

Other ‘introduction to polytunnels’ articles

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