Page 18 - ALG Issue 4 2017

Expert Guide to...
Polytunnels
One thing we can rely on with the weather here in the UK is that it is completely unreliable. Winter, spring, summer and autumn can all be early, late, long, short, cold, hot, wet, dry. We can also add to the list: cold drying winds, lashing gales, heavy rain, little rain, snow and of course the gardeners’ curse - frost. Frost can arrive in August and hang around as late as June. It all depends in which particular part of the country you happen to garden.
One way to overcome these little ‘inconsistencies’ of our
weather is to invest in a polytunnel. Polytunnels are much more versatile than glazed greenhouses, creating far more plant-friendly growing conditions. Glass may seem to have the advantage in light transmission and overnight heat retention but the direct glare of the sun shining through glass on to plants can damage plant tissue, and a damaging hard frost can cut straight through glass. A warm sunny winter or spring day can quickly create a desert-like atmosphere
in a glasshouse. It quickly becomes too dry and hostile for plants
to survive and often they can suffer beyond the point of recovery. I always remember one spring day early in my gardening career, being told to go and attend to a greenhouse by the Head Gardener who said, “I just went into greenhouse number ve, laddie, and a camel rushed past me to get out!” Enough said, point taken. I was outside watering the frames, enjoying the blue sky and warm spring sunshine but the plants under glass were screaming out for air and water.
The modern generation of polytunnels are much more sophisticated structures than their predecessors: plastic sheets stretched over a frame that were no more than walk-in tunnel cloches. A few minutes spent looking through the catalogues of polytunnel suppliers is an education in just how smart today’s plastics are in terms of light transmission, heat retention and how they protect plants by removing the damaging rays of the sun and, most surprisingly, with pest and disease control. The best quality polytunnel covers have a life of around 5 years but I have personal experience of them lasting double that.
Advantages of investing in a polytunnel
• Protection - Polytunnels will protect the overwintering crops from the worst of the bad winter weather. It will shelter them from damaging winds, heavy rain and sodden soil. It guarantees a harvest. You can safely bring a selection of healthy, fresh vegetables through the winter.
• Extending the season – Apart from the obvious of crop protection, another major bene t of a polytunnel is to increase the length of the overall growing season by allowing earlier sowing/transplanting times and a longer growing season with later harvesting dates. With appropriate cultivar selection it should be possible to grow crops all year round.
• Hardy crops – The excellent light transmission of the modern skins produces plants with thicker healthier growth, including the root system. This makes the plants better able to survive the shock of transplanting.
• Pest and disease control – Research has demonstrated how crops, pests, diseases and pesticides respond to ultraviolet – UV light. The modern polytunnel skins create the optimum growing environment by managing the light transmission, diffusion and temperature. Condensation dripping on to growing crops used to be a major problem. Modern plastics reduce larger, heavier drops into smaller, lighter droplets that cling to the skin until a sharp tap on the framework causes them to run down, draining into the side border. This reduces the risk to plants suffering from dry and powdery mildew and improves the admission of light to the growing crop.
• Temperature control – As well as being able to manage the UV light the newer generation of diffused skins also offer an element of control over the day and night temperatures of the polytunnel.
• Excellent crop quality – The nal result of all this technology is to produce the healthiest crops that are
packed with anthocyanin and better keeping life.
Pick a sheltered, open, level site well away
from overhanging trees or in the shade of
other buildings. Take care to avoid a windy
position and frost trap. Align the tunnel along
an east – west axis. In winter the sun is low in
the sky and most of the available light comes
from the southern direction, even when the
sun isn’t shining. More light will be able to
enter the tunnel/greenhouse in winter if one entire side is facing
to the south. Also, the light intensity will be more evenly spread throughout the length of the tunnel which is ideal for growing winter crops. When the tunnel is sited running north – south just one end of it will be facing south. The light levels inside will be low and mainly near the southern end.
Buy a polytunnel with straight sides. This will allow you to cultivate right up to the edges of the structure. Fit double or a single wide door at either end of the tunnel to allow a ow of fresh air if required. Fit lifting mesh side ventilation along the sides to help with ventilation.
You may need to improve the quality of the soil inside the tunnel by adding organic matter or importing clean, healthy top-soil. You can sow directly into the soil or better still produce your own plant plugs to transplant. This will give you a quicker turn round between crops at the same time increasing the overall weight of harvest. Cultivate
the crops exactly as you would outdoors but always avoid leaving water on the foliage of plants over night to prevent mildew developing. Be careful with bamboo canes inside; it is so easy to pierce the skin. But, if you do, you can purchase a tunnel repair tape which is like a giant roll of Sellotape. Make the repair as soon as possible before a small hole becomes a apping sheet.
Polytunnels make an excellent choice on the allotment because they are practical and ef cient to use. They are pretty low maintenance when compared to a traditional greenhouse. They don’t require painting, replacing rotten wood and are very strong storm resistant structures unlike an aluminium greenhouse. And most importantly they don’t use dangerous glass. Glass becomes brittle with age breaking more easily and becoming impossible to cut. Shards of glass in the soil are a danger to gardeners, their families, friends and pets.
Don’t think of a polytunnel as a plastic greenhouse. It is something completely different. It is a walk-in structure that allows you to garden standing up and in a Mediterranean climate in the depths of winter. Not bad!
Mike Thurlow
Take care to avoid a windy position and frost trap.
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