By David Milton

Inside the polytunnel. Photo: David Milton

Not many of us live next door to a constant source of locally grown, organically produced, fresh fruit and vegetables. The solution is to grow as much as possible of your own food but the further north and the greater the altitude of your garden, the more difficult it becomes. You want the biggest range of fruit and vegetable types, continuity of production and food transport measured in yards rather than miles. The winter months are the ideal time to consider investing in a polytunnel to help meet those requirements.

Investment is the right word because a polytunnel will cost you money and its potential cannot be realised without time and effort. However, it can totally transform what, when and how much you can grow. You will soon wonder how you managed without one. Your homework should include visiting a few polytunnels in other local gardens to pick up some do and don’t tips; I am always happy to show strangers around ours and I’m sure you will receive a welcome elsewhere if you express an interest. Polytunnels are all basically the same but you will need to consider size and especially ventilation. Even here in the Scottish Highlands the temperature inside can reach 40°C with all doors open and that is when plants begin to suffer. Models with side ventilation panels would prevent that excessive heat build-up.

Wider range, longer season

With your new polytunnel you can consider extending the range of fruit and vegetables that you can produce.

Plants that are almost impossible to grow successfully outside in the garden.

Most of these will be annuals and among them will be pumpkins, melons, peppers, aubergines, cucumbers and tomatoes. Even if you can sometimes get a crop from them outside, they will perform better inside and regularly repay your efforts. It could be worth sparing permanent space for perennial crops such as asparagus and globe artichokes that perhaps would fail outside; the former grows very well in a polytunnel and you will end up exploring more recipes for cooking with it.

Plants that will grow faster and be healthier and more productive inside than out.

These include onions grown from seed, beetroot, celeriac, sweetcorn and strawberries. Onions that are grown this way store over winter much more reliably than those grown from sets, which can carry neckrot fungus spores.

A polytunnel enables you to not only extend the range of fruit and vegetables you can grow, but also to extend their season. The plants grow more quickly and will mature in a shorter time so you will, for example, be eating French beans long before the outdoor plants have even thought of flowering. It’s easy to achieve earlier crops but a little trickier to extend the season into autumn. You need to remember to make later sowings and hope there is still time for the plants to catch up and mature. I use this technique with climbing French beans and runner beans and yes, you can grow the latter in a polytunnel especially if you can have doors open to allow bumble-bee pollinators to enter. The other type of season extension is through successional sowings and this applies to all the quick-growing salad crops such as rocket, lettuce, mixed salad leaves and so on. You should find that you can make these later sowings because you have freed up ground space during the summer by harvesting early potatoes and shallots.

Keep it fed and watered!

Having extolled the virtues of the polytunnel, I must now point out a few pitfalls. When it rains outside it does not rain inside and it will be your job to provide the plants with their water. If you are connected to the mains water supply the easy option is soakahose laid throughout the polytunnel, with the flow rate adjusted to keep the soil moist but not flooded. If (like me) you have no mains connection, you’ll probably collect all your water from rainwater run-off from buildings, store it in several water-butts and transfer it by bucket and watering can. I have more than ten water-butts and many buckets! You need to work out a watering regime that suits you. Daily watering by hand does take time but puts you in a good position to spot anything that’s going wrong or needs attention.

You will also need to consider food for the plants, which will be growing and producing crops at a far greater level than those outside. In the winter I dig in a large amount of organic material to replenish the soil, so you could need several compost heaps to supply that material. If you can set up a water-butt of comfrey liquid it can be used to benefit the summer cropping of tomatoes, peppers and so on.

Remember that the plants will grow faster and bigger than they would outside. This makes a difference to the way you organise the plants. Courgette plants in a polytunnel will become huge, so allow plenty of space and do not have too many. Bush tomatoes will take up more space than vertically trained varieties so are not as productive overall. Some plants find conditions so pleasant that they just grow and grow and forget why they were put there: my summer cauliflowers grew to about five feet high with no sign of a flowering head and now they are excluded. This rampant growth might have to be controlled by you but you can turn this to your advantage by, for example, training pumpkins to a single vertical shoot and cutting off all side shoots. That way they do not spread sideways and suffocate other plants and they quickly get the message about flowering and fruiting. You should stop picking asparagus at the beginning of June, rather than later in the month, to leave enough aerial growth to build up underground stores for next year’s shoots.

Conditions that are good for the plants could also be good for pests and diseases, so be vigilant and ready to nip such problems in the bud. The good news, though, is that your polytunnel plants should be growing so fast and be so healthy that they are better able to cope with such problems than their cousins outside battling against wind, hail, drought, rabbits and pigeons! Meanwhile you can continue gardening even when it rains and be proud of all the fresh organic food you put on the kitchen table. Your investment will be worth it.

Erecting your polytunnel

A polytunnel at Joe and Kate’s Manchester allotment. Photo: Peter White

If you have decided to go ahead with a polytunnel you have probably already considered the eco-worthiness of a plastic-based structure. I would argue that over the years you will produce so many fruit and vegetables that you will offset the carbon cost of a similar amount purchased conventionally, with its accompanying production, harvesting, packing, distribution, refrigeration and transport burden. The more you grow, the more you save in that respect and this is in addition to the personal benefits to you of readily available fresh organic produce.

The lifespan of the plastic membrane is many years and I have found that the manufacturer’s literature seems to err on the conservative side. The first cover on my polytunnel remained perfect until it was nine years old, and only needed to be replaced after the entire structure collapsed under the weight of a metre of snow in December 2009. The actual membrane did not rip, tear or puncture during the collapse therefore it could normally be expected to last at least ten years. Collapse under snow accumulation is not normally a problem. You must have the membrane really tight upon erection and expect to wash off dirt, stuck vegetation and any algal film from the inside in early spring to maximise light penetration. During use the membrane will survive accidental damage by forks, penknives and so on.

The erection process is in three stages. Firstly prepare the site, which must be level and larger than the actual polytunnel dimensions to allow for outward-opening doors at both ends and for the membrane-burying border along each side. Do your best to clean and prepare the soil, especially to remove stones and perennial weeds. The rectangular shape is marked out and this must be accurate because any error will be compounded when the structure progresses. The ground poles can simply be driven into the soil but everyone I know has elected to dig holes and use a bit of concrete to bed in each pole. The second stage is like grown-up Meccano, involving building the tubular metal framework using a few simple tools and a stepladder; you need two people. Finally you wait for a warm day to put on the membrane – if you put it on when it’s cold the membrane will stretch later when warm, become loose, then flap and tear off in a strong wind (I’ve seen it happen). I have erected two polytunnels that were 54′ long and needed four people for the final membrane installation so I assume that two people could manage a small- to medium-sized version. If you have already inspected an existing structure then the instructions should make sense. You will probably find the pleating at the door ends the trickiest procedure. Incidentally, the manufactured doors can be expensive. Your own DIY version could be a sensible way of saving some money without jeopardising structural integrity.

Enjoy your polytunnel and enjoy even more all the vegan organic additions to your diet.

This article appeared in Growing Green International magazine Num 26 (Winter 2010/11), p30.