Polytunnels and The Greenhouse Effect

With its standard apex roof and rectangular shape, my catalogue ordered greenhouse has all the aerodynamic profile of a Sherman tank. This certainly isn't helpful in the early year gales which have been causing chaos round these parts of late with120 km per hour plus blows..

But unlike said Sherman (which weights 34 tonnes and is made with three inch thick steel plates), my greenhouse comprises a light aluminium frame filled with clipped in sheets of flexible feather light plastic polycarbonate sheets. My greenhouse combo thus has all the spine of a Rolf Harris wobble board and a kite enthusiast would struggle to come up with a more suitably airborne design.

When the really big gales blow, my greenhouse shivers and shimmers to a blur. And just like said octogenarian Aussie’s preposterous musical instrument, the polycarbonate sheets also emit a disconcerting “woppa, woppa” noise. But like a WWII doodlebug, it’s when the noise stops that it's time to panic. The lack of sound means the wind has gotten inside, and popped out a panel. When this happens I have to leg it outside as fast as I can with a light hammer, a bone spanner and a gun containing heavy duty silicon sealant before a vortex builds inside and sends the rest of the sheets flying around the garden.

Woppa, Woppa .... Don't buy a catalogue sold polycarbon and  aluminium kite

Having spent a year trying to come up with varying ways to hold it together, I finally seem to have cracked it thanks to a combination of super strength industrial silicon glue that the shop guy said could hang a man from a helicopter - this I've fed into every joint. The whole lot is reinforced with strips of heavy duty waterproof duct tape which has gone on around the joins. Now it still goes woppa woppa and it trembles to a visual haze, but it holds firm.

So don't buy a light aluminium greenhouse with polycarbonate sheets, especially ones with rubbish "clip on" panel systems. Before I finally sorted it out I'd spent much of January last year chasing errant polycarbonated sprites around my garden. 

But if my greenhouse happened to be glass paneled like those owned by associates and relations of mine, then I would have certainly had different problems. The glaziers would have had to have been called around at least twice in the last year thanks to heavy snow - we’ve had two serious doses in the last fourteen months. With both glass and polycarbonate you have to spend blizzards raking snow off the roof - the only difference with poly is that the sheets don't break when they do fall out.

If I were back again I would have ditched the greenhouse idea altogether and bought a polytunnel instead.

Polytunnels are a relatively recent experience for hobby growers in Ireland. Until about five years ago they were both too expensive and impractical to be used for anything but commercial growing.  We first saw them first with a hobbyist on telly thanks to the inimitable Hugh Fearnley Whittingstall - seen naked and using his as a sauna in his River Cottage tv series - and more recently our own Richard Corrigan (clothed) - employing one on his popular City Farm tv series.

But now, after a year of weather hell, we greenhousers are starting to notice that the polytunnel pioneers still have their clingfilm tents intact. 

Allen Garrard of Polytunnels Ireland, a Galway based supplier has installed 500 poly tunnels all over Ireland through the last two years. Perhaps it's the dome-like shape and flexibility of the polytunnel form that has seen them remain resolutely standing, but Gerard claims he's only had problems with one of his customers thus far. 

Hugh Fearnley Whittingstall (pic is from his Paupered Chef website) fully clothed (this time) in his polytunnel

"This guy put his tunnel up high in the Wicklow mountains. He left it unattended over Christmas and the heavy snow over Christmas saw two and a half foot of the stuff pile on top of it. The sheer weight cracked one of the supports so we had to replace it. Otherwise all the rest appeared to have weathered the snow quite well."

Apart from it's resilient shape, a polytunnel's other main advantage is the value for money on offer  - you can have a fifty foot long version for around a grand and a half although most non commercial growers start with the 14 ft x 20 ft version (E850).Another aspect of the polytunnel’s new found popularity in Ireland is the recent ironing out of one of its two major failings.

For many years the outer skins had the reputation of perishing far too easily. It's not that it's expensive to replace, but rather a major inconvenience.

Thanks to improved skins and cut prices we don't need to be commercial growers to get the polybenefits 

The second issue is one which polytunnellers everywhere will simply have to put up with - the semi circular shape means you can only stand up in the middle portions, and growing space at the edges is limited to low ranging food plants only. But that semi circular shape also gives it a natural strength in January's crazier gales.

Nowadays however straight sided tunnels with standing room all round have been developed without losing too much stability inherent in the basic structure while the high tech skins deployed today should last you ten years if properly installed. It is these developments that have taken tunnels from the sole realm of the smallholder or commercial market gardener for which they were intended and made them so recently popular with medium to large sized urban garden owners.

Unlike a greenhouse, which takes from two days upwards to assemble, a polytunnel goes up in a few hours. A simple frame is screwed together and sunk two foot into the ground. Then a one foot deep trench is dug on both sides. You place the edge of the skin sheet into one and back fill it with soil to keep the sheet in place. Then you throw the rest of the skin over the top of the frame and stretch it. You lay the other side it in the second trench and backfill again. The weight of the soil keeps the skin stretched taut. The door at one end is a simple timber frame coated with plastic and inserted into a timber frame, also coated.

Wind proof! This one's located at Stornoway on the outer Hebrides

Because the skin is buried a foot deep on either side there's no way the wind can wheedle it's way in. The foot deep plastic also tends to stop tunneling pests dead in their tracks whereas generally, pests find it easier to find their way under or through the chinks in a greenhouse frame.

"If you lay a piece of plumber's copper pipe across the ground at the door, slugs or snails can't cross it and it's effectively pest proof," says Garrard. He also advises using opaque foggy plastic rather than the clear version. Polytunnels get really hot inside and in my view, the clear plastic can cause the crops to burn" hence HFW’s penchant to use his as a sauna.

For this reason there's no point in growing certain crops in polytunnels as many would wilt, burn or bolt too quickly. "I wouldn't grow potatoes, carrots, onions or anything like that. It's really more suited to tomatoes, chillies, courgettes and so forth.”

The growing warmth they generate and their now proven resilience to Irish weather has given Garrard a brainwave which he says will allow Irish families to sit out in tropically warm gardens in all weather conditions. Gerard, who manufactures his tunnels in Clarenbridge and hopes to expand his business to employ more people in the future, is now planning to start manufacturing geodesic domes in Galway.

Woppa! Woppa! Nnnnnghhh! Only Rolfie's board should make that music.

Anyone familiar with the Eden project in the UK will recognise the sort of space age domes which allow tropical climates in temperate conditions. He believes they will allow us to have enclosed and warm gardens from 30 feet in length. "You'll be able to sit out in warmth at this time of year."

And thankfully they don’t go “Woppa, Woppa” in a gale - in fact they go “ZZZZZZzzzt!!” as the wind ripples over the skin. But most importantly they don’t go wallaby either..