At Earth Ways in north Scotland, we have designed and built a versatile polytunnel. To get maximum use from it, we have made one section of it completely frost-free, we harvest rainwater for irrigation, and we have equipped it to provide plants with enough light for year-round growing.
Rainwater is often the best and easiest source of water for polytunnels in the UK. At Earth Ways we do sometimes run out of it because we also use it to make and dilute liquid fertilisers, so it is good to have mains water as back up. We fill barrels with it as it is better than using tap water directly because it allows some of the chlorine to evaporate and the water to warm up.
We looked into self-watering systems like drip feeds, but I have been told by experts that they only really work well with mains water at mains pressure, so we didn’t try them. Self-watering beds are handy, so an alternative that we tried was the traditional African ollas,1 a porous terracotta pot, kindly provided by Thys Berkers. This gradually releases water into the surrounding soil and the roots from plants nearby grow towards the pot. We loved it but found it too pricey for a whole tunnel. We did an experiment creating a self-watering bed, using a pond liner (more about this later). In the end we chose to use watering cans. The main argument in favour of this choice is that it forced us to visit our plants daily. We spotted pests, diseases and nutrient issues much quicker than we would have otherwise.
The logical place to harvest rainwater is from a convenient clean surface area high up: the roof of the polytunnel. Plastic tile trims work really well as flexible gutters that can be taped to the outside of the tunnel. Use the ones that go on top of a row of tiles in the bathroom. They are shaped like a little gutter if you turn them upside down and are much cheaper than commercial polytunnel gutter kits.
Clean the plastic well and use transparent tunnel repair tape. Coloured tape blocks light and heats up in the sun, shortening the life of your plastic. Tape the gutters in a V-shape above a barrel, with the ends sticking out a little. Make a slit in the hose and slide it on to the gutter and fasten with wire. The hose will guide the water neatly into a barrel.
We placed five blue 225 litre (50 gallon) barrels on both sides of our tunnel and two more inside. We used second-hand blue barrels because they are cheap and sturdy. We found them for £10 each. Sometimes they are used to transport toxic material, so check what they were used for previously.
Moving the Water
It is simple to move water between barrels by siphonage. When taking water out of one barrel, most of it will be replaced by gravity and air pressure. There is a need for a bit of extra pressure as the narrow diameter of the hoses creates friction which needs to be overcome by creating a hydraulic head. To achieve this we have a height difference of 180mm (7in) between each barrel.
Barrel 0, where we take water out, is dug 180mm into the soil, barrel 1 is at ground level, barrel 2 is raised 180mm and barrel 3 is raised 360mm (14in).
To connect the barrels, submerge the ends of a short hose in each barrel with a piece of metal wire at the ends to weigh it down. Make sure there is no air in the hose or the syphon won’t work. We use a water hose to ‘blow’ the air out.
Having a couple of barrels in the tunnel makes the water easy to access and it warms up a little, which is better for the plants as cold water can be a shock, especially to seedlings.
Pond Powered Self-watering Bed
One of the barrels from outside the tunnel leads into a small pond inside. When it rains, it slowly refreshes the water in the pond and when full it overflows into a self-watering bed.
For the full article, which includes the design and build of a polytunnel heating system, off-grid power and diagrams, check out the latest issue of permaculture magazine (PM88) out at the end of April.
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The Earth Ways Team consists of Ludwig Appeltans, Suzanne de Waard and Jack Lennon. Earth Ways grows food and teaches permaculture in the north of Scotland: www.earth-ways.co.uk