Polytunnels are wonderful structures and are good value for money compared to greenhouses. Once assembled they will be there for a number of years so check out my advice before you buy.
I have used plastic tunnels since 1983 when I bought three, each 18×55ft (5.5x17m). They served me well, except their 1-inch tubes were too thin for my exposed site; in a gale in March 1988, one of them collapsed after the wind ripped off the polyethylene on the other side.
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I would buy the largest one that can fit in your area, because the space is useful for storing and drying, washing, as well as growing crops and there is always more to plant in a tunnel. The hoops are usually 5 feet (1.5 m) apart, so the length is a multiple of that. I reckon 20-30 feet is a good length for even ventilation, while tunnels say 60 feet long can have pockets of unvented “dead” air in between.
• North to south is ideal for tall crops in summer to cause equal amounts of shade on both sides, also to reduce overheating in summer.
• An east-west structure faces the midday sun and in a hot climate this is not ideal during the summer. It also means some shade from crops on the north bed when summer crops grow tall in the middle.
However, any orientation is possible and I have tried many, with success. Other important factors to consider are lining a tunnel up and down any slope (beds along the slope are more difficult to irrigate and cover) and having gates at convenient points for access and irrigation.
Plants like cool air and it’s important that they get enough of it, rather than straining for maximum heat. The air in a polytunnel circulates from one end to the other, and if possible I recommend leaving a gap between the top of the doors and the frame above them, say 6 inches (15 cm), so that it can always flow a small amount of air and increase. carbon dioxide levels, without shot at ground level. I find that winter salads stay healthy, with no mold problems, but no need to open and close doors for much of the winter. A job saved!
In my opinion the option of side vents (instead of the poly going to ground level) is not necessary or suitable for vegetables as it reduces the temperature too much in windy weather and the doors offer enough possibilities for air intake fresh. It is cheaper, simpler and more effective to cover the hoops with polyethylene only, up to and below ground level. Keep side vents only for long tunnels, above 40 to 50 ft (13 to 16 m) in length.
Polyethylene adhered to the ground level or buried?
I recommend burying the poly throughout in shovel-deep trenches, to have a poly barrier in the ground. This protects against:
• incoming pests such as rabbits, slugs, cats and dogs
• creeping weeds, including couch grass, buttercup, bindweed
• too much draft and cold air around plant leaves.
It is sometimes claimed that it is easier and faster to lay polyethylene at ground level on battens or rails. But having tried this I don’t agree, because before you do that you need to dig holes for the foundation tubes instead of just pounding them in, you may also need some concrete for windy sites. All the fixings you need involve additional expense and are only worth it if there is simply no room to dig a trench.
When burying polyethylene in a trench, the pipes simply need to be hammered into the ground 12-18 inches (30-45 cm), where their function is to give some rigidity to the structure, that’s all. When the polyethylene is buried, the tubes do not anchor the structure in place, but instead keep it stable in the wind.
In contrast to this, when the polyethylene is fixed to a rail and not buried, the foundation tubes are more important, as they are the ones that support the structure in the ground (rather than the buried polyethylene) and therefore more expensive tubes with metal supports attached are needed. its ends, and each requires a hole dug, sometimes with concrete.
While watching this in one of my courses, a participant lamented how her new tunnel had blown up in a month: the polyethylene was attached to rails, rather than buried. The exceptions to the above are sheltered gardens and sites where a trench cannot be dug, eg too much gravel, concrete, etc. But it is also difficult to dig holes for foundation pipes.
Crop Support Bars
Although it’s an ‘extra’, I would always buy them, so it has support for climbing plants, from tomatoes and cucumbers to beans and melons. They spread from side to side to about 6 feet (2 m) high or a bit more, so you can walk underneath, but also reach to tie ropes or stakes.
Poly should be replaced every 5-8 years depending on how windy your site is and how tight you can stretch it. Polyethylene is easier to squeeze when the cover is placed in hot sun, if possible. I’ve done it often in January and managed fine, but the skin is definitely looser and more prone to damage than when I’m covered in the hot sun.
What kind of polyethylene?
There are more types available and the choice is difficult. Of the tunnels that I have seen with the most expensive thermal or anti-condensation polyethylene, I doubt that it is worth the extra cost. I recommend standard light-diffusing polyethylene that admits about 89% of daylight. Always use horticultural grade polyethylene, which has a UV stabilizer, without which it would be brittle and break within a year.
A friend recently noticed the great clarity of my standard light-diffusing polyethylene, which is now three years old. He had recently washed it with a damp sheet and brush to remove lichen and moss.
You use these a lot, so suffice it to say that it’s worth spending money to have the ones you like. The homemade is certainly possible, take into account my tips on ventilation. It is the area where you can be most creative, especially if you like woodworking. For door frames, 4 × 2.