Monday, 29 May 2017

P-P-P-Put up a polytunnel!

Back when it was shiny and new.
We're now just over a year into our experience of polytunnel growing. Definitely a success. Very happy that we decided to go for one. It extends the growing season considerably for a number of crops, and expands the range of growing possibilities.

If you're a grower of veg living some place with lots of wind or short summers and long winters, a polytunnel is a great option.

Polytunnel vs greenhouse (or glass house) is a classic debate. What it really boils down to is cost. To cover a given area, the polytunnel comes in a lot cheaper, even if you go (as you should) for the highest spec you can afford.

The chosen spot.
Before even getting to the tunnel, selection of a site is key. A bit of shelter is helpful, if available, but direct sun is probably more important. The ground doesn't have to be exactly level, but shouldn't be too far off, and there has to be sufficient space on all sides to work when putting the thing up.

Choose a tunnel

As usual, we did quite a lot of research before selecting a tunnel. There are a lot of options out there. For us, strength was right at the top of the desirable qualities list, as we knew it would have to stand up to some very strong winds here, particularly through the winter. That helped to narrow the field for a start. A good rummage around through gardener's forums etc. online soon flagged up some brands to forget about and a few to look more closely at.

Putting it up.
This leads directly into the nitty gritty of what the tunnel is made from and how it's anchored and, again, there's considerable variability on these points. Gauge of steel used and tube diameter of the hoops, how many sections they come in, what cross members are used, anchoring system, thickness of cover plastic - all affect the strength, durability and, of course, cost of the unit.

Cost, naturally, came next on our list of considerations. We're always looking for value, but will pay what we have to, to get good quality. There's no point in 'saving' a third on something that will only last a quarter as long.

Securing the cover to the frame.
Our final criterion was ease of erection. There are quite a few different systems for securing the cover to the frame, some more, some less user-friendly. Getting the cover on really snugly is a critical determinant of its lifespan, so we wanted to go for one that would give us the best chance of getting it right on the first attempt.

Put it up

Our chosen tunnel is 20 x 8 feet, with screw anchors at each hoop base and a clever wire-clamp system for securing the cover to the frame. We put it up over 2 days, the frame on the first and the cover on the second. It's a huge sheet of plastic, so it's vital to do this on a windless day!

The first critical step, and one that was surprisingly tricky, is getting the positions of the anchor points correct, so that all four corners are exactly 90° square. It's really important to get this right, so mark it out with stakes and string, then check and re-check using the Pythagorean 3-4-5 method on each corner. For those whose geometry might be a little rusty, the length of the sides of a right angle triangle are always in the proportions of 3-4-5.

Finally: the door.
Putting anti-hotspot tape over the steel frame everywhere it will contact the plastic is a simple way to extend the life of the cover: don't skip this out.

Make sure you have as many helpers on hand as necessary to get the cover on nice and tight. The design we chose was okay for two people to put up - one to hold the plastic while the other did the clamping - but I've heard that other types, such as the traditional trench-fixing, are better done with at least three.

So, two day's effort to finish putting up our very first polytunnel - not bad. The second would obviously be easier with the experience, but I'm in no particular hurry to do it again.

The finished article fits in nicely.
I must confess that there was a certain amount of trepidation in the air when the first of the big winter gales struck, but that tunnel is rock solid and has so far stood it all without complaint. Well, almost without complaint. The one thing that couldn't quite take it is the velcro fastening on the vent panel cover at the back end of the tunnel (visible in the photo above). No matter how firmly we press that on, it will blow off with a strong wind pressing against it. My solution has been to screw a couple of 'storm boards' into the aluminium frame across the vent. It does the job, but that fastening seems to me to be the one design flaw in the whole setup. Perhaps a zip fastener would have been a better bet.

Next time: Growing in the tunnel
Extend your growing season and range.

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