What to look for when buying a polytunnel

[bluebellParticipant]

Having had a bit of a disaster with the polycarbonate green house I am looking getting a a poly tunnel.

I think I will have access to an additional half plot to put it on so size could be rather large – although Im only really trying to feed 2 people so Im thinking that a 12 x 15 or may be 20 would be sufficient.

It will need to be able to survive without my visiting every day, so the questions are:
What covering – are the non drip ones worth the extra cost?
What door set up? Im thinking of a screen for most of the summer and full plastic for the winter on double width door frames, is there any advantage to having sliding over opening?
I see that some now have an aluminium fixing system to pull the cover taut which looks like a good idea? Would it be best to still bury the edges to help keep it in the ground?
What about fixings? Should I buy base plates or use dry mix to secure the footings? Site is exposed but not extremely so.
I see that some now come with side ventilation panels – are these necessary?
So many questions! Sorry to bombard you but I would love some comments from people with practical experience.

[sarah-off-gridParticipant]

Hi bluebell,
I’m a newbie to the forum and to PT’s as we’ve only had our 20’x 40′ PT for 6 months- which is 800 sq feet- maybe 6 times the surface area that you are looking at. I’m sure you will get replies from more experienced members here
Nonetheless here’s my two penneth worth…
As much ventilation as possible. That’s hard to automate- unless you have some boffin friends who can rig up a system for you. If you can’t visit often I think that is problematic not just in summer but also in autumn/winter when you don’t want any moist stagnant area rotting your autumn salads. I am opening up every day, even on days like this where the fleeces are on at night.

Here in Wales, the temperatures can rise really high when the clouds part which means an awful lot of adjustment trips. In summer we got frequent peaks of 35’C+
We have wind-up one side ventilation plus doors front and back, we haven’t fitted an extraction fan yet but will do. Even with everything open and some shade netting it can get very hot.

Sliding double doors -essential for us as we are very exposed to wind.
Concreting fixings in? We did – same effort and cost as base plates but then we needed solid fixings due to size, wind factor, high water table with clay pan in places plus high rainfall area.
If you’re not using a water tap nearby gutters connected to large butts as they can fill quickly . Gutters really beneficial even though the rig-up is fiddly with repair tape which channels the rain over the gutter lip. ( Any better suggestions would love to know?)
Non-condensation covers – ours is , still get the odd drip onto the beds but I have no experience with the other cover to compare it with.
Cover tightening – you should be able to do that with a helper at that size, plus online video guides. Ours took four people, a hot day and a lot of sweat and swearing with a subsequent re-tightening from the side rails but, so far, it’s as tight as a drum.
Nicky Kyle’s website is also good for organic, non-commercial PTs – I don’t think she does any hand pollination and it might seduce you to go a little larger if you can.
Overall: expensive but well worth it, even with a second-hand frame like ours.

[bluebellParticipant]

Thank you Sarah – off- grid, that is very helpful.
I was wondering about the side ventilation, especially in relation to convection currents: With a greenhouse it is normally the top windows that open thus letting out the hot air that has risen in the greenhouse. However if the side opens from the bottom presumably cold air has a tendency to roll in? I can see this being a potential issue if left over night in March, when there may still be just be a frost.
Iif that is the case would it not be better to have the bottom half of the doors covered and the top open? or would that not provide enough ventilation during the day?

[JayjayParticipant]

Hi Bluebell, I put up a 10′ x 20′ polytunnel in May 2014 and wish I had been able to go to 12′ wide because my paths are very narrow if I have 21/2 to 3 foot wide beds either side and a bed in the middle for tall stuff such as tomatoes, it’s a bit of a squeeze getting along the paths when everything is growing strongly. I would also say go for the largest you can afford and you will still feel you needed to get a bigger one!
I bought my tunnel from First Tunnels and went for the Thermal anti drip cover, (I think called Super Therm now), I find I do get condensation and some drips and this seems to happen where the polythene touches the Twin Support Brace Bars and the drips fall from the bars. I have since put anti hot spot tape along these and that seems to have made it worse as it’s created a barrier for the condensation to hit against, but then, the metal bars were touching the polythene and it’s recommended to fit the tape to stop the polythene deteriorating prematurely due to heat build up.
I went for single sliding doors because this means you don’t have to worry about where a hinged door would swing out to, either inside or out, as I don’t have a lot of room, so the sliding doors just run along the ends not taking up further room.
I have put half inch square solid metal mesh on the top panels of my doors and on the outside I have put runners to hold 4mm twin walled polycarbonate sheets sliding up and down for ventilation, i just screwed on some vertical runners with holes drilled about every 4 inches and I fashioned some 2mm metal rods into stops that I just insert where I need to hold the bottom of the sheets.
My allotment is on a very windy site and is orientated with the ridge east/west (with a southwest prevailing wind). I have been worried about gale force winds meeting a solid object, so I leave a 3 inch gap at the top of both doors all the time to allow any wind to blow through and thus far, it’s been okay.
I have anchor plates and wooden base rails and have buried about 6 inches of the polythene in the ground to tidy it up and also create a bit of a physical barrier for bugs. We are not allowed to use concrete on our plots so the anchor plates are just buried 600mm down. Another plot holder has Aluminium base rails and regrets it, he says the locking centre rails are really difficult to press in and once the polythene is trapped, the locking rail is really difficult to get out.
I didn’t use the supplied nails for all the battens holding the polythene, my thinking was it would be easier to change anything if I used screws all around, this being the first time I had put up a tunnel. This may prove to be useful for the ventilation screen and side vent. I found this to be a bit ‘Heath Robinson’ because the rod holding the 20 foot of polythene is only supported at the winder end and is floating at the other end, just relying on the polythene holding it level all along it’s length and this was quite difficult to get evenly wrapped around the rod. In practise, when I wound it up and down at various times, I trapped my Melon leaves around the polythene, so at the moment I have decomposed leaves wrapped around the polythene either side and I can access the inside to clean them off but not the outside as it has the ventilation screen, (net), fixed via the battens, so it means that I will have to remove the battens to take the net off in order to clean the outside of the polythene screen. If I had used the nails there would have been a danger of splitting the battens with the constant removal of them over time.
I will be looking to try and get some sort of solar powered fan for further ventilation because you certainly need ventilation/cooling in a tunnel most of the time, especially when it warms up and removal of condensation when it gets cold. I figured that I could fit a large sheet of ply on the east side from the continuation of the ventilation panel support rail and the base rail as it shouldn’t affect the polythene cover, a fan could then be fitted in this ply panel. I just need to find a source for a fan a bit stronger than those tiny ones fitted to boats or motorhomes! (Any suggestions would be really helpful).
I think, like Sarah said, you will need to open up every day because the temperatures and humidity change very fast and the air needs changing frequently. I did think about putting up another side vent and screen to get more ventilation and rang up the Company to order it but they said as the tunnel had been up more than 6 months they couldn’t recommend messing about with the polythene cover and I wouldn’t get it drum tight so I haven’t gone down that route. In your case, it may be worth thinking about getting it for both sides.
With regard to the cover tightening, it was only my sister and I who put the tunnel up and we got it almost tight but needed the extra strength that only men can supply in order to get it drum tight. The wooden base and side rails made this tightening easier than just burying the polythene without base rails I feel.
So a lot to think about but it really opens up more possibilities and gives you a month more growing time at each end of the year, and a lovely shelter from the wind and rain! At the moment I have salad leaves, onions, cabbages, pak choi, pea shoots all coming along nicely and today I started on building the frame for my hotbed in there. The possibilities are endless! Good luck. Jeanette.

[bluebellParticipant]

Thanks JayJay That is really helpful – nothing like doing it to find out the pitfalls. Especially about the side vents. I did wonder if it may be better to have double width sliding doors so I could just have one or all doors open with a drop down fly screen, turning a door sideways if it was a bit inclement to stop the cold air running it – it did occur to me that being low down the ventilation may let more cold in than I would like?

I could get a 10 foot on my existing plot but think I may wait and see if I am offered an additional half plot reasonably close before I order as one plot is just not enough! (it would also allow me to store more manure and compost). But I would not be able to get on the plot until may – having said that I can see advantages to building it in good weather.
Jan

[charlesModerator]

Good info here.
I would add that in certain weathers, condensation is pretty well inevitable, but I have not noticed problems from it. My polythene is just the standard, also I have never used hotspot tape and my last tunnel had the same polythene for 8 years.
Jayjay’s idea for a gap over the door is excellent, it allows air all the time and on windy, winter days I leave doors closed as there is plenty of air filtering through at a level where plants are not being hit by it. Few growers use side netting, I think that is more for raising plants.

[JayjayParticipant]

Just for clarity, the adjustable gap is in the top panel of the door, not over the door. An improvement would be to put some sort of vent as high up on the ends as could be, to allow for the rising hot air. I have pondered this but can’t see a solution because this is where a lot of the polythene has been gathered up because of the curve of the hoop so even if one constructed a wooden frame to put a vent in, it would be difficult to fix this frame to the hoops.
I would think that it would be a definite advantage to have double sliding doors, but at the time I couldn’t afford to get them, c’est la vie! Jeanette

[bluebellParticipant]

Thanks guys that is really helpful. now just need to remove the old one and get it to the dump…(not in a state to sell or I would!)
You have got me thinking about the possibilities of fitting a smaller frame above the doors that could potentially either be an opening window or hold a solar powered fan…..This from the person who has ery little idea about woodwork and DIY!

[MPGParticipant]

Hi everybody,
We are about to order a couple of polytunnels and I would like further information.
Charles, did you get a feedback from OGA regarding UV blocking polythene? does this affect the growth/taste of plants?
Jayjay, you said you have rails and burried the polythene 6 inches to act as a pests barrier, has this depth proved efficient? Any comment in this matter welcome as it sounds that building a tunnel with rails is easier than digging a trench and bury the cover.
Thanks,
Luciole

[charlesModerator]

I suggest you read my polytunnel article under Learn – Articles
If you have access to soil for making a trench, I think it’s easier and cheaper than rails.
And keep pests out! But 6in may not be enough for keeping it in place when windy…
Sorry no feedback on that poly, I still reckon the standard & cheaper, normal one is best.

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