Multi-span Polytunnel or Glasshouse

[dieselthedemon]

Looking for some advice, we are looking to install a 24m x 19m (approximate sizes) protected area. Our options are a multi-span polytunnel or a multi-span commercial glasshouse, quote for the glasshouse is 34k installed (excludes groundworks and plant hire). I am not a big fan of the look of polytunnels but do appreciate it is a cheaper way to cover a large area, which means the return on investment is achieved in a shorter term. My concerns about the poly tunnel option is that the site is very exposed, i.e. flatland in banking onto a hillside, so when the wind comes over the hill it sounds like a 737 taking off. Plus the risk of losing the covering and it ending up causing an accident on the A75.

Just really need some guidance.

[Kristen]

My concern would be how you ventilate the greenhouse.

I think roof ventilators on glass allow a "hot air rises" approach, whereas with plastic its normally "Open the ends and get a draught through".

Probably depends on the crop, but I think the draught approach causes more transpiration (and potentially stress) than the hot-air-rises approach.

Other factors might be the weight that the structure can take - i.e. crop supports in addition to glass / plastic.

Also the cost of re-doing the plastic (how often is that, once every 3 or 4 years?)

[SarzWix]

I would also consider the possibility of vandalism and wind damage - how much would repairs cost for either structure? For a polytunnel, this could mean replacement of the entire cover, although emergency repairs (tape & small polythene patches) would be quick and easy. For a glasshouse, this would be replacement glass panes, and emergency repairs would be more difficult.
You don't say whereabouts you are, how important is heat retension? Have you had the ground surveyed to see what sort of anchorage you will have to use?
If I was you, I would go down the route of the Pro' & Con's list for each structure - only you can decide which is best for your situation!

[dieselthedemon]

Kristen, SarzWix, thank you for the responses.

Our location is north of Dumfries, outside the village of Thornhill.

I think wind damage could be a problem with a polytunnel at our location, vandalism I am not concerned about we have two Dobermans and no one ventures onto our ground.

Ultimately we want to grow a variety of crops for the local market, those that require protection for growth and those that will benefit form the addition of protection. Our ground is very stony and direct planting on our site may not be very successful.

We are also interested in growing cherry trees under protection, and moving them outside for the winter for the frost to get at them and bring them in early march until the fruit has been harvested. The only problem is I can't find much information on this technique.

What retains the heat better polythene or glass?

We have just received information on semi-commercial glasshouses, which are glazed with toughened safety glass but on a cost breakdown it would be a lot more expensive to cover the same area as a true commercial structure.

[Kristen]

"Our ground is very stony and direct planting on our site may not be very successful."

Hydroponics? (NFT / Nutrient Film Technique, if that's still practiced?)

"What retains the heat better polythene or glass?"

I would think that Polythene would be more air tight, but I don't know about U-value / heat loss. We had a multi-spam plastic greenhouse back in the '70s which had two layers, the bit in between was inflated by a very small fan - this provided insulation, and also extended the life of the polythene - which tends to "rot" first where it is in contact with the metal structure.

[dieselthedemon]

Funny you should mention Hydroponics, as I was just looking into it last night in particular NFT. I have ordered a couple of books on crop production using hydroponics and sent a couple of emails to UK suppliers of hydroponic/NFT equipment and systems.

I think I would trial the technique on the two small glasshouses before committing to it as the system for the future multi-span.

[sewer rat]

Another option that you may want to consider are the Keder Houses, which have a sort of bubblewrap cover.
I have a 24' x 60' ft polytunnel and it lost the cover in February - £275 to replace it so not expensive, but the time element is a factor as you cannot recover when it's windy - my site too is very exposed.
As regards stony ground - raised beds is the simple answer.
Another quick thought - if you are growing veggies to sell and you wanna grow organically and become certified as an organic grower, Hydroponics is out - if it's not grown in soil, the SA will not classify it as organically grown.

[dieselthedemon]

Thanks for the response Sewer Rat.

The Keder Houses, I requested information on yesterday and will get a comparative quotation for a multi-span structure.

I am keen at looking at the hydroponic system, I have a degree in applied plant science and the technique and yield increase potential interests the lingering scientist in me. I realise that the produce wouldn’t be able to be certified as organic by the soil association, but I think I am correct in saying that if I used raised beds it wouldn’t be either.

There does appear to be organic nutrient solutions on the market for use with hydroponics so I could be organic without marketing my produce as organic.

Lots of research still to be done.

[Kristen]

"I am keen at looking at the hydroponic system"

I was involved with an NFT set up in the '70s, about 1/2 acre I think. It was on an industrial site so nothing remotely like "soil" available! however they did have some posh analytical equipment, so it was easy enough to get accurate testing of the nutrient.

The "reservoir" had a ball valve, to keep the water level constant, and some sort of salinity probe that injected more concentrated nutrient, and similarly for PH adjustment. Back then I think the recommendation was to "chuck" the reservoir periodically and refill from scratch, to get back to level-playing field of nutrient balance; of course having proper test equipment meant we didn't need to do that, and it was noticeable that the Toms would strip certain nutrients within hours when they got to that stage of their growth ...

Power loss, or a blocked nozzle, was the biggest concern, but in practice once the plants were established their root mat could hold quite a lot of moisture, so there wasn't any need for emergency-response for an hour or two, but after that wasn't be so good!

[dieselthedemon]

Hi Kirsten, once the multi span is up and we are confident that hydroponics is the route to take I would be looking at spending some money on automation, sensors and automatic dosing pumps.

Just spoken to keder and they are going to send me some quotes and information on the commercial structures. Compared to glass it is about 40% cheaper.

[dieselthedemon]

Hoping to go and view a commercial keder this weekend, this will give us an idea if it is something we would want to progress with.

We also have Bridge Greenhouses coming out next week to look at our site and provide us with options.

The thing I am most excited about is our first glasshouse arrives a week on Thursday, a 20' x 10' rhino in green powder coat. Work on erecting it starts the next day.

[SarzWix]

Good luck with it all Diesel, whatever you decide to go with

[dieselthedemon]

Thanks SarzWix.

Will keep this thread updated as everything progresses.

[Galileo]

Hi Diesel

I just put up a Rhino 8x16 two weeks ago if you need any help put a post on the forum. The instructions could be a bit clearer. I could send you pictures if you need them.

I hope you ordered the optional assembly kit or already have a glass sucker and glazing paddle. Make sure your base is level and square (Really I mean it!)otherwise the glass won't fit. If the base is a little bit off (even a few mili metres) you won't notice until it comes time to put the glass in. Then you will notice!!!!

Regards

Galileo

[dieselthedemon]

Thank you for the advice Galileo.

We are using one of rhino's recommended installers, so hopefully he will be well prepared to deal with any issues.

It would be great to see some pictures of your glasshouse though.