Cooling off question for Newbie.

[Johan]

I would like to build myself a polytunnel as my shade cover after 9 years are nearing the end of life.

Here in South Africa my problem is the heat and I am full time working. I will not be able to open anything daily as I leave for work 05h45 on week days.

It is so hot here that I need to have a ventilation fan to force a wind flow through the whole year, except winter. Our winters is your growing season (almost).

I want to keep a net over the doors to keep fruit- and pumpkin flies out.

I am looking for ways to do, perhaps a solar power fan with a thermostat. The problem is I am not used to solar panels.

I am looking for a 12 x 16 feet (4 x 5 meter) polytunnel. It will also have 40% shade cloth over between September a year and May the next year.

[Glutton4...]

Hello, and welcome to the Madhouse! Don't expect any sensible answers for the next twelve hours, or so - Happy New Year!

[Bigmallly]

I'm trying to get my head around why you need a polytunnel in your climate. We obviously use them very differently to yourself.

[polc1410]

The solar panel is not the debate. Thats just a power source so for now lets assume you could plug into the mains, or a car battery or whatever as that doesn't matter.

What you need to determine is what the fan might look like.

Trouble is moving hot air from one place to another doesn't make it cold so will a fan actually achieve what you want. A fan makes your skin feel cooler because it replaces 'saturated' air with 'drier' air. The drier air allows evapouration of sweat which makes you feel cooler...

If you are sure you want a fan - define the fan - preferably 12Volt to work with the solar panel and I can probably provide some guidance about what you need in terms of a solar supply. Need to know how many hours it needs to run and also if its sunny for all those hours (sunny means > 1 hour after sunrise and < 1hr before sunset not the sun being out).

[Johan]

The commercial farmers still do better than open farmers. As well as protection from insects and birds in a tunnel.
In the summer the D shape sides (front and back will have shade net only. The top and sides will have plastic and shade cloth.
It is not every day that it gets hotter than 40 degrees of C. In spring it gets colder at night. In fall it is normally hotter than in summer.

The fan I want to start turning in daylight above 25 degrees of C until nightfall (12 Volt).

As long as I can keep the roots wet, the plants actually excell in the heat. i will have an automatic watering system, Gravity feed, 12V control station. Mulch as well.

There is only one drawback: Cucumbers gro fatter than in the UK (Open pollinated ones). Sometimes too fat if not picked very young.

Hygrotech is experimenting on their experimental farm with hobby tunnels as well and they really perform with that.

Here we can plant in winter as well without heating as it will get hot enough to cool circumstances. Frost is very rare where I live, I have not encountered it before. Snow only occurs on high mountains.

With a tunnel we can plant right through the year with tomatoes, cucumbers and squash as well. Most of the other you can plant throughout the year. Peppers last two years or more when pruned. In the second season it bear fruit like if there is no tomorrow. So if I pland peppers today, it will still bear befor the hard winter, but it will go slightly dormant in a tunnel and start bearing in August again. I can have a 10 - 12 month season.

Before I will add plastic, I have to cover 100% against insects. I don't want cross pollination or fruit flies. I also need to keep cats away.

[polc1410]

Do you have your control system sorted? Do you have a control system for the watering sorted?

Have you identified a suitable fan?

Those three things would define your power requirements.

How long after sunrise is it likely to take till 25C? Or will that be at sunrise ;-)

[Nicos]

Just a thought...I presume you will need special plastic which can stand higher UV levels than in the UK?

I am in northern France and even here I've noticed that plastic disintegrates much quicker here than when we were in the UK.

[Bill HH]

I have usualy got something to say about everything but I will have to pass on this one. I think I would be more concerned about elephants and lions etc than flies.

[RedThorn]

Where in SA?

I'd be more inclined to go 70% with solar motorised blind effect....

In polytunnel you'll increase humidity a lot

Perhaps a polytunnel with netting rather than plastic to stop bugs but allow air movement with motorised plastic cover to seal when temps colder?

A lot depends on where u live

[Johan]

I have watched some videos and did some research.
My approach will be to make the structure from galvanised steel and wood, cover it completely with 40% shadecloth (That will last about 9-10 years),
I will then cover tha top and sides with Polytunnel plastic, but not the font and back. If I feel it gets too cold when winter approaches I will cover that up and take it from there.
The shade cloth (40%) structure I have currently is now lasting since December 2005 and the cloth (shade net) is still gowing strong. It is properly UV treated.
The thick spolytunnel plastic available here is also UV treated and last about 5 years if properly spanned and looked after. You do get thinner ones, but that will be a waste of money.

I am specifically thinking of steel construction only and not plastic hoops as I want to add plant staking rails, that will have to take a big load of pants and fuit at a given time.
Wind can also getting very violent here in Cape Town.

Bill,
On a lighter note:
There are lions in the Western Cape, but only in sanctuaries / game rances.
The fruit flies / pumpkin flies is vitually uncontrollable and they deatry your complete crop. A cucumber sometimes got stung up to five times. Since I have covered (they still get in some kind of way - I dont know where or how) up they were less. By using a poisonous bait mix of sugar water, I managed to control them to a manageable percentage.
For the purpose built tunnel I will seal better to attemps to keep them out permanently.

[RedThorn]

The wind will Deffo be a factor, when the cape doctor blows

Have you thought about maybe putting paving slabs down the centre with a drip feed to dampen them for evaporative cooling?

Coming back to it then what are you asking, I'm a little confused now

[Dorothy rouse]

Just wanted to say 'hello' Johan, I know Cape Town reasonably well now, as my son, moved out there 6 years ago, just beginning year 7, and I have a grandson of 2+, have visited SA, several times, spending time in different areas, as well as time in Cape Town, last visited in Feb.

You never know, you might even have met him!

I love SA, beautiful country.

DottyR

[Bill HH]

Even without the lions I take my hat off to you for gardening under such extreme conditions.

[Johan]

I live in Cape Town.
The suburb is Brackenfell, 30 km North-East from the City.
I gets hotter and colder here than in Town. The average weather forcasts are always for Cape Town International Airport. Different here.
Fortunately here is almost always a breaze of wind blowing to cool things off a bit.

I won't say it is extreme conditions. I think we are fortunate that the weather is not so cold as in UK or North USA or Europe.

[Dorothy rouse]

My son is now living in 'Plumstead' , anywhere near you, his office is in Cape Town.