Shade netting for heat/sun in greenhouse

Troubleshooting Stress
1

Hi all,
I grow at a latitude of roughly 25°S (Pretoria, South Africa). Our climate is comparable to Texas or maybe Southern California. Growing in a greenhouse (6m x 4m for 24m²), but have blackout covers to control sunlight. One part of the greenhouse is for 18 hours of sun (I supplement it with a few hours of lights at night) and the other part gets only 12 hours of sun for the flowering plants.

Since September (beginning of spring for us) daily highs (lol, pun intended) have been around 30° Celsius OUTSIDE. Inside the greenhouse it goes to at least 5° to 10° hotter. Got vents, got fans moving air inside, even got vent fans for air-in and air-out.

Just purchased a 24000 BTU aircon that runs flat-out from 09:00 till 16:00 daily, and this helps, but only to the extent that it brings the temps close to outside temps. It NEVER gets it colder inside than outside. In desperation, I’ve put some 40% shade netting over the roof of the greenhouse (nothing on the sides). This does not seem to make any difference. Temps are currently hitting highs (sigh…I know…) of around 34° outside. These are quite extreme for us, as normally, at this time of year, our top temps would be around 30° at most. In Jan and Feb (our hottest months) temps may sometimes hit 34° or 36°.

The question:

PART 1 - Could I go as far as 60% shade netting, or even 80% netting, or will that take away too much UV that the plants need?

PART 2 - Since I have 40% up already, am I an idiot for wanting to simply add a 20% “OVER TOP OF THAT”, to get to 60%, or should I remove the 40% and simply replace it with 60% ?

For our American friends: 30° C is about 86° F, and 34° C is about 93° F.

Thanks all.

2

sure hope you are growing sativa strains that can handle the 30c degree + temps.
suggest you try 60%shading first…and if it too much, back off to 40% again.

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3

Did you say what the construction and covering material was? R value in a greenhouse works both ways , for heat and cold. Many greenhouse operators have two layers of poly, an opaque diffuse om the outside and regular greenhouse poly on the inside to trap air as an insulator. This may or may not help? Ventilation is also critical, you need a large volume fan place high on one end and a louvered intake on the other. Combine that with passive venting if possible.

4

If you are running AC and have vent fans running, you might be dumping your cool air.

If you close all vents does it get cooler? Ac in a closed system and Co2 might be the answer

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5

Yeah you don’t want to be venting AC! Another trick some greenhouses employ is to bury a 6" hose underground and run an inline fan to circulate the cooler ground temp while at the same time sequestering heat for use in the cooler months. May be an option too?

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