Dealing with high temperature and low humidity in small greenhouse


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I have a 2x2 meters polycarbonate greenhouse and I’m struggling to keep temperatures in the 23-28 °C range during the day in direct sunlight. Between 12 pm and 3 pm it can get as hot as 35° C even with door, side vent and roof vent open. Naturally, that interferes with the humidity level that drops to 20-30%. I’ve found that opening the greenhouse does reduce the temperature a bit but it also lets the humidity out (I’ve a 4 L humidifier but does nothing in the day, as the humidity is blown away by the air. It does work in the morning and night when I can close the openings). Even with morning waterings to the floor (dirt and grass) and pots do almost nothing in those hot hours.

I place a shade cloth that works to some degree, keeping the temperature below 30 °C, but it loses some of the sun rays that my plants need. Also, that doesn’t fix the issue with humidity. Everywhere I read it says that ventilation is key in order to keep the temperature down but that way the humidity just can’t keep up as it escapes the greenhouse.

What can I do? I’ve think of foggers, industrial humidifiers and more solution relying on increasing the humidity so the temperature drops but the greenhouse is so small and crammed that it seems difficult to implement those ideas. Besides, there are fans, lights and a heater inside that can get wet. I plan to upgrade to a bigger greenhouse in a few months but everyday I have to deal with this issue until then. Please help! My plants aren’t very happy.
 
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Greetings, welcome to the Forums.

Which plants are you growing that require high humidity? Do you live in a very arid region? Usually glass or polycarbonate greenhouse are not needed in the tropics, though perhaps you circumstances are exceptional. In any case, it would be better to keep the plants outside in drier air than cooking them in an excessively hot greenhouse.
 
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Hello Marck, thanks for the reply.
I’m growing tomato and cannabis plants. It’s not arid but right now we are experiencing some heat waves. The humidity outside has been low, between 15 and 30% in the day. I began using a greenhouse with the idea of having high humidity in these cases but it has been really challenging. I don’t know what to prioritize, light level (outside of greenhouse, full sun for some hours) or humidity levels (inside greenhouse, where at least I can keep humidity high in the morning and late afternoon). I give them 15 hours of light and night temperatures over 17 (here it gets below 12) so either way I have to place them inside the greenhouse for the night.
I’ve grown these plants indoors with high humidity and perfect temperature under led lights and I was astonished by the rate of development, so I know humidity is very important. I can’t give the plants perfect conditions but what should I do for them so they can grow as healthy as they can? Maybe a shade cloth that provides more cover? So I don't need to open all the vents and humidity can stay, even if that means the plants won’t get that much light. Again, light levels vs humidity.
 
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I would try growing some of your plants outdoors to learn how well they will grow. Tomatoes (Solanum lycopersicum) are often grown outdoors in areas of low humidity. As long as irrigation is is sufficient they can do quite well. Overall, the same is true for Hemp (Cannabis sativa), though perhaps some specialized, modern cultivars are more particular. Cultivar selection is an important point to consider for any crop. The varieties that grew best indoors in another place, may not be the best kinds for outdoor (or greenhouse) production) in Zone 12a. I suppose Cannabis crops are often grown indoors for security reasons. Likely, many modern cultivars are selected for those conditions. Both tomatoes and hemp/marijuana will need high light levels to produce a good crop.
 
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I’ve grown both crops outdoors at this time of year at they do well, but they tend to grow slow with few side branches. I do have to keep in mind the different strains there are that are best suited for the time of year. Maybe an everyday light watering can help with the low humidity issue? Either inside or outside the greenhouse. The thing is that I’m in search of the perfect conditions to grow these plants and humidity definitely plays a huge roll. Maybe I’ll stick to outdoor growing until I have a bigger greenhouse (at least 3x4 m) where I guess I’ll be able to manage the humidity and temperature better.
 
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Overall, that sounds good. By 'everyday light watering', do you mean just spraying the leaves?
 
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Overall, that sounds good. By 'everyday light watering', do you mean just spraying the leaves?
No, I mean to water the pots lightly everyday, instead of watering every 4-5 days (at least now that they’re seedlings in big pots). I would consider misting constantly but I fear direct sunlight could burn the leaves.
 
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Watering lightly everyday is usually not a good technique. It can lead to soil that stays overly saturated. It is better to water deeply and then let the container drain and partly dry before the next irrigation.
 
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Regarding your solar loading, two things might be controllable and suit you.

The solar spectrum heat comes from roughly 49%Visible, 49%IR, 1UV.

To cool while maintaining visible light you can increase glare of angled light with frosted film.

Glass absorbs a surprising amount of IR, but further reduction can be made preferably externally with solar films. Plastics can pass most IR or be treated to stop most IR.

We put 80 % vlt solar films on car windshields because they absorb 90% of any IR that makes it through. Car glass is usually 75%VLT and at least 60%IR filter if not 80%IR on solar ray types. The films handle the difference coming through onto you and add interior absorption where the shiny glass was acting as a retentive mirror for heat. This helps heat leave for passive cooling. It never equals ambient temps but is much better.

There are also uv sensitive films of PET and Urethane that self darken in sunlight to a specified range of VLT.

There are also electrically switchable frosted films that can be controlled via thermostat if you have power or batteries out there in the greenhouse. They are clear or frosted based on the switch on or off. They need a glass substrate for mounting.

Anyway, IR control can be a challenge because it is invisible. Heat flows from the high to the low so interrupting that flow is to control emissions from one source to another, radiantly, convectively or via contact. Heat rejective surfaces are used for retention in cold climates by facing them inwards so usage depends on orientation. In warm climates they are faced or layered on the outer skin of a structure. LOW-E glass for you would have 2 layers of glass where the outer layer has the IR mirror coating. This would produce a 40f degree difference between the outer and inner layers of glass here at 33° latitude. A space separated outer layer of frosted translucent plastic can act similarly in your environment. Its best to have some electrical, even battery fans, to mechanically balance heat but controlling the radiant barrier helps also.
 
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