Temperatures get steamy during the dog days of summer. Without proper cooling, your vulnerable greenhouse plants will soon become shriveled up, heat-blasted husks.
The best ways to cool a greenhouse without electricity are: maintaining good ventilation, periodically wetting your greenhouse surfaces (damping down), and blocking solar radiation through strategic use of shade,either from shade cloth or other sources.
Yes, cooling a greenhouse is expensive. Bills from greenhouse fans and other powered cooling systems can easily become frighteningly high.
Since you’ll probably want to be saving all your money for frozen margaritas, it begs the question: How can I cool my greenhouse without using any electricity?
The Ventilation Situation
What's the purpose of a greenhouse? To trap heat from the sun.
But what if you trap too much heat--like what happens in the summer?
Well, you need a way to let that heat back into the atmosphere.
Enter ventilation: the low cost way to get sweltering, used-up air out of the greenhouse, and let fresh air in.
Fresh air from outside the greenhouse is not only cooler than the air inside it, but carries vital carbon dioxide which plants need to conduct photosynthesis, as well as oxygen, which plants (and humans) need to conduct cellular respiration, the process wherein living organisms produce ATP.
Why ventilation cools a greenhouse
The most cost effective way to vent your greenhouse is to let the physics of air flow do all of the hard work for you by taking advantage of a concept called Thermal Buoyancy.
(Be sure not to confuse this with Thermal Beyoncé, which is where the sun’s rays cause all of your plants to break out into Single Ladies at the same time).
What is thermal buoyancy?
You already know: hot air goes up, and cool air stays down.
Manual ventilation to cool a greenhouse
So, we need a vent, or vents, on the roof and high up on the walls to give the nasty ascending air a place to escape.
The hot air leaving from the top will create a vacuum near the bottom of the greenhouse. And that's where we’ll want some more vents so the cool, CO2-rich air from outside can enter the greenhouse. This sort of passive, or manual, ventilation will work best when there’s a decently strong wind blowing.
Manual vents, as you may not be shocked to learn, must be opened and closed manually. Open them during the day after sunrise, and close them at night once it starts to get cooler. Be sure not to forget, or your plants will pay the price!
So, how big do the vents need to be?
A good rule of thumb is to have your overall ventilation area equal 40% of the area of your greenhouse floor. Typically, that gets divided into 20% for the roof and 20% on the walls (make sure the roof and wall ventilation are equal).
Let’s say your greenhouse floor is 10 ft x 10 ft, or 100 square feet. You would need a total of 20 square feet of roof vents and 20 square feet of side wall vents.
The process of damping down is a simple one. But, it's very effective at helping protect your plants from the intense heat of summer.
All you need to do to damp down is get your hose and spray down hard surfaces. For example, stone walkways, cinderblocks, and wood--just spray them down with water a couple of times a day. That way, the evaporating H2O will create a moist, humid environment.
Remember: evaporation--turning a liquid into a gas--requires energy. So, the remaining liquid--or the solid on which the liquid evaporated from--cools down.
Basically, water takes the energy needed to evaporate from the surrounding materials, thus cooling them off.
Making the air in your greenhouse more humid is miserable for humans, but quite helpful for plants.
During hot periods, increased humidity slows down the rate at which they perform transpiration. Transpiration is when plants release water from their leaves into the air around them. You can think of it sort of like sweating for plants.
Rates of transpiration increase in dry environments, and decrease in humid ones. If the air around the leaves is already full of water, then the water inside the leaves is more likely to stay there. And keeping water inside the plant means it stays where it's needed. This is what makes damping down an effective way to decrease the water loss your plants experience during time of heat stress.
There’s really no limit on how much you can damp down your greenhouse. The more the better. But for the sake of convenience, doing it twice a day when you’re opening and closing your vents should be sufficient for most climates. And best of all, damping down is totally electricity independent.
All the heat-related woes you & your plants experience during summer are caused by solar radiation. That's just the scientific way of saying “sunlight”.
So it stands to reason that blocking incoming sunlight will help in keeping temperatures down. This is where using shade can come in handy.
The main two ways shade can be provided to a greenhouse are via curtains/cloth or via paint. Which one you decide to use is based largely on personal preference, future greenhouse plans, and budget. Both are way less expensive than trying to cool a greenhouse using power, though!
One thing to keep in mind when utilizing shade is to not block out all of the sunlight.
Reducing incoming solar radiation is great because it keeps temperatures down, but plants need light to photosynthesize and stay alive.
If you block out all of the sunlight your plants will be nice and cool but they’ll also be dead. It can be tricky to work out exactly how much light you should be blocking and when. You’ll have to have an intimate knowledge of the light needs of your plants and the typical amount of light they’ll get in your specific climate.
Shade paint, often referred to as something like “liquid shade” or “shade compound” is inexpensive. And, it blocks solar radiation and is applied to the outside of a greenhouse like a normal coat of paint.
Shade paint needs to be reapplied every year. And, once applied you can’t adjust how much sunlight you’re blocking and how much you’re letting in.
In fact, shade paint used to be the standard method for reducing summer temperatures and overheating inside greenhouses. However, shade cloth and ventilation are better, more modern methods that cool greenhouses more efficiently.
Shade cloth, on the other hand, is basically a big curtain that can be opened or closed at will, and is therefore more versatile than shade paint.
Whether the curtains go inside or outside is up to you; if you are expecting heavy wind or rain, inside may be better but in gentle conditions having the cloth outside helps reduce heat even more.
Shade cloth has a few advantages over shade paint. Shade cloth:
- Is available in varying strengths.
- Can be added or removed more easily than shade paint.
- Is installed outside the greenhouse, which prevents heat transfer onto the greenhouse itself.
As you may remember from the preceding paragraphs, transpiration is kinda like plant sweating, and increases accordingly with increased heat.
Water travels from inside the plant to the surface of its leaves, and then evaporates. Just like how humans need to replenish water lost via sweat, transpiration needs to be answered by more frequent watering. But be sure to use normal water, not Powerade.
Misting is similar to damping down in that it increases the humidity of your greenhouse, but is more direct. Get a spray bottle or misting pump (they exist, the moms at my summer band camp used them to spray down the flute section) and go crazy.
Other problems from excessive heat
Warm Weather Pests
Increased power bills aren’t the only thing that breeds during the summer. Pests thrive in the hot humid environments inside your greenhouse. And, they’re often able to sneak in through open vents. Since they’re all inside, there are no natural predators to take care of them for you. Aphids, sweet potato whiteflies, leaf miners… all sorts of creepy-crawlies love a hot greenhouse.
It’s next to impossible to fully prevent insects from entering your greenhouse. But you can cut down on their invasion by thoroughly inspecting any new plants you buy, and keeping your vents well maintained so they completely shut when closed. It’s also a good idea to try to sweep away any puddles of standing water caused by damping down.
Certain fungal infections do well in summer, too. Check and see what sorts of disease are prevalent in your area, and learn how to identify them. There’s no quick fix for disease, unfortunately.
Your best bet? Expect what’s coming, and prepare for when it does.
Can I revive an overheated plant?
Common symptoms of an overheated plant include flaccid stems and wimpy, wilting leaves. When plants don't get adequate water, they get stressed. If you think that one of your plants is suffering from overheating, don’t despair. Move the plant into the shadiest part of the greenhouse for a little while, and give it extra water. This may be enough to revive it.
If you notice however, that major parts of the plant have become brittle and crumbling, or squishy and rotten looking, it’s too late. Sing “Amazing Grace” and get the tombstone carved.
What temperature is too hot for a greenhouse?
Photosynthesis decreases above 94 degrees Fahrenheit, so a greenhouse should kept cooler than that--unless you're growing cacti or other hot-climate plants. Plant tissue for most temperate-climate plants begins dying around 115 degrees, but cucumbers can sunburn in 100-105 degrees.
How to keep a greenhouse cool in the desert?
Keeping a greenhouse cool in the desert is done mainly by evaporative cooling. Since, by definition, deserts are dry, the air can hold more moisture. So, using an evaporative cooler can be very effective. Other methods include shading and ventilation (which is a component of evaporative cooling).