Greenhouse & Indoor GardeningVentilation, Heating & Cooling

How to Keep Your Greenhouse Cool in Summer

During the first summer with my new, clear greenhouse, I quickly discovered that keeping it cool was going to be my main priority if I wanted any chance of growing any plants, let alone heat sensitive plants like tomatoes and cucumbers. Remember, just because it is 70 degrees outside, which is comfortable for most plants, in a greenhouse that can equate to 120 degrees or more, depending on glazing/outer covering, exhaust fans, base venting (for cross circulation), humidity and growing medium, soilless, or hydroponic to name a few.

Therefore, my first accessory was a shade cloth specifically designed for greenhouses. I had witnessed over the years prior to my foray into greenhouse gardening that tarps do not hold up to the extreme sun or wind that we are prone to in South Dakota. Plus, intuition told me that a tarp would only trap more heat between it and the greenhouse, and that was something I needed to avoid. So, I bought a shade cloth that provided 50% shade, because I didn’t want to completely eliminate the beneficial light of the sun — just the heat.

In addition to the shade cloth, I began to utilize a technique called dampening down by spreading 25 bags of play sand in the greenhouse walkway. With this technique, you wet the floor and inside of the structure with water, with the understanding that as the water evaporates it cools the greenhouse. Of course, I had to be extra careful not to get the electrical plug-ins wet, because this created a whole different set of problems, like shutting off the exhaust fan until the water evaporated.

This technique was somewhat helpful, but only when I was available to consistently hose down the inside of the greenhouse, which on a day with outside temperatures climbing into the 100’s meant running out to the greenhouse every 15 minutes. Now, I felt like I had two jobs instead of one, and it was quickly becoming an unwelcome chore. After that first summer, I spent the winter months trying to devise a more practical method of cooling the greenhouse for the next summer. I made several trips to the local commercial greenhouse in the hopes of formulating a workable plan.

My next expense was to invest in an evaporative cooler, because that is what the commercial greenhouse utilized, though on a much larger scale. However, after buying four small units, then investing in a much larger unit without much success, the idea was scrapped after the second year, along with the greenhouse structure itself. It became clear that a hobby greenhouse wasn’t going to be compatible in South Dakota. After the second frustrating, unsuccessful summer “growing” season, I bought a greenhouse that offered diffused light, which turned out to be the smartest investment I’d made.

Then, after attending a trade show, I switched directions completely with regards to cooling systems for a hobby greenhouse. I started experimenting with various types of misting systems but unfortunately this had limited success. That is when the light bulb went on, and I researched the difference between misting and fogging systems, and I had that Eureka moment.

You see, there is a difference when it comes to misting and fogging systems and that is the particle of water emitted by each system. The fogging system reminded me of the times I’d been to a rain forest, and the fine mist that hung over everything and it was always hot and humid. But I knew the principle would work here too, because in a semi-arid environment, there is little to no humidity, so the evaporation would really cool the greenhouse — and it did!

From the beginning, it was necessary to tweak the design of my fogging system. First it had too many fogging heads, which kept my tomato plants too wet, so I created a barrier or dry zone around the plants that needed less moisture. I then wired the system into a Humidistat so I could control the amount and duration of the water, then stood back in the dry zone and proceeded to be amazed.

As I stood in the dry zone watching the system cycle on and off, I was able to determine just how long the water particles hung in the air, before being dispersed by the constantly running exhaust fan, and set my Humidistat at 30% for optimum cooling. With the use of the fogging system in conjunction with the exhaust fan and shade cloth, I can keep my greenhouse, on average, 20 degrees cooler than the outside temperature and by creating the dry zone; my tomatoes produced more fruit than ever before. I am assuming they received the benefits of higher humidity levels, without having to deal with the stressors of heat and the hot southerly winds, which wick the moisture from everything.

I also learned that the calculating formula of length x height x width, used by most greenhouse manufacturers to determine the size of an exhaust fan was an oversimplified system at best. This only taught me that when it came to exhaust fans, the old saying “bigger is better” was the only axiom that stood the test of time in my semi-arid part of the state. So, if you can afford the next size or more up, buy it and then have it fitted with a variable speed motor that is thermostatically controlled.

Paula M. Christensen has over 20 years of experience gardening year-round as a hobby greenhouse gardener and incorporates natural and organic feed and insect management practices.

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