In colder climates, the fall is a good time to kill a few plants. It happens when you close your greenhouse at night because a frost is forecast and then forget to open it in the morning before the sun heats it up very hot. I did that a few weeks ago when I had an early-morning appointment. When I arrived back home, the greenhouse plants were pathetically wilted in the 130oF (60 oC) heat. If this is what happens on the coast of Rhode Island in early autumn, I can only imagine the struggles of a greenhouse owner in Virginia or Arizona during the fall and early winter.
What should you do in this situation? First, open all the greenhouse windows and let cooler air in. Unfortunately, however, this may not be enough when the outdoor temperature is 80oF (27oC) or more, as it was for me that day. To lower the temperature further, I hosed down the entire greenhouse, including the stone walkways. Because these stones were hot, the water on them evaporated quickly, bringing down the temperature with it. The spraying also helped to cool the plants themselves as the water evaporated off them. This is the same cooling effect you feel when you step out of the water after a swim. It’s why luxury ships and yachts often install misting systems to cool the air on deck. As the tiny water droplets evaporate, the temperature drops, often quite dramatically. A misting system in your greenhouse can provide the same benefit if the air overheats. So can bowls of water placed near your plants.
Another method of preventing overheating from happening in the first place is to install automatic window openers in your
greenhouse. These are typically set to open a window when the greenhouse temperature reaches 80oF (27oC) and then close the window when the temperature drops. If you decide to install them, be sure to pick something that can support the window weight. Also, you’ll need at least two of these devices – one near the floor to open a window that allows cool air in and another in the overhead to open a window that vents hot air out. Fans linked to a thermostat can also turn on to blow hot air out of the greenhouse. You might also run fans to distribute air around inside your greenhouse. They will help to keep it cool and help to reduce molds and mildew bit that is a whole ‘nother story.
What if you don’t want to invest in automated equipment despite knowing that you sometimes forget to open your greenhouse? If you’re very forgetful, you might consider changing what you grow. There are many quite attractive cactuses and other succulents that can tolerate both high heat and low humidity.