One of the most challenging tasks of indoor gardening is keeping the temperatures from becoming too hot or too cold for the plants.

“When it comes to cooling your greenhouse, there are many different options, but perhaps the simplest is natural ventilation.”

— Jessie Kelias, “Not Too Hot, Not Too Cold: How to Manage Your Greenhouse Gardening Temperatures, 2023

This spring, I’m experimenting with starting my warm-season plants from seeds in a sunroom that’s only a couple of steps away from my living room. This location is more convenient than my greenhouse, so I’m hoping it will be easier to keep the temperatures in the sunroom from becoming too hot.

At 1 p.m. on Monday, the temperature outside was 58 degrees, but it was already 80 degrees in the sunroom. I opened the screen doors and the windows, and the temperature dropped to 72 degrees within 20 minutes. Clearly, natural ventilation is an effective way to keep indoor temperatures cooler, so my seedlings won’t fry in the middle of a sunny day.

However, according to author Jessie Kelias, natural ventilation for enclosed gardening spaces is more than just opening the doors and windows. The process of natural aeration requires the use of vents strategically located on the sides of the greenhouse or other growing space, as well as on the ceiling or as high up the sides as possible.

When cooler air enters through the side vents and makes contact with the warmer air inside, the denser cool air sinks while the lighter warm air rises and is pushed out of the room through the top vents.

My sunroom doesn’t have ceiling vents, which makes it more challenging to expel hot air. The best I can do to maximize natural ventilation in my sunroom, short of installing ceiling vents, is to open the screen doors at either end of the room and use the two ceiling fans to help push the hot air out.

One of the reasons my greenhouse becomes unusable at this time of year is the lack of ceiling vents and ceiling fans to help move the hot air outside. Another reason is that I have only two vents positioned at the front of the greenhouse and no vents on the sidewalls.

Kelias says, “The number of vents you…have in your greenhouse is incredibly important … If you only have one vent in a giant greenhouse, then you are not going to get the degree of air circulation that you will need for your ventilation system to be effective.”

Fortunately, there are five sidewall vents in my sunroom that allow cool air to enter. The trick is to open the vents and the screen doors before too much heat builds up inside the room, creating a situation in which the cooler air coming in is not enough to offset the warmer interior air.

Wind speed and outside air temperature also affect natural ventilation. If I open my vents in the sunroom tomorrow at 9 a.m., the outdoor air currents will be moving less than 1 mph, thus limiting the amount of cooler air that’s pushed through the vents. By noon, the breeze will have picked up to 3 mph, but the increase in air coming through the vents is partially offset by the rise in outdoor air temperature — from 47 degrees at 9 a.m. to 60 degrees at noon.

Warmer outdoor air entering the sunroom through the vents is not as effective at moving the warm interior air out. Air that’s 60 degrees is still pretty cool, but as outdoor temperatures rise to the 70s and beyond, it will become increasingly difficult to cool the sunroom in the afternoon.

Of course, another factor to consider is the optimal indoor temperatures for my seedlings to grow. My warm-weather seeds (beans, cucumbers, eggplant, peppers, squash, tomatoes) will germinate and the shoots will grow happily in 85-degree temps, so I’m able to allow more heat build-up in the sunroom than I could for my cool-season crops (lettuce, peas, spinach, broccoli, cabbage).

In fact, my biggest challenge right now is keeping my seedlings warm enough at night — at least 60-65 degrees. I’ve had pretty good success by closing the windows and doors and setting an electric radiant heater close to the plant shelves to 70 degrees. Having energy-efficient windows in the sunroom also helps.

I’ve already discussed the importance of my ceiling fans in the sunroom to help move air around the room. The exhaust fan in my greenhouse is not as efficient because, as Kelias tells us in her book, this type of fan ventilation is best at creating airflow in specific areas — like “a beam of cool air” — rather than circulating air throughout the entire growing space.

Shade for my greenhouse and sunroom is another factor to consider. I’ve used a knitted shade cloth over the entire top to extend the use of my greenhouse in the spring, and it does help to reduce the direct sunlight coming in. Unfortunately, the greenhouse is exposed to late afternoon sun, and the heat buildup in the greenhouse outweighs the shade cloth’s cooling effects.

More effective are the two large sycamores that shade my sunroom when the trees leaf out in May. While the sunroom still receives sunlight in the morning, the sycamores help shield the room from the sun’s hottest afternoon rays. During the latter part of spring and throughout the summer, my sunroom is at least 20 degrees cooler in the late afternoon than the greenhouse.

Clearly, managing temperatures in a greenhouse or other enclosed growing space largely depends on knowing what you have to work with — in terms of your greenhouse and the external conditions that affect how air and sunlight move in and out of the structure. Next week, I’ll discuss how different types of greenhouse materials play a role in maintaining favorable interior temperatures. Stay tuned!