Our greenhouse isn’t much to look at, just some old redwood fencing and plastic sheeting that we repurposed with the help of my dad, but its creation several years ago revolutionized my efficiency. Tom went to the trouble to figure out mathematically how to build the shelves inside to best capture the angle of the sun, particularly in winter. I don’t have anything in there right now, but starting March 1, the first batch of pepper seedlings will need to move in here to make room for tomato seedlings in the house. I’ll also start a lot of flower seeds, and squash/melon seeds at the same time, and just keep them in the greenhouse.
In an effort to maximize efficiency, I decided to buy a cheap greenhouse thermometer from Govee. This has allowed me to see, from my phone, the temperature and humidity inside it, without opening the door and letting out all the heat. If you allow Govee to have a lot of information about you, you can also make some neat charts and get a history of temps over time, wherever the thermometer is placed. I didn’t allow that, but I’m still able to collect enough data to get an accurate picture of what’s happening in there. Last week, I added a bowl of water to see if that would affect the humidity. I’m not sure it has, actually. But the data is interesting and it caused me to ask some more questions about humidity and its relationship to heat.
Here’s a morning reading:
You can see that temp and humidity are at about the same level.
Now, here’s an afternoon reading (the door is closed, remember):
Notice how, when the temperature rises, the humidity level goes low. I couldn’t figure this out, so I asked Tom to explain it to me. That allowed us to do some research together about it. Now, I know some of you probably already know this, but I did not, and I’m guessing some other folks don’t either. So bear with me as I share what I learned.
Relative humidity represents a percentage of water vapor in the air that changes when temperature changes. Warm air holds MUCH more moisture than cold air. So when it’s very warm, the air is ‘holding on’ to the moisture, and when it’s cool, the air can’t hold on to it, releasing it into the atmosphere. For instance, a rain cloud can only release its moisture when it cools down enough. That’s one of the reasons rain clouds are higher in the sky - it is colder as you get higher. 100% humidity makes a cloud, but it won’t release water until it’s cold enough to do so.
So, roughly, when the air in the greenhouse is at a very high temperature, it is ‘holding on’ to the moisture and I get a low humidity reading. But when it cools off at night, the air ‘releases’ the moisture, allowing it to be read by my thermometer.
Does that make sense?
Now, when there are plants in the greenhouse, they will be photosynthesizing; taking the sun’s energy and converting it to sugars to be taken to the root of the plant. While this is happening, the plant is transpiring - releasing moisture from the leaves. Photosynthesis ‘pulls’ the water up from the root of the plant to the leaf of the plant, where it is released into the air (there’s a complex process of using some of the water in the process of photosynthesis which we can leave out for the purposes of this conversation). Will this increase the humidity in the greenhouse? It won’t be happening at night, because there is no photosynthesis at night (for the most part - some plants do this but not the ones I’m talking about, usually only desert plants). It will be interesting to see the results of that.
The thing to remember is that air circulation is the MOST important thing about greenhouse management. If the air doesn’t circulate, the plants will have too much humid air around them, therefore they won’t be able to make any more water evaporate, and that means no nutrients will be coming up from the soil. So open your greenhouse doors during the day when plants are in there, no matter how much you want to keep them closed. Open the greenhouse during the day and close it up at night to keep things warm and snug.
