Most people who mist their plants do so with the intention of raising the humidity around them. This is a great goal, most of our houseplants are tropicals and do like a more humid environment. This is because plants thrive with the right level of humidity because their pores can be completely open, letting them "breathe" deeply without excessive water loss.
Unfortunately, misting our plants is not an effective way to raise the humidity around them. The humidity is only raised so long as the mist stays in the air, once it falls to the ground, that humidity is gone.
That water falling onto the leaves of your plants can actually have some unintended detrimental consequences too. It can lead to leaf malformation, rot, mould, or the germination of fungal spores, which thrive in a moist environment. It can also spread disease - especially if your plants are closely grouped. Water from an infected plant could drip down onto a healthy plant, causing it too to become infected.
Now, you may be thinking "That's BS. Plants in nature get rained on and deal with mist and heavy fog all the time, and they're fine"
You're absolutely right. Plants in nature deal with all of that, but they have one major advantage over houseplants when it comes to moisture. They're living in the open air. There is constant air movement outdoors, which means that moisture is going to evaporate off of the leaves quickly. Now, you could put a fan near your plants and leave it going, or keep all of your plants near an open window at all times, but most people aren't going to do that.
There's another reason for not misting I want to talk about. In many plants, exposure to rain or misting can rapidly slow photosynthesis. To understand why that is, we're going to get sciency for a minute.
The surface of a leaf looks smooth but is actually lined with tiny little openings called stomata. Each one is guarded by two cells that expand and contract, controlling the opening and closing of the stoma. Stomata have a couple of functions:
Function #1: During photosynthesis, stomata allow carbon dioxide to enter the leaf and oxygen to escape. When photosynthesis is not happening, they allow oxygen to enter, and carbon dioxide to escape.
Function #2: Transpiration. Transpiration is to plants what evaporation is to the water cycle. During transpiration (when the stomata are open) excess water that has been absorbed by the roots and not used by the plant is expelled as vapour through the stomata.
Plants open and close their stomata in response to the environment around them, for example, many plants close their stomata at night. Other environmental factors such as humidity, wet or dry weather conditions, light, temperature, and wind, control the rate of transpiration (how fast water is lost).
Now that we understand what stomata are, and their roles- let's talk about photosynthesis. Very briefly, photosynthesis is the process of taking the sun's energy and using it to combine water and carbon dioxide into glucose (a type of sugar). Then oxygen is released as a waste product of photosynthesis. I probably don't have to say this, but photosynthesis is how plant
s get the energy to grow.
When it boils down to it, even though stomata do not directly participate in photosynthesis they do have some control over it because photosynthesis only happens when they are open.
In nature, photosynthesis fluctuates as stomata open and close through the day. Often they open in the morning, close midday, reopen mid-afternoon, and then shut again in the evening.
Lastly, let's talk about boundary layers. The thin blanket of unstirred air (air that is not moved by wind) surrounding the surface of a leaf is called the boundary layer. It helps to insulate leaves against too much water loss because it becomes almost saturated with water vapour. Boundary layers are minuscule, we're talking millimetres here. Basically, it's the microclimate of the leaf and its thickness is dependant on the plant, air moisture, and how much the air
around it moves. Still air = thick boundary layer. Wind, or lots of air movement = thin boundary layer.
This is important to know because the air inside of our homes is fairly still, so when we mist, we're creating an almost perfect boundary layer. Meaning that the humidity inside the stomata and the humidity of the boundary layer are nearly equal. This closes the stomata, which means that the influx of carbon dioxide is not happening, which means that photosynthesis is not happening.
TLDR; Leaves are full of water, which diffuses through the stomata (the pores) of the leaves in accordance with the level of humidity in the air. Misting for as little as two minutes can slow transpirational loss by as much as 30 - 40 percent, which can result in slowing photosynthesis for the better part of an hour. No photosynthesis, no growth.
When it comes to adding humidity to our spaces, I am a huge advocate for humidifiers. They do the work we need them to do with minimal effort - and they don't cause the loss of photosynthesis like misting does.
Below are a few more resources, or in-depth explanations of each process I mentioned in this post, if you are interested in more information!
More in depth on transpiration -
More on boundary layers -
More on photosynthesis -