Summary: Relative humidity levels directly influence the transpiration process. Excessive dryness slows down and often halts the rate of transpiration, resulting in curling tips, browning, and wilting. Establishing the right indoor humidity level will enable our plants to stay supple, be healthy, and flourish.
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I don't know about you, but I can tell when my plants are happy.
I'm by no means an expert, but it's been a few years since I started to really look after my plants more seriously. I took a neighborhood botany & indoor gardening class, and to this day, one of the most important and impactful concepts that help shape my plant care routines is humidity management.
Don't get me wrong - humidity is just one part of the puzzle. In order to really establish the proper water balance in your plant's environment, there are so many factors like plant type, stage of development, soil moisture, soil pressure structure, and air & soil temperatures (just to name a few) that go into determining how often and how much to water your plant.
My goal in this post is to boil down the science of humidity into a (hopefully) easy to digest explanation of how and why humidity has an impact on your plant's health. Feel free to skip to the second half where I share my tips & tricks for keeping humidity levels just right in your household, but I encourage the full read!
What is relative humidity and why does it matter to my plants?
Here's a quick explanation of relative humidity.
Compare how much water vapor is in the air now with the maximum amount of water that the air can hold, and that's your relative humidity. The only catch is that the relative humidity can change with temperature. It's a bit confusing at first, but long story short, if you live somewhere dry, your relative humidity level is more often than not much lower than what it should be for most indoor plants to thrive. This problem can easily get worse if you're cranking the A/C or heater throughout the summer/winter. I'll leave a link at the end of this post to a hygrometer, which is what you can use to monitor the humidity level near your plants. Before we get into any ideal numbers to strive for, let's go over how plants behave differently in more humid environments.
So why does relative humidity matter?
A plant's transpiration process is very much like what breathing is to a human being. Air flow is important to us as humans, and similarly, water flow is important to plants. The easiest way for us to influence how much a plant "breathes," is through temperature, light exposure, and humidity.
Bear with me as we get a bit more scientific, but I promise it will all make sense in the end!
Plants "breathe" through the undersides of their leaves - stomata. The opening and closing of the stomata is what governs the overall water flow throughout your plant. Your plants get their water from the soil through their roots, and most of this water actually ends up getting pushed up and out onto the surface of its leaves, eventually evaporating (cohesion-tension theory). As water molecules evaporate, they pull adjacent water molecules to replace them, and this is what creates the continuous flow of water throughout our plants.
I've attached a link at the bottom of this post where you can learn more in depth about how the stomata enables H20, CO2, and O2 movement, but for all intents and purposes of this explanation, you can think of the stomata as "waterways." When the weather gets warm, the stomata will close more frequently in an attempt to reduce water loss. When this "waterway" is closed for too long, your plant can begin to suffocate itself within its own transpired gases. This is why we have to create environments that allow for our plants' stomata to remain open and for water to flow comfortably - in other words, we've got to help our plants breathe when it gets too hot or dry.
When you increase the relative humidity, the stomata "waterways" stay open longer, allowing for transpiration.
OK - so more humidity is generally a good thing in the context of indoor environments, but I do want to quickly go over what too much humidity can do to your plant.
Remember how I said that what keeps everything flowing is the evaporation of water? If it gets too wet & humid, then water actually won't evaporate fast enough or sometimes even at all. This creates an issue called cavitation which is basically a blockage of large chunks of water vapor molecules. This is really bad, because if your plant can't break through this blockage on its own, you'll find yourself Googling "how to revive a dying plant," but don't worry. Most of us raising indoor plants would literally have to recreate a tropical rain forest in our homes to get anywhere close to this kind of danger. Plants have a much higher risk of dying to dryness and heat than to an overly humid environment. All in all, a large goal of every plant caretaker is to balance the relative humidity to create a proper water potential gradient to allow for ample evaporation and transpiration rates.
Like humans, plants have to sweat and breathe to keep their systems going.
And that's it for now - no more scientific talk.
I've attached a bunch of links at the bottom of this article for those of you who want to do a deeper dive, but I imagine that most of you reading this post are looking for ways to impact the relative humidity in your homes to try and offset the impending summer heat waves & winter dry spells.
Here are my top tips & tricks for keeping humidity levels just right.
1. You guessed it - add a humidifier or two to your collection and run them throughout the day.
Don't run them throughout the night, as there won't be enough sunlight or heat to ensure a healthy evaporation rate. The last thing we want is to suffocate our plants with their own water vapor. An industry classic is a Honeywell humidifier, but we actually recommend our very own Stella humidifier, which is a dual mist volume-adjustable ultrasonic humidifier. You can find one of these here. Humidifiers are by far the easiest way to add humidity to your plant's immediate environment.
2. Arrange your plants in clusters / groups.
By grouping your plants together, they will essentially serve as humidifiers for themselves. This is called "green-walling" to create a micro-climate. When one plant transpires and water evaporates into the air, it increases the relative humidity of the area, so the transpiration of one plant ends up being the humidification of another. Be sure to leave enough space between your plants so that there is still a good amount of air flow. A general rule of thumb is to make sure that your plants aren't touching each other and this should ensure that they have more than enough space to breathe. By grouping your plants together, you will have created an area where the relative humidity is higher than that of the rest of the room.
3. Get a spray bottle and mist away.
Not to be confused with running a humidifier throughout the day, morning mist is just taking a spray bottle and mist-ing the exterior of your plants in the mornings. This is an easy and quick way to get your plants' days off to a good start, but it's high maintenance because you'd have to mist them every hour or so to keep the effects going. It's a good idea to mist your plants at the start of a season, but definitely do not mist certain plant types (i.e. ones with hair) because that will surely harm them. You can check out Gardenista for plant-specific guidance.
4. Pebble trays aren't just for show.
I used to see this used by a lot of the plant bloggers I followed, and I honestly though it was purely for aesthetic. However, using a pebble tray and filling it up so that it's damp in the mornings make it so that throughout the day, the evaporation of water makes its way up to your plants' space and serves as a humidity buffer. This is a great complement to having a humidifier or misting throughout the day, but keep in mind that if you don't keep your tray hygienic, it can serve as a breeding grounds for bacteria & insects. Definitely clean your pebble trays at least once a week.
5. Measure & make adjustments.
I mentioned this earlier, but definitely pick up a hygrometer if you're new to plants. Although it's often intuitive to be able to tell if your plant is "happy" or not, it's almost impossible to micro-manage the humidity level without a hygrometer to tell you what the percentages are.
When your plants are in a growth/vegetative stage, preferred relative humidity can range between 50-60%. This number tends to drop down to around 40% for when they're in a more mature stage. Even if you're raising succulents, a little bit of mist can really go a long way during the colder months.
Some symptoms of low humidity are brown edges, wilting and/or yellow leaves, and crisping. Nothing is sadder than watching the plant you've grown attached to die, so be sure to catch these issues early on and make adjustments!
For seedlings and especially after cutting/trimming, stomata is closed to prevent excess water loss. Run a humidifier throughout the day to help offset this!
One last thing while I still have your attention. For those of you who follow me and are familiar with my posts - you know the deal. For you new readers out there, I want to be completely honest and transparent with you!
Although we do in fact offer our own humidifiers here at Missed Global, my intention is to really just compile my learnings & experiences and share them with you! I really encourage you to learn and read more on how to establish and maintain a healthy environment for your plant buddies to grow and thrive. Even if you choose to add humidifiers that aren't from our collection into your homes, my team and I will be more than happy just to hear that our post made a difference!
Humidifiers are really incredible plant and human companions. As promised, I've attached a number of other cool sources and sites to help kick-start your reading journey and explore the heaps of information that is available out there!
If you have any questions, thoughts, or concerns, feel free to reach out to our team via our contact page or leave a comment below. Thanks so much for reading, and I wish you all the best in your plant-caring journey!
- Written by Henry J.
Lead Product Specialist & Orchid Enthusiast