Indoor plants do better with higher humidity and there are several recommended ways to create this increased humidity. Some of these suggestions, like misting and pebble trays, have been used for many years and are still being recommended today. But do they work? Do they increase humidity and are they good for houseplants?
The heating we use in winter dries the air in our home and this makes it more difficult for plants to grow. The same thing can happen with air conditioning in summer. Tropical houseplants are particularly vulnerable.
What is the best way to increase the humidity around your indoor plants and how high should the humidity level be?
Best Humidity for Indoor Plants
There is no such thing as a “best humidity” level because this depends very much on the origin of the native plant. Cactus in a desert need very different growing conditions than a plant from the jungles of Central America.
Plants growing in the tropics experience humidity levels in excess of 80%. Many of these plants will do very well in the 70 to 80% range, but that is too high for most heated homes in winter. Most common tropical houseplants will do well with a humidity around 50%. As humidity drops below this, plants start having trouble.
Plants collect water through their roots. This water moves up the stem, into the leaves, and then it escapes into the air. Low humidity levels can be compensated with more water at the roots but each plant has a limit as to how far you can take this.
Tropical indoor plants will struggle at humidity levels below 40%. Low humidity causes plants to dry more quickly, have a lower sheen to their leaves and increases the likelihood of spider mite problems.
Homes heated in winter tend to have humidity levels in the 10 to 20% range, but this does depend very much on the local climate.
Plant Signs of Low Humidity
Low humidity will cause the following.
- brown edges on leaves
- wilting plants
- leaves turning yellow
- foliage dries out and becomes crispy
Humidity in Relation to Temperature
When people talk about humidity, they are really talking about “relative humidity”, which is the humidity relative to temperature.
When air is saturated (i.e. can’t hold any more water) it is at 100%. Warm air holds more water than cool air. Air a 50% humidity and 80 F contains about twice as much water as air at 50% humidity and 60 F. For a given amount of water in the air, lower temperatures will provide plants with a higher relative humidity. Cooler temperatures also slow down growth making any humidity issues less critical.
Evaporating Water and Humidity
The air in your house will contain a certain level of humidity. As the amount of water vapor goes up, humidity rises. When the amount of water in the air goes down, humidity goes down. What causes these changes in water?
Cooking and showering are two significant contributors. They both create steam which is just water vapor. Breathing also adds water to the air. Anytime water or a liquid containing water is exposed to air, some of the water evaporates, turning into water vapor which increases humidity. The heating system reduces humidity.
Air coming in from outside can either increase or decrease humidity depending on the outside weather conditions. Cold winter air can be very dry (cold air holds less moisture), but a warmer rainy winter day can be quite humid.
Increasing Humidity With Pebble Trays
One of the most common recommendations for increasing humidity around plants is to provide a pebble tray, as pictured above.
I learned this was a complete myth 40 years ago when I was growing orchids and it surprises me that most of todays houseplant advice still promotes them. They don’t work and here is why.
Water does evaporate from the pebble tray and this increases the humidity right above the water (purple dots in the left side of the diagram). Once the water is in the form of a gas (water vapor) it quickly moves away from the pebble tray, in all directions, by a process called ‘diffusion’. The water molecules spread out throughout the entire room and if the door to the room is open, they spread throughout the whole house. The amount of water added to a room from a pebble tray is so small it does not change the humidity of the room. The humidity a few inches above the pebble tray is the same as the whole room.
The same thing happens when you cook or have a shower. The humidity from the steam does not stay above the stove or in the shower. It quickly moves into the rest of the house. Long before dinner is over the humidity above your stove has disappeared.
This change in humidity above a pebble tray was tested by an orchid enthusiast. He found that in winter (see top meme), the humidity at 1.5″ above a pebble tray was 3% higher than the rest of the room. At 4″ this dropped to 2% and at 1 foot above the tray there was no increase in humidity. Increasing humidity by 1% or 2% has no significant effect on plants.
Pebble trays don’t work.
Increasing Humidity By Misting Plants
Another common technique is to mist plants. Misting does increase humidity around the plant. But for how long?
When the mist lands on the plant, it sits there as water droplets. Over time, this water evaporate into vapor and spreads out in the whole room just like the water from the above pebble tray. That means the humidity around the plant will only be higher while there is water sitting on the plant. Once the water evaporates, the humidity quickly drops to that of the room.
How long does your plant stay wet after a misting? Give it a try and see.
The time will depend on how large the droplets are and on your room humidity. At high humidity it evaporates slower and at low humidity it evaporates faster. At 100% humidity, like you find in some tropical greenhouses, it never dries up.
In a dry winter home you will be lucky if it lasts 15 minutes. That means you need to mist every 15 minutes to keep the humidity high. Who is going to do that?
Misting can also cause fungal growth on some plants.
Unless you are growing some special plants like air plants, misting is a complete waste of time.
Use a Terrarium to Increase Humidity
You can set up an aquarium, put some stones in the bottom and set your plants inside the aquarium. This will provide much higher humidity. If you cover the aquarium, you can get close to 100% humidity, even without water siting in the bottom.
The problem with this setup is that some plants will get fungal diseases and die in such humidity. Humidity can be regulated by using different types of covers or by partially covering the top. A full top will give you 100% humidity. A humidity of 50% is achieved with 30% covered and you get 70% humidity with 90% coverage (these numbers are approximate).
Why Do Terrariums Work and Pebble Trays Don’t?
The air space right above liquid water does have a higher humidity but as soon as the water is in the form of a gas it starts moving in all directions. In a pebble tray it can move up as well as sideways so it leaves the area quickly. However, the glass walls of a terrarium prevent sideways movement. The water gas can only go up, which slows down their movement and results in the space inside the aquarium having a higher humidity. Once the water molecules reach the top of the glass, they start moving sideways as well, so the air above a terrarium is no more humid than the air above a pebble tray.
Give Plants a Bath
Some people suggest giving plants a shower to increase humidity. This might be a good way to clean indoor plants, but it does nothing to increase the humidity long term. As soon as they are dry they are back in a dry environment.
Place Plants in a Humid Room
Bathrooms and kitchens tend to be a bit more humid and some suggest placing plants there. How beneficial is this? Right after a shower the bathroom is humid, but unless you close the door to trap that humidity it soon dissipates into the rest of the home and modern homes have a system for pulling out the humidity even if the door is closed. Kitchens are similar. They get humid around meal time, but they are dry the rest of the day.
This suggestion may work in some cases, but I suspect it makes little difference.
Many Plants Increase Humidity
Putting all your house plants in one spot can be effective. Each plant transpires water out of its leaves, creating a higher humidity layer around leaves. When several plants are growing close together, they can generate enough water to keep the area more humid. Unless you have a lot of plants, or a small room, this effect will be limited.
Increasing Humidity for Indoor Plants
So how do you increase humidity? The only way that really works is to increase the humidity of the whole room or house. This is best done with a humidifier placed either in the room or on the furnace. Such a system not only benefits the plants, but it also makes you feel more comfortable.
The other suggestions given are not going to be of much help, except for the terrarium, which causes problems of its own for many plants.
there are a lot of believers on this thread of Lamarkian evolution! the seeds of plants do not reflect the growing conditions of the parent plant. That having been said, I have found that many plants are highly adaptive to indoor conditions and it is always worth experimenting if you have your heart set on a tropical plant. I live in the northeast with extremely dry conditions indoors (about equivalent to high desert climate) and I grow SOME orchids (oncidium, dendrobium, phalaenopsis) successfully, for example. I don’t do anything special for exactly the reasons described in this post but luckily the plants seem to respond nonetheless. Otoh, I have also had failures (virtually every other kind of orchid I’ve tried; gardenias)
It is my understanding that orchids like high humidity and collect most of the water they need through humidity around their roots. Here is what I’ve done in an effort to create a humid, but not overly wet environment, around my orchids’ roots. I choose a tall nonporous container, such as a pot or vase, and place several inches of stones in the bottom, sometimes filling up to half of the container. On top of the stones I plant my orchids in their bark based potting medium in a well ventilated grow basket. When I water the plants the drained water collects in the pebbles at the bottom so that as the excess water evaporates it is forced to pass through the growing medium above to diffuse and thus exposing the roots to higher humidity. The more closely the top of the grow basket and the top of the stones’ container fit the more humidity is trapped. This may also somewhat increase humidity around the plants foliage as evaporated water escapes at the surface, but this primarily benefits plants that appreciate high humidity in their root zone. Thoughts?
What you describe works – you are trapping the humidity around the roots.
Hi there,
So what I have been doing for my plants is placing some crescent/ moon shape trays on top of the soil next to the stem inside the plant potsbso that when the water evaporates it’s gets some direct humidity. I feel like this works better than pebble trays which are on the bottom. .what’s your take on this?
Thanks
Said
It is better than a pebble tray, but it is better to just humidify the room.
which is needed more light or humity
i have Calantheas and Maranta- i just bought a light from amazon-EZORKAS 9 Dimmable Levels Grow Light with 3 Modes Timing Function for Indoor Plants. My room does not have good lighting. i have it set for 3hrs starting 4-7 is that enought to do anything…
i do have a pebble tray. i like it because i combine three plants together.
Plants grow best when they get everything they need. Nothing grows in the dark. Nothing grows without water. Neither is more important.
Nice! The information I got through this blog has really helped me in understanding this increasing humidity for indoor plants. That was something, I was desperately looking for, thankfully i found this at the right time.
Thanks for another great post. I have never tried humidification of the air, of any sort, since I cannot see that I need it (except for brown leaf tips on my Chlorophytums!).
I would suggest that in the winter in cold climates, a window sill will often have higher rh, if there is no heater panel below the window. I know that in my home, some parts of the window sills approach 100% rh, since I sometimes observe condensed water on the window glass.
I would also suggest that a water (pebble) tray could work better than reported in the study, if placed in a window sill with a heater panel below. Because the convection air flow from the heater would make the gaseous water pass the leaves.
Thank you so much for posting this. I now have my old humidifier hooked up and going. This was the one I believe I had when my children were babies. I am using it to keep my begonia brevirimosa subspecies exotica in top form. This plant is worth it with itś unreal iridescent pink leaves.
Or add multiple open top aquariums to your room. And have hanging plants and plant shelves over the aquariums. That is what I have in one of my rooms. But then I am in Louisiana and we nearly always have high humidity. I don’t think I could live in a desert – I would dry up. 🙂
Thanks for an article that explains “why” , rather than just “how”. I will revisit my house plants and provide a better atmosphere for them – at least they mostly all live in the same room! Since it is late October, it is entering a great time to take advantage of this info.
Thank you for the precise explanations. it confirms my suspicions. In the late ’80’s I had the opportunity to spend some time at a research camp/tourist destination on the Rio Madre de Dios a tributary to the Amazon river. I had experience in sub-tropical climates but never tropical…to this degree. One of the things I recognized were the myriad “houseplants!” Some time later when it was time to leave the “jungle,” I was as rank smelling, damp, bitten and moldy as the scientists were. But I had learned some things that would have evaded me had I not been in that environment. It may sound kooky, but trying to maintain plant specimens that struggled to survive in the Mediterranean climate I called home, now seemed to be a moral issue. Just because we could transport them out of their best environment, didn’t mean we ought to do it. I’m not a fanatic about this. I feel the satisfaction of keeping an exotic plant just like most of my gardening friends do. But at least I am aware now, and I understand. If I can’t give them what they REALLY need, I ought to move on to something that can adapt to what I am able to provide. Again, thank you for condensing all that information and delivering it to the rest of us in such easy-to-understand terms.
Hi Mary you say “I understand. If I can’t give them what they REALLY need, I ought to move on to something that can adapt to what I am able to provide…”
My understanding is if you can collect seed from any given plant & grow it (the seed) on successively, year by year, multiple times, eventually you will have a plant that is tolerant of the particular conditions to some degree that you choose to keep it in.
Hi Jo,
Well I think you are talking about natural selection, if one chose the seeds from the hardiest plants, which is how agriculture worked/works.
The development of wheat is a good example. Imagine having to gather nutritious kernels from the ground, because earliest forms of this specialized grass, “shattered,” (a term for all the kernels falling off the head while it’s standing in the field). Oh lordy! So a grain crop that didn’t shatter, but would release its kernels after harvest, Bingo! But it would take generations for me to develop hardy plants that I would love to be able to grow.
Plants don’t “learn” to tolerate the climate, but if it’s possible at all, genetically, hardiness can be improved by selecting the most hardy, or the mutations which make it hardy, in successive generations. I’m just repeating what I learned in High School science classes and a few univ. courses in my major. I’m sure there are some experts who could weigh in on this and give a much better response.
What I was trying to say was: We’re all limited in time and resources. I’ve grown to believe that I should put my efforts into what is suited to my climate, wherever I am. And I’ll admit, I’m seriously considering trying to grow a lemon tree by wrapping it in xmas lights as some folks have done with success here! (Vancouver Island, west coast marine climate). I would love to once again have a lemon any time I wanted one 🙂 With that confession, I’m trying to learn to LOVE rhododendrons, maples, rugosa roses, lavender, green leafy veggies and potatoes…..and forget eggplants and a whole, long summer of sweet tomatoes! (sob!) (I’m a transplant from northern California, mediterranean climate).
Hi Mary Ann, as you say “…some experts could weigh in on this and give a much better response.” We have very poor soils where I live in WA & we are told to collect the seed from tomatoes successively each year ie year after year, & eventually one will get a tomato that is suited to local conditions, soil & temperature etc
Regarding Lemon trees I believe some varieties are hardier than others, ever though of have a pot grown one on a south wall, & bring it in on the coldest of days.
You’re both right. Fundamentally, changes within an environment alter the structure of DNA, which can be inherited by progeny.
By growing a plant in ANY environment you’re changing it; either trying to replicate it’s desired conditions or adapting it to yours. Harvesting the compatible progeny then reinforces these traits, rinse and repeat.
Personally I gain the greatest enjoyment adapting exotic plants and animals to my conditions and watching how they evolve.