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
- 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.