There are some things I feel the average houseplant grower doesn’t quite get and one of them is the importance of good atmospheric humidity.
And not just in preventing wilted leaves, brown leaf edges and aborted blossoms and other symptoms of water loss due to evaporation. Because good humidity improves photosynthesis: the plant’s use of light. And this is the ideal season to talk about this, as, with autumn upon us in the Northern Hemisphere, both light and atmospheric humidity are presently on a major downward slide.
Humid Air, Happy Plant
Under high humidity, most plants actually photosynthesize better and thus grow better. Improving your plant’s humidity is therefore the equivalent of giving your plants an extra dose of sunlight … even if you don’t give them more light!
Can Plants Obtain Energy From Humid Air?
It’s not quite that. But high humidity can help compensate for decreased photosynthesis.
It’s important to understand that when the air is too dry, the plant reacts by trying to reduce water loss. It does so by closing its stomata (singular, stoma) or breathing pores so no more water can escape. Found on plant leaves, mostly on their underside, the stomata shut down either partially or entirely. Under home conditions, plants already lose about 97% of all the moisture they absorb to transpiration (evapotranspiration to be more precise). They can’t afford to lose too much more!
So, when the air is too dry, they slow down their respiration or even stop it entirely. Therefore, with little or no respiration going on, photosynthesis is not being carried out efficiently. And your plants therefore aren’t using the sunlight that’s available to them.
So, your plant could be sitting there in what should be just the right amount of natural sunlight for perfect growth … and actually be going downhill rather than growing because the air is too dry!
Terrariums Show the Way
Terrariums are great examples of this phenomenon.
You can’t put a terrarium in bright sun, especially not a closed one, as heat will build up and the plants will bake. We all know what happens when you park a car in the sun, so you get the drift!
That means gardeners are obliged to put their terrariums in shady spots, far back from the window. Usually spots where they receive little to no rays of direct sun. “Too bad for the plants!” you might think. “They’ll have a hard time!” But then they don’t. In their (fairly) dark corner, they positively thrive!
Because terrariums are humid. Very humid. Even open ones are usually offer more than 70% humidity. Closed ones, probably in the range of 90%. Plants can keep their stomata fully open almost all the time and no serious amount of moisture will be lost to transpiration, since the humidity inside and outside the leaf are almost equal. So, for 12 hours (even more in the summer or if you grow them on a timer under lights), photosynthesis races along at full speed.
Too Much of a Good Thing
This actually becomes a problem. Plants in terrariums grow far more quickly than the same plant in open air would do and soon outgrow their space. If you want to create a terrarium that doesn’t turn almost immediately into a jungle, you therefore really should stick to naturally tiny plants. Otherwise, you spend your time cutting tall plants back and digging overgrown ones out.
Succulents Are an Exception
The main exceptions to this rule are succulents such as cacti, aloes and echeveria. They are adapted to low humidity and will do perfectly fine at a humidity of 20%, less than the average relative humidity in the Sahara Desert (25%)!
That’s because they do things differently: they carry out CAM photosynthesis. That stands for Crassulaceae acid metabolism. It was first discovered in plants from the Crassula family, whence the name Crassulaceae, but it occurs in many plant families.
Succulents have learned to carry out respiration separately from photosynthesis. So, they close their stomata during the heat of the day so as not to lose water. They then only open them to exchange gases at night, rejecting oxygen and filling up on carbon dioxide. They can then use the stored carbon dioxide to create sugars while the sun shines during the day (photosynthesis). Ingenious, isn’t it!
However, succulents do not like terrarium culture. In our homes, they’d much rather be in full sun right near the window where their growth, although slow (CAM photosynthesis is less efficient than regular photosynthesis), is a least steady.
Helping Your Other Plants Survive Fall and Winter
Most tropical plants originating from humid climates—and that would include pretty much all houseplants other than succulents—will do better with high humidity even if that means decreased light.
Those with thin leaves, like fittonias, polka dot plants (Hypoestes phyllostachya), brugmansias, flowering maples, ferns, etc., struggle even more than the average houseplant. They rarely thrive outside of the summer months when the sun is bright and the air is humid. They’ll often tell you they’re unhappy with leaves that curl slightly down, or whose edges or tip dry out or blacken. The leaves may hang limply even after you water or, most obvious of all, simply fall off.
Plants with thick, leathery or waxy foliage are comparatively resistant to dry air, as are plants with very hairy leaves. Many of these are of arid origin—mostly succulents as well as plants that spend the winter dormant—and they want their full ration of sun, but will put up with drier air in return.
Heat = Dry Air
As we start heating our homes, relative humidity drops like a stone. When you heat cool air, it becomes “thirsty” and absorbs whatever moisture it can. Water molecules in the air become few and far apart. The more you heat, the drier the air becomes. So, if you want to keep your plants happy as heating rises, you have to increase atmospheric humidity.
Your home probably has a humidifier of some sort. Maybe a heat pump. They’re usually not all that good at their job, though, and struggle to stay at a relative humidity of 40%, whereas plants and humans do better at 60%, with 50% being “barely enough.” In fact, 80% is not too much for plants! So, you’ll probably want to adjust yours to “maximum.”
If you have a plant room (or an office: humid air is good in the workplace too!), consider adding a room humidifier and keep it filled up. Your plants will be so happy!
Also, add more plants, as each plant loses humidity to the air. Therefore, the greater number of plants you have, the higher the humidity.
For the same reason, grouping your plants together, especially around thin-leaved specimens, can help them.
Laundry rooms can be a bit more humid than other rooms, especially if you’re always hanging clothes there to dry, so make good plant rooms. But keep the door closed or all that great humidity will diffuse through the entire house or apartment, dropping as it goes!
Humidity trays (pebble trays) are of some use for low-growing plants, but the slight increase in humidity does little for taller ones.
Misting plants, on the other hand, although often recommended by Internet experts who actually know very little, is a total waste of time. Read why under Horticultural Myth: Misting Your Houseplants.
And finally, you may have to bag the ones the least able to adapt to dry air (calatheas, fittonias, etc.) for the winter. That is, seal them inside a clear plastic bag or a mini-greenhouse to give them the equivalent of terrarium conditions. Read more about that technique in Bag Delicate Houseplants for the Winter.
So, your houseplants probably need a humidity boost at this season to reduce evapotranspiration. And this is also just when day lengths drop, so light decreases, so your plants can use your help with photosynthesis too.
By improving the atmospheric humidity, you can kill two birds with one stone!