Dew to Humidity
Houseplants are about ninety-percent water, so where does that leave humidity?
source: The SpruceYesterday I opened up a large, clear plastic bin full of rubber-tree cuttings.1 It’d been sealed for a few winter months. The results looked splendid. Dozens of leaf cuttings were showing new tips; some even had new leaves already (see images below). Only a tiny percentage had withered away.
But looks can deceive and once I began pulling them free of the propagation mix, I found that about a quarter had no roots at all, not by any standard. But how can this be? How could these ‘resisters’ not only stay upright and green, but actually produce new growth, all without any hydration from roots! Magic? Not at all. As many readers will know, the key is in the environment — yes, the plastic bin.
Bins are great for propagation because they provide a micro-climate where the light comes in and heats up a small space (with a lightly moistened mix), creating constant, high humidity. Much like a terrarium. These cuttings carried on growing because the moisture in the air was doing the job of the non-existing roots, bathing the leaf in moisture.
Sometimes when you’re doing cuttings of a trailing, leafy plant, you lose track of which end is up. By which I mean you plant the darn thing in the mix upside down. So here’s a question: what happens? If you answered that the plant doesn’t care, try again. In many instances, the plant simply grows upside down: the foliage grows down into the soil, and the roots grow up and out into the air. It looks quite strange. Again, this is possible only because of the humid conditions. The high humidity allows the roots to drink from the air, and the plant has little choice but grow out into the soil (how long this can carry on, I’m not sure).
When planted upside up, once the roots are established, the plant can be removed from the humid bin and placed in a pot of growing mix (as shown above). Now out of the bin, the humidity around the plant falls off considerably unless you live in a tropical climate (and keep the plants outside). Of course, in this case, this won’t pose any issues, as ficus trees have no trouble growing in conditions of average humidity, indoors or out. Indeed, this is true for most tropical plants.
All of which brings us to the issue at hand: if we care about houseplants, including tropical plants, how much should we worry about humidity? The short answer is: not all that much. Let me explain.
Paying Homage to (Relative) Humidity
When you go out in the morning and the grass is covered in water, it has probably rained, or the sprinklers have been on. If not this, it may be dew. ✻
Dew is great because it dumps water on my bonsai, but also because it can teach us all about relative humidity (RH). And it’s really simple. The atmosphere/gases around us can hold goodies like oxygen, but can also hold water vapour (aka, humidity2); how much water can be held in suspension depends on the air’s temperature. Hotter ‘air’ holds far more water — that’s physics. According to Wikipedia: “A parcel of air near saturation may contain 28 g (0.99 oz) of water per cubic metre of air at 30 °C (86 °F), but only 8 g (0.28 oz) of water per cubic metre of air at 8 °C (46 °F).” That’s 3.5 times more water at 30 °C than at 8 °C.
So where does dew come from? Easy. Warm air often cools at night and, as such, it can’t hold as much water. As it cools down, the saturation of this cooling air goes up and up — because of all the moisture in it from when it was warm. At some point it reaches 100 percent, the dew point. The ‘air’ simply cannot hold any more water. The water vapour settles out of the air and hits the ground.
When we are outside working, as the day warms up, the opposite happens: there is a proportional increase in how muggy it can be, for the warmer ‘air’ can carry more evaporated water. It can be sticky, and if you’re trying to exercise (and need to perspire) this can get in the way, as the sweat has nowhere to go.
But what does dew and RH tell us about caring for houseplants. Alas, not very much. And whatever you do, don’t ask Google. On this topic, confusion is the rule, not the exception.
Plants don’t perspire, they transpire. We often wonder about the humidity needed for our indoor plants, but also wonder about how indoor plants affect overall humidity (due to soil and transpiration). Just know that the more plants you put in your home the higher the humidity will be.
I must say, it’s all a bit bizarre. When you read online about humidity and indoor plants, as people do, there’s the usual homage paid to RH followed by a range of RH levels that are ideal for indoor plants, with some suggestions on how to raise humidity for them. Not only do the suggestions have nothing to do with RH, the numbers given are pretty useless. It sounds like science but it’s really just pseudo-science.
If the moisture in a room is constant, what does the plant care if, as the room-temperature changes, the RH goes up or down (given that the available moisture is constant). Indeed, how confusing can it be to say that most houseplants are fine with, say, 40-60 percent RH, when that amount of available moisture (absolute humidity) represented by this range differs dramatically depending on how warm it is.
In this example here, it’s suggested that you might consider having 60 percent RH in your home, with no mention of the ambient temperature, which is like saying I drank a glass of vodka without saying the size of the glass. If the house is kept cool, the numbers might be useful. However, on a hot afternoon in, say, Australia, 60 percent RH is like being in Singapore (where the average RH is about 60% in the afternoon).
Humidity is a slippery subject. When Google advice tells you, for the sake of your houseplants, to turn on a humidifier on a hot afternoon, you know something’s amiss.
Think ‘Relative Hydration’
Let’s get away from the nonsense of RH and consider real life. Say you love the frail beauty of the maidenhair fern (MHF), but you find the leaves too often go brown. If you take your frustrations online, you’ll see many comments about light (no direct sun), water (regular but not excessive), humidity (high), air flow (avoid drafts). In truth, these variables are all one thing (see video below).
The leaves of the MHF are so fine you cannot even detect them when holding them between your finger and thumb. The thicker a plant’s leaves, the less prone it is to dehydration; think succulents. As such, the thin leaves of the MHF need constant, uninterrupted hydration from some source or sources. This demand for moisture can be met by the roots, but as suggested above, the moisture in the air can also help. So instead of thinking of ‘relative humidity,’ think ‘relative hydration.’ How much moisture the leaves need from one source is relative to how much they’re getting from others. All the variables that affect hydration work together to affect the eventual plant health.
Sometimes and in some plants, the leaves can be so well hydrated that moisture droplets actually form (common on big leaf plants like philodendrons or monsteras; see the top image). This is not condensation (which has to do with the dew point). Rather it’s called guttation, resulting from transpiration, and it’s essentially the opposite of dry, brown leaf tips. Feel free to consider it a good thing, for it means that all systems are go. The humidity, the roots, the stillness of the air are all combining to provide more than enough moisture for the foliage, at least for the moment. Who could complain about that.
Of course the condition are not always so perfect for MHF. If you have long periods of sun entering a room, the room will warm up and the moisture in that room will decrease. Regardless of whether the plant is being directly hit by the sun, there will be less moisture in the room (it’s the same for heating and air-con), which means less hydration from the air. It’s not that heat or direct sun are necessarily a problem for MHFs, in other words, but rather that, in terms of relative hydration, they put more onus on the roots. At some point the air can be so dry that it’s impossible for the roots to keep up. (Incidentally, the same thing happens to a potted maple tree outdoors in the sun and wind; the roots will not be able to keep up, and the leaves will dry out.)
In short, while humidity is important for plants, it’s only one factor in the hydration equation.
In the YouTube video below, a MHF is shown happily growing in quite low humidity (35 percent RH under warm but not hot conditions) with an hour or two of direct sun a day. How can this be so? It’s possible because, as some factors challenge the plant, other factors are clearly coming to it’s aid. We can assume for instance that there are no drafts, that there is the steady watering that ferns enjoy, and maybe also some help from neighbouring plants offering shade and local humidity.
I know what you’re thinking: forget all this fancy talk. Just chuck the darn thing into the bathroom.
I’m not sure where this urban myth came from. Probably it’s the fact that showers and baths produce a lot of moisture for a small room. Whatever the case, whether the bathroom may be the solution will still depend on relative hydration. A shower a day may make the difference, but several would be better. 😓 On the other hand, if you open the window or skylight, or run the fan, to deal with the extra moisture, the draft may offset the benefits. The bathroom may also be a favourite because there’s no direct sun, so the room doesn’t become arid. But if the opposite is true — if the bathroom goes from steam room to sauna each afternoon — the bathroom may not be the panacea it’s held out to be.
Get Tropical
Tropical plants evolved in humid areas, just as cacti evolved in arid ones. Each are well suited to the moisture of their own climates. This is an obvious fact, but from this fact come many wrong conclusions.
Consider cacti. We have cacti at MONSTERA that are over 50 years old. During most of their lives these plants lived outdoors and they were often damaged by frost and animals. When people ask how old these large cacti are, I give them a rough age. “Wow,” they say. But then I tell them something else. While the plant might be 50 years old, the same plant could be grown in a third of that time if grown under optimal conditions.
In the case of cacti, optimal is not natural. A cactus growing in the desert is growing under the natural conditions from which it evolved, it’s true. But those conditions are stressful, not just for cacti, but for any plant. If you grow cacti indoors (as, e.g., in humid Japan), under hot-house conditions, watering the soil the minute it’s fully dry, the plant will grow many, many times faster. And look far superior.
The cactus example does not apply perfectly to tropical plants, but there are similarities. Tropical environments are quite ideal for plants, which is more than we can say for the desert. However, just as cacti don’t mind living in (non-native) humid areas, tropical plants can cope with conditions that are far from tropical.
I suppose it’s common sense to assume that a plant from high humidity will benefit from high humidity. But, as with cacti, it’s a mistake to confuse what’s natural with what’s necessary. Do tropical plants really need that much humidity? I don’t think so. Consider epiphytes.
Epiphytes are essentially non-terrestrial plants; that is, their roots, if they even have roots, don’t require soil. As such, they are a bit like my ficus cuttings: they need a humid home. Some houseplants that are epiphytes, or at least start out as epiphytes, are aroids, including some anthuriums, monsteras, and philodendrons.3 Here’s a summary of how humidity affects the ecology of epiphytes, and how they come to inhabit life above ground:
Humidity more than any other climate variable favors species richness, up to 107 species on a single tree where dense evergreen canopies feature multiple microclimates and numerous alternatives to earth soil. Specializations for narrowly defined substrates (e.g., ant carton, suspended humus, naked bark) and the distributions of these media and microclimates often structure communities of epiphytes and allow them to accommodate extraordinary high concentrations of species. (source)
There are many useful lessons to learn from epiphytes, the most obvious of which is how amazing it is that a plant can thrive and even flower without roots. But many epiphytes do have roots, they just don’t go into the ground. Orchids are a common example.
Most of us are familiar with orchids, although few of us have any sense of their native, epiphytic lifestyles. Orchids are typically displayed and sold in a pot, which immediately makes us think of them as terrestrial beings, even though it’s not soil in those pots. Rather, they contain loose nutrient-rich materials that quickly drain but hold back some moisture, thus simulating an epiphytical — that is, humid — environment. What, though, do orchids and epiphytes generally tell us about houseplants and humidity?
As noted, just as cacti make us think of dry conditions, tropical plants make us think of humid ones. But epiphytes reveal quite clearly that this may be wrong. Which is to say, many of the tropical plants we worry so much about as houseplants need high humidity, not for their foliage, but for their arial roots. And as the orchid example shows, we can compensate for this by placing their roots in a micro-epiphytic environment. (Incidentally, this also suggests that, as with orchids, we don’t necessarily want to be putting tropical plants in traditional potting soils).
Perhaps a more conventional example of epiphytic plants sold in pots are ferns like the elkhorn ferns illustrated below (see here for more photos of epiphytes). In their natural setting, the elkhorn creates a natural habitat in which it attaches to the tree trunk with what’s called a ‘basal frond’. Whether in a pot or on a tree, this green saucer-like frond covers or shields the roots. On the tree, it also traps leaves and other materials as they travel down the trunk — in a nest-like fashion, capturing detritus to form a natural plant-pot that also adds nutrients to the rainwater. Of course, such a system works where it works only because there is ample natural humidity to keep it going.
Hardly could there be greater contrast between this habitat and the way in which we encounter the plants at the store. As with orchids, by imitating the native, epiphytic environment in a pot, we create a surrogate environment that works reasonably well. Most importantly for the present discussion, we also create an environment that essentially removes the need for a tropical climate.
Orchids and elkhorn ferns also remind us that there are plenty of tropical plants that we don’t even think of as tropical. Perhaps if we did, we’d be less prone to assume that tropical plants need to live in high humidity. So when you think tropical plant, don’t assume your urban jungle needs a tropical climate. It won’t do your home’s interior or your respiratory allergies any good (and might anger a landlord). Instead, think of relative hydration and how you can optimise it for each plant, knowing some are more delicate than others.
And finally, don’t forget about bugs and spores. These guys love the extremes, from spider mites when it’s too dry to mildew when it’s too wet.
Before breaking out the spray bottles, humidifiers, and pebble trays, which probably won’t help much, make sure you have done the easy and the obvious things. You have to live and thrive too, so find the right balance between what works for them and what works for you.
Note that, despite what you may read, these rubber trees are not the rubber tree. The trees from which rubber is famously made are not from the ficus family but from the euphorbia family. From Wikipedia: Ficus elastica yields a milky white latex, a chemical compound separate from its sap and carried and stored in different cells. This latex was formerly used to make rubber, but it should not be confused with the Pará rubber tree (Hevea brasiliensis), the main commercial source of latex for rubber making.
Helpful article: https://www.scientificamerican.com/article/what-causes-humidity/
https://plantsarethestrangestpeople.blogspot.com/2010/12/list-houseplants-which-are-also.html