For Houseplants, Light Is a Shady Business
When it comes to plants and the indoors, we love to teach nature a lesson. A plant may have evolved over millions of years, but somehow we’re going to persuade it to change its stripes just for us — so we can have it cheering up a dark and depressing bathroom.
The results are usually depressing too. A plant starved for light is like anything starved; diminished in all ways. It sounds simple: we need to match our houseplants with the conditions they naturally require. But no matter how many times we hear this, or say it to ourselves, we persist in the same quest, praying for a botanical miracle, hoping that our closeted plant will somehow see the light.
There is another side of this story, too — the opposite problem, where a shade plant ends up getting too much direct sun. At least this is an easier problem to fix. Every home has somewhere nice to put plants out of the sun, I would think, and if for some reason there isn’t, you can always use larger, sun-loving plants to create a canopy of shade for others.
So that’s the pep talk. If you want to know more on how to use light to make big, bright, and beautiful houseplants, read on. There’s more to light than meets the eye.
When buying plants, instead of asking how much light does a plant need, ask a better question: what are the light conditions that will make this plant thrive?
Or the same question in reverse: what plants will thrive in such and such light conditions?
Seeing the light
Part of getting a good match between our plant and the place we put it doesn’t have to do with plants at all. It has to do with our own subjective assessment of how much light there is in a room. It’s easy to make mistakes here because our eyes adjust to brightness, so as the room gets brighter, at some point we don’t see it as brighter because our eyes constrict. One footcandle is roughly the brightness of one candle, one foot away. Outdoor, direct sunlight has a peak intensity of about 10,000 FC.
Another way to think about this is to recognise that we don’t really see how powerful the sun energy is, because our eyes limit our experience of it. This is one reason why we might confuse the intensity of sunlight versus the intensity of artificial lighting. The latter might seem almost as bright, but from the plant’s point of view, the energy is far superior from sunlight.
It seems obvious when you say it, but we don’t always realise that most of the light feeding our plants is sunlight, even when the plants are in the shade and there is some artificial lighting. Indirect light simply means sunlight reflected off other surfaces — and into your eyes and onto your plants.
When we talk about sunlight and houseplants, it’s helpful to use three categories: sunny, bright (indirect sun), and shady (moderate to low indirect sun). As a basic rule, it’s also good to judge brightness with any artificial lighting off. Unless you have lamps that are specifically tailored to keep plants healthy, and directed right at them, it’s unlikely they will make a significant contribution to plant health.
At home, follow this basic rule: if, with the lights off, there is enough light on a bright day to read a book, that will be sufficient for most shady houseplants. If there isn’t enough light to comfortably read a book, or the light is of short duration (say, under 5 hours), best to choose a low light plant (for a list, see here). As the seasons change, you might have to reassess them. This is a basic rule and we should always remember that, for the best results, the brighter end of the spectrum of what a plant can tolerate gives the best results. We also have to consider the duration of light a plant receives (long durations can compensate somewhat for a lack of bright light).
A chart like the one below can help. It uses the categories above, where the sunlight in a room goes from shade to bright to sunny conditions. It also lists some plants suitable for each category. Note, however, that just as there is no perfect, linear relationship between sunlight intensity our our experience of brightness, there is also no perfect, linear relationship between sun intensity and a plant’s absorption of energy. Not at all. This is why houseplants became houseplants in the first place: they have acquired a greater ability to gather and utilise light than those that evolved in sunny conditions.
Succulents and cacti do well in full sun, for example, but even these plants won’t be able to use all the energy that a full day of sun offers. It’s more than enough. On the other hand, like the eyes of nocturnal animals, houseplants have evolved mechanisms to capture better what light there is. Shade is quite inferior in light intensity compared with direct sun — nobody ever got a tan sitting in the shade — yet many plants can be very lush and healthy even though they only ever receive indirect light.
Bringing balance to the force
Light affects the growth of plants, but not just in the simple sense that they won’t grow, or fruit, or flower — or live! — without it. There’s quantity of growth, but there’s also quality. Which is to say that, if you want a healthy, lush plant, you have to grow it a certain way.
The first step in doing this is suggested above, namely: putting a plant in as much light as it could ever want. If the available light meets the plant’s basic needs, a plant may grow for years under these sup-optimal conditions. But it won’t thrive. To avoid leggy, dull, and drab plants, you have to go one step further and get the plants in light where there’s energy to spare.
Consider the Aeschynanthus, lipstick plant shown above. Although far from being a spectacular plant, it has an illustrative history. Notice the older growth, which is larger and darker green, versus the newer, light green (Spring) foliage, which is smaller. In other words, this plant shows two different grown habits from two different growing seasons. The larger, older foliage was from a year ago when the plant was growing in bright conditions. The new, smaller growth is from more recent months when the plant was moved so it would receive part sun.
As you can see, partial sun makes a more compact plant. It’s not just that the foliage is smaller, which you may or may not prefer, but it’s also more lush because the distance from one leaf to the next — the nodal distance — is shorter. This variation reflects a common process: less and less light means a greater nodal distance, which will result in a leggy plant. Instead of dense foliage you end up with long, spindly stems populated with sparse foliage.
Much the same results with regard to flowering. For plants that flower, if you want a greater density of flowers, you also want to optimise the light to shorten the nodal distance of the buds.
But optimise doesn’t mean maximise. Remember, we want a good match. Especially here in New Zealand, where the sun is treacherous, a good match means just the right amount for that plant. Some, like the Tradescantia zebrina, Callisia Repens bubbles, or Aeschynanthus, love a good amount of sunshine and become lush and dense with proper sun. Others, like all the Scindapsus (pothos) and many ferns, will only yellow and fade from direct sun. It may seem counter-intuitive, but as with all things, you can have too much of a good thing. So don’t maximise, optimise. Every plant, depending on type, and how it was originally grown, with have its sweet spot. You just have to find it.
And don’t minimise light any more than you have to. A lady Palm or ZZ plant may tolerate low light, but that doesn’t mean that’s why they want. Think of the camel: just because it can cross the desert without drinking water doesn’t mean it wants to.
Shady, but smart too
While it isn’t my goal to add complexity to this whole light thing, there is something else that you have to keep in mind — something I slipped into a sentence just above: ‘how it was originally grown.’ It’s fine to make generalisations about plants, as I just did regarding plants like the Scindapsus. But keep in mind that these are general rules. Many houseplants adapt to the light conditions they grow up in, so if you alter those conditions, problems may arise. For instance, I can show you a photo of a Ficus lyrata growing in full sun (as above), but that can be very misleading. Most lyratas are grown in shade houses and will burn like a potato chip if they are put out in the sun later (also above). Confused? Let me explain…
I probably did a lot of stupid things last autumn, but one I particularly remember was taking a handsome Ficus benjamina from the nursery, where it was happily growing outdoors in full sun, and putting it in the store where it was in full shade. I should have known better given that there is a whole literature on how to get sun-grown Ficus trees from the nursery to the market without having all their leaves fall off (the trick: go in stages). Which is what happened to me. The tree was several years old and was not at all happy.
The good news is that plant defoliation of this kind can teach us about how houseplants adapt to the light in their environment.
Like most other indoor plants, a Ficus benjamina is perfectly happy growing in bright shade. So why does it drop its leaves? If you look this up online, you’ll see a lot of possible reasons for why this might happen, most of them relating to how moving a plant can stress it. But in my case I know the problem was one thing: not enough light. This, though, raises the question — if Ficus benjamina are fine indoors in the shade — why did moving one to the shade cause its leaves to drop? Saying this was due to stress is vague and unhelpful: why did its leaves drop, because it was stressed; how do we know it was stressed — because its leaves dropped. Stress is not a real explanation.
So let’s ask a better question: what did this tree accomplish by dropping its leaves? From a distance, this seems nuts. Not enough light, might as well give up altogether and drop every last leaf. If we look at my case, all the evidence is actually before us if we think it through.
First, we know that the bright, glossy, green leaves that fell to the floor were of a certain kind. That is, they were leaves that had emerged and grown under full sun conditions. When the tree grows its new set of leaves in the store — assuming it stays alive long enough to pull off this trick — these leaves will develop under shade conditions (i.e., no-direct-sun). Will these two sets of leaves behave the same, given they’re from the same plant? No, they will not. The new leaves will be bespoke for the new, shady conditions.
So this is what the tree accomplished: like changing your clothes for a change of seasons, the plant changed its leaves for a big change in light. It’s a risky move but given that the tree does it, it must be the best tactic, evolutionarily speaking. Here’s more on how and why this happens…
If we know one thing about nature it’s that living things have evolved to adapt. Not only do plants grow and orient their foliage according to the prevailing light conditions, they can also build different kinds of leaf profiles to assist in acclamation. Even a cactus does this. As much as we think a cactus loves sun, when you shift a cactus you actually should mark its orientation because when you place it elsewhere, the northern side needs to be kept north. Why? As the cactus grows, the areas exposed to direct sun will have more ‘sun blocker,’ whereas the shady side of the cactus will have less (so as to maximise its light absorption on that side). Thus, if you move it 180 degrees, the unprotected side will be in the sun and could burn.
So what about my Ficus tree? Here the story is similar. Without going deep into the physiology of all this, suffice it to say that shade-grown leaves are designed like the shade-side of the cactus: i.e., to absorb more light. First, under shade conditions, some plants will grow larger leaves; second, sometimes the actual leaf itself will form so as to alter absorption based on the prevailing conditions: when there is essentially too much sun (all day) you have one profile, and when there really is not enough (full, canopy shade), you get another. Here’s a summary from an article on Ficus benjamina, in case you love the details (the conclusion is in italics).
Peterson et al. (30) found that trees grown under high light intensities had smaller thicker leaves with two distinct palisade layers, while shade grown leaves had only one palisade layer. Fails, Lewis and Barden (18, 19, 20) studied the anatomy and morphology of sun and shade grown foliage and confirmed these findings. They also reported greater stomatal density in sun grown leaves, although shade grown leaves had more stomata per leaf. Sun grown leaves were small and thick with 2 layers of elongated palisade mesophyll cells and chloroplasts were aligned along the radial cell walls. Shade grown leaves were larger, thinner and darker green with a single layer of short palisade cells. Chloroplasts were dispersed throughout the palisade cells and appeared to be larger than in sun grown plants. When net photosynthesis of plants grown in full sun and 50% sun was compared under various photosynthetically active radiation (PAR), shade grown leaves had a photosynthetic advantage over sun grown leaves at PAR comparable to lighting found in interiors.
This acclimation process also shows up in leaf colour. Take the case of Strelitzias. Birds-of-Paradise are sun-loving plants, but they can also grow under a canopy of shade. In the latter conditions, there is less light so any new foliage adapts to by having a greater mass of that lovely green pigment we call chlorophyll. Differences in chlorophyll density mean that the foliage of Strelitzias can vary from a greyish blue to a deep evergreen, corresponding to their past. Chlorophyll is of course what allows plants to convert sunlight (when combined with carbon dioxide) into plant food (sugars/ carbohydrates) and oxygen. This is the process we call photosynthesis.
To sum up, then, you get the best houseplant results when you optimise growing conditions. But to do this, you need to take into account the conditions the plant grew up in. If, for example, you want a Monstera deliciosa that will thrive in your sunny lounge, the plant you choose needs to have been raised in the sun.
A final note — on watering
Every article about houseplants has to mention watering. Fortunately, there is an actual connection. Sunlight translates into heat, obviously, so much of what’s said about how much light a plant needs also reflects on how to water it. This is for two reasons.
First, as a plant moves out of direct sun, the plant and its pot won’t get nearly as warm. That’s one good thing about sun-loving indoor plants, like cacti or succulents: they dry out so quickly, you’re less likely to over-water them. Still, a room that gets sun will warm up during the day, so regardless of whether or not the plant is actually in suns, the humidity will go down and it will dry out faster. At the other end of the spectrum, when a room receives no direct light, there is a real risk of over-watering, especially during the cooler times of year. The exception being, of course, air conditioning, which basically changes everything. Any plants in the path of air conditioning — hot or cold — need to be assessed with that in mind.
The second and final point is this: more light energy usually translates into more growth. And more growth means the plant will drink (and transpire) more water. Conversely and perhaps more importantly, the less light energy, the more likely the plant will sulk in the cold and grow slowly or not at all. This is the most dangerous situation for a plant: with little or no transpiration, the plant has no way of removing water from the soil. Evaporation will help but it too will be minimal. So, houseplant lovers, careful as you go…
All of which is a very longwinded way of saying, if you love houseplants, give them the greatest gift of all — light.