When caring for your urban jungle, providing the proper light for each individual plant is one of the most important requirements for their prosperity. Here is an in-depth guide for all things light-related for your plants!
Why do plants need light?
We meet again, 10th grade biology! The short answer: food. All living beings require nutrients from food in order to maintain their bodily functions and survive. Plants create their own food through a process called photosynthesis. They absorb light through chlorophyll, the pigment that gives plants their green color. You can think of their leaves as solar panels absorbing light to be used as energy. Plants use that energy to convert carbon dioxide (from the air) and water into glucose (a type of sugar). This glucose combined with other nutrients drawn from the soil is used as food.
If plants do not receive adequate light, they are being starved of the energy they require to make food. Imagine being hungry but not having enough energy to physically go to your kitchen to cook for yourself!
Every plant needs some amount of light. Even for plants that are considered "low light", this usually means that they will survive in low light environments, but surviving does not equal thriving. If given their ideal amount of light, most plants will grow faster, larger and more vibrant than with minimal light. The ideal amount of light is, of course, different for every individual plant, sometimes even among the same species depending on their acclimation (and ability to acclimate) to certain environments throughout their life.
General Light Requirements
Most plant shops, nurseries and experienced plant owners will generally describe 4 levels of light in which to place a plant:
(1) Low Light: areas that receive little to no direct light at any time of day (example: the corners of most rooms farthest from a window)
(2) Moderate/Medium Light: areas (whether at a window or a few feet away from a window) that receive some direct light (even just a little) at some point during the day
(3) Bright Indirect Light: areas that receive lots of bright light throughout the day that is filtered (example: windows that have blinds, sheer curtains, frosted glass, other furniture/barriers, or outside spaces with tree or percola shade, etc.)
(4) Direct Light: areas that receive the maximum amount of light throughout the day with no shade or filtering
If a plant can handle a broad range of light, it is labeled with a spectrum (example: low to bright indirect light). Again, in most cases, it is still better to give it the higher end of the spectrum for ideal conditions. A caveat of this is that there is also an upper limit as to how much light a plant can absorb. If given too much, it will show signs of sun stress (often by irreversibly changing leaf color) or develop actual sun burns on its leaves. For this reason, few plants can handle true direct light 100% of the time. Knowing both the minimum and maximum light requirement for your plant will allow you to figure out where to place it in your space.
Light Intensity by Direction and Time of Year
Keep in mind that not all sunlight is the same. All light levels outside are stronger than their indoor counterpart. Sunlight in the summer also tends to be stronger than sunlight in the winter. Indoors, the strength of the light will decrease as you move further away from the light source (ie. window or grow light). Additionally, due to the direction in which the sun rises, travels across the sky throughout the day and then sets, morning sunlight tends to be less intense than afternoon/evening sunlight. Therefore, plants in bright indirect light in the morning are essentially receiving less light than plants in bright indirect light in the afternoon/evening.
Because of the path of the sun, the direction in which your windows face plays a big role in the quantity and the strength of the light that your plants receive:
(1) North: never receives direct sunlight; more suitable for plants who tolerate low or moderate light
(2) East: receives direct morning sunlight, which tends to be less intense and cooler than afternoon/evening; suitable for moderate and (lower end) bright indirect light plants
(3) South: receives the most consistently bright light throughout the day; suitable for most plants from moderate to bright indirect light levels (avoid plants that prefer shade)
(4) West: receives direct light from midday to sunset, which tends to be the warmest and strongest light; suitable for plants who need (and can take) intense bright indirect to direct light (this area has the most risk for sun damage)
Lastly, remember that light also bounces, which means that surfaces directly surrounding your space or outside your window will affect the light your plant receives, such as water, snow, sand or a brightly-painted wall. Glare from the sun will intensify the light.
Take all of these factors into consideration when placing a plant by a specific window. For example, say you have a plant that needs bright indirect light but is sensitive to burning from too much light, and you only have space by a west-facing window. You can minimize the strength of that sunlight by using blinds that you can open partway, possibly in conjunction with sheer curtains to further filter the light. Then place the plant a few feet away from the window, rather than directly in front. You can always shift it closer little by little if you think it can take more light.
How Much Light Does My Plant Need?
A good rule of thumb to use when trying to determine how much light a plant needs is to consider the environment in which it grows natively in the wild. The two most common climates in which house plants originate are (1) arid and (2) tropical.
Light for Arid Plants
These are your cacti, succulents and other plants generally found in dry, hot, bright (and often harsh) environments. Think of sparse deserts, rocky crags and sandy lowlands. In the wild, these plants are generally accustomed to the bright, hot, direct light of the sun with minimal shade. This means that plants who fall into this category will require the most amount of light, from bright indirect to direct light (as a rule of thumb). Some of these plants have developed natural sun screen to protect them, such as the thin layer of white powder found on some succulents like echeveria. However, this only helps to a certain point, so my advice is to start with bright indirect light, then give it more if it seems to need it.
Light for Tropical Plants
These plants are typically found in wet, humid rainforests and jungles, which includes most large, leafy foliage plants. While the tropics usually get tons of sunlight, a big factor when considering light for tropical plants is that these environments generally have large amounts of tree cover. The house plants originating from these areas are usually found either on the forest floor or attached to rocks and tree trunks, all of which are beneath the tree canopy. This means the light that they receive is almost always filtered by the trees above, and therefore most tropical house plants generally love bright indirect light (with exceptions of course, such as some ferns who prefer more shade).
While these 2 climates encompass most of the plants commonly found in houses today, you can still use this rule of thumb for plants that do not fall into either of these categories. Find out where your plant thrives naturally in the wild and try to replicate that environment at home.
The Extra Variegation Requirement
Plants with variegated leaves have an extra need for more light than non-variegated plants. Variegation simply refers to pigmentation that is not one single solid color. This can occur as (1) a defining feature of the entire species (example: all stromanthe triostars are variegated plants with leaves containing different shades of green, cream and white, with magenta undersides) or (2) as a separate distinct version of the same species (example: there exists the regular, non-variegated solid green monstera deliciosa, as well as a few different variegated versions, such as the monstera albo borsigiana that has white/creamy patterns across its green leaves).
In order to maintain the colorful patterns and bright pigmentation of variegated plants, they require more light than most other plants. Additionally, the lighter the color (such as white variegation), the more light it will need. This is because these areas contain less chlorophyll (or none at all) to absorb light, which means more light is needed so that the green areas that the plants do have can optimize their energy production. With this added pressure, variegated plants tend to be slower growers than non-variegated plants. If variegated plants do not receive enough light, they will often "revert", which means they will start to lose variegation. New leaves will grow in with less and less variegation in favor of greener leaves with more chlorophyll to work with. Once reverted, depending on the genetic stability of the variegation (species by species), some plants' variegation can return with proper light, while others will not.
Lastly, when a variegated plant is stressed in some way, this will often show up first in its variegated areas (example: a pothos N'Joy receiving too much light will start to brown in the white spots of its leaves). These are essentially the most fragile parts of the plant and are good indicators of its overall health.
Supplementing with Artificial Light
Grow lights are not just for indoor agriculture! When you do not have an ideal space for your plants to get enough light, you can often supplement by using grow lights. These are different from your average light bulbs, because they emit light in specific wavelengths that are beneficial for your plants, and they produce much less heat than regular light bulbs that would otherwise damage your plants. (This means, no, that plant in your dark office will not survive with just those ugly fluorescent ceiling lights. It will need real sunlight and/or or a grow light specifically for plants).
There are a myriad of factors to consider when choosing the proper grow light for your specific plant in your specific space, ranging from different types of light emitters (HID, CFL, LED, etc.), to different wattage and intensities, to different light emission colors that are suited for different types of plants and different stages of their life. This topic is large enough to warrant its own post (to limit the length of this one), so if you're interested in using grow lights, do your research (and I will update this post with a link when I write up my own guide)!
Signs of Too Little or Too Much Light
All plants have their own way of showing signs of stress, including with their light intake (even with artificial lights). In general, lack of light will result in slow, small, stunted growth. As stated earlier, variegated plants will lose their variegation and bright colors and become more green. Some plants like succulents will grow long and stretch out (called etiolation) as they attempt to reach for more light.
Again, an over-abundance of light will show up as discoloration in different ways. Some leafy foliage plants will turn pale, either a lighter green or even yellow, in order to reduce the amount of chlorophyll to absorb light. Some cacti and succulents will turn shades of orange, red or brown from sun stress (some plant owners actually do this deliberately to achieve a desired color). Variegated areas can develop brown spots. All plants, from arid to tropical, can develop burn spots in various colors from too much light.
A Couple Final Tips and Tricks
(1) Dust Off Your Plants
Because plant leaves are like solar panels, it's important to make sure there is no barrier between their leaves and the light they are trying to absorb. Leaves should be cleaned regularly through various means depending on what works for you and your plant. A fresh, slightly damp rag can easily be used on plants with large leaves to gently wipe off any dust. You can add a bonus spray of preventative pest repellent (I recommend a homemade neem oil solution) onto your rag to do double duty as you clean! Make sure to wipe both sides of the leaves, as some pests like to hang out inconspicuously on the undersides.
If your plant's leaves are too small or numerous for a rag to be effective, you can also give it a nice gentle shower, either in the bathroom or outside with a hose. This method is great for tackling multiple plants at once, and the strength of the water can knock off pests clinging to your plants (though a more targeted approach, like wiping down each leaf, is still better for pest management).
For plants that want to avoid water being left on them but also cannot be wiped down with a rag, such as succulents and cacti, try using a soft small brush, like those for paint or makeup, to dust them off. This method also helps with any soil or pebbles that may have ended up stuck in their spines and nooks and crannies. Make sure to be gentle, especially with echeveria that have a white sun screen coating - once that layer is removed, it cannot be replenished.
(2) Rotate Your Plants
Most plants will grow towards their light source, meaning leaves will often face in the direction of the light. In order to keep your house plant growing evenly on all sides, it is a good idea to rotate your plant every so often so that its leaves grow in different directions (or in the case of succulents to grow nicely centered). Some plants can change the direction of their growth over time, while others will grow in a specific direction and stay that way forever (ie. once a leaf comes out in a certain way, it can't move, and you will only be able to encourage the growth direction of new leaves). Alternatively some plant owners prefer training their plants to grow in a specific direction for displaying or taking photos from a specific angle (similar to bouquet arrangements), allowing them to hide things like stakes and ties in the "back" of the plant.
Sunlight plays a huge role in the life of this planet. I hope this in-depth look into lighting for your house plants helps your urban jungle thrive, and I hope you can take a leaf out of their book and enjoy some sunshine yourself! The vitamin D that we as humans absorb from the sun through our skin has been proven to be essential to our physical health and our mental well-being. What other lessons can we learn from Mother Nature?