Light is the life source for all vital plant functions.
During photosynthesis the energy of light is captured by chloroplasts, which sparks a ton of different metabolic reactions. One of those reactions is the creation of sugar (or food) for plants. These sugars fuel plant growth, so the more light a plant is exposed to, the more energy it will create, & the faster it will grow!
THE END.
Just kidding, there's a ton more to learn silly goose.
TABLE OF CONTENTS
So you have
Quality --> Color
Quantity --> Intensity
These characteristics of light are critical in understanding how & why a plant will behave in your home. Basically, if you want your babies to thrive, pay attention.
QUALITY
First off there's quality. Quality is based on the color & the type of light. Light is energy that comes in varying wavelengths, each with a corresponding color. Colors we can see with our own eyes may not be useful for plants, because they 'see' light differently than we do. Plants appear green because they reflect green light, so green light is useless to plants. Instead, plants need light they can absorb & make use of like yellow, orange, blue, red, & violet, as well as invisible UV light from the sun, as well as some infrared.
QUANTITY
Next, there's quantity. Quantity of light is based on intensity or the brightness of light that reaches the leaves. The more light photons that hit the leaf - the more energy captured & the faster the growth. Oxalis or Begonias, for example, depend on intense light to sustain their fast paced growth. Any plant that produces flowers or fruits depends on intense light too. These plants are working with basic ingredients like water, CO2, sugars, & nutrients that are chemically built into complex molecules, like flower pigments, but only when the right light conditions are met. Some plants have self-regulating mechanisms & will even refuse to flower or will try to but fall short halfway through the process if there isn't enough intense light.
Are we following?
Summary so far -- Plants need quality light they can absorb & make use of, such as yellow, orange, blue, red, violet, invisible UV light, & some infrared. The quantity of light is based on the brightness that reaches the leaves. The brighter the light, the more photons it has, & the more photons that hit the leaf means the more energy the leaf captures, thus accelerating the plant's growth. Some plants depend on brighter light than others. Some plants use specific light conditions to help them use other basic ingredients, like water, carbon dioxide, sugars, & nutrients to produce fruits or flowers. But! They can only produce said flowers or fruits when the right light conditions are met.
INDOORS vs OUTDOORS
When a plant is outdoors there are no boundaries. Light is bouncing from all angles -- from 360 degrees around & from the 180 degree arc above the sky. When a plant is indoors the light is usually only coming from one source, such as a sunny window. This massively reduces the angles light is bouncing off from, as well as reduces the amount of light & vital photons the plant needs. In conclusion, when you bring a plant indoors you are exponentially reducing the photon exposure a plant receives.
The darkest shade outdoors is still brighter than the sunniest window indoors.
BASIC LIGHT TERMINOLOGY FOR OUTDOOR PLANTS: Full Sun, Partial Sun, Partial Shade, Full Shade
Full Sun: Plants that require at least 6 hours of direct sunlight per day
Partial Sun: Plants that thrive in between 3 - 6 hours of direct sunlight per day
Partial Shade: Plants that thrive in between 3 - 6 hours of direct sunlight per day, but need protection from the intense mid-day sun.
Full Shade: Plants that require less than 3 hours of direct sun per day
FULL SUN
The term "full sun" describes the type of light that is found in open areas where there are
little to no objects to block it. Plants that thrive under full sun do so for a large portion, if not all, of the day. There are some plants that are labeled as "full sun" that cannot tolerate these conditions for the entire day & may need a bit of a break. If a plant is labeled "heat tolerant," "drought tolerant," & "full sun," then it is more than capable of withstanding intense summer sun day in & day out.
With that being said, there are some plants labeled "full sun" that can stand more than 6 hours of direct sunlight in some areas, but cannot in others. For example, 6 hours of direct sunlight in Georgia may have a stark difference in temperature to 6 hours of direct sunlight in a more northern state. For this reason it is important to research a plant's requirements in from more than one source if dealing with an unfamiliar plant. Knowing the conditions in your own yard is as equally important.
PART SUN & PART SHADE
By definition the terms "part sun" & "part shade" have very similar meanings with subtle differences that define the differences in the terms. Generally speaking, a plant that requires
partial sun or partial shade will thrive in filtered light throughout most of the day, or direct sun during morning or afternoon hours. Bear in mind, direct sun temperatures during morning hours are going to be relatively lower than those during the afternoon.
Plants that are labeled "part shade" are going to be more sensitive to receiving too much light, especially direct sunlight & particularly in the afternoon. These plants will need shade during the hottest parts of the day. Lack of shade may result in burn spots or discoloration of foliage, wilting leaves, etc.
Plants that are labeled "part sun" will typically be able to stand more direct sunlight than the former & need a minimum amount to thrive. Plants that require partial sun & fail to receive enough direct sunlight may result in failure to bloom properly.
From what we've learned so far we can deduce that for either group (partial sun & partial shade), providing direct morning sun is beneficial.
FULL SHADE
Plants that fall under the category of "full shade" can require a number of different light
conditions. These circumstances may include anything from the indirect light found on the north side of the house to the deep shade found under the canopy of evergreen trees.
True shade plants, such as ferns, will perish if exposed to too much sun.
Filtered light, such as that found beneath a tree canopy, is a good setting for a full shade plant. This type of light is referred to as dappled shade (which we will discuss later).
HOUSEPLANT LIGHTING
Similar to outdoor plants, mimicking the lighting of a plant's native habitat is crucial in keeping it healthy & thriving. Many popular houseplants come from a variety of habitats, including those whose temperatures never fall past freezing, are adapted to living under the canopy of taller trees, & thrive in an environment fill with high humidity & filtered sunlight. Other houseplants may flourish in a sunnier, less humid climate. Ultimately, the closer you come to replicating a plant's natural habitat, the more likely your plant is to succeed.
WINDOW LIGHTING
While plants will survive from sunlight passing through a window, it is important to consider how said light is altered.
Light that passes through glass isn't as strong as direct, outdoor sunlight.
Levels of brightness will change throughout the day depending on the direction the window faces.
The size of the window, eaves & overhangs, as well as shading by trees & buildings all affect the amount of sunlight that comes through the window.
In the winter, the sun is lower in the sky, so there may be more direct sun, however the intestine is weaker than in the summer & there are fewer hours of daylight.
BASIC LIGHT TERMINOLOGY FOR INDOOR PLANTS
BRIGHT LIGHT or FULL SUN
The terms "bright light" or "full sun" means there is no barrier (ex. curtains, blinds, trees, buildings, or anything that creates shade) between the plant & the light source (a sunny window). This is where your plant will receive the most bright or direct light while indoors. The most sunlight will come into a south-facing or southwest-facing window. However, the brightness & duration will depend on the factors above. West-facing windows offer shade in the morning, but direct sun in the afternoon. During the summer, this is usually the hottest part of the day, so plants will receive a surplus of heat, as well. Ficus, succulents, & Monstera are all sun-loving plants that should be placed directly inside or no more than 2-3 feet away from a window. Ultimately, stick those suckers in the brightest spot in the room.You may be able to start seeds without artificial lighting, but seeds that need more time indoors, such as tomatoes or peppers, may become leggy without extra light. High-light areas can be warm, making plants dry out faster. Check these plants more frequently & water when soil is dry.
MEDIUM LIGHT or FILTERED SUNLIGHT
These terms relate to light that has been diffused between the plant & the light source. For example, if you hung a sheer curtain in a sunny window. Some refer to this as "dappled sunlight." Anything partially obstructing the path between your plant & the light source creates medium light. As we know, mimicking a plant's natural habitat is the more effective way to create a climate it may thrive in. Plants such as ferns & aroid plants have evolved to live on the forest floor, so they are used to being shaded by the sun. They have not evolved to handle the harsh rays of direct sunlight, so they prefer medium light conditions.
LOW LIGHT
If a plant thrives in a low light environment it prefers that no direct sunlight will tough its foliage. It will thrive in an area a few feet away from a window or any space where it can see outside, but cannot see the sky. Low light means less energy & less food. This usually means plants that thrive in low light do not grow at an accelerated pace. North-facing windows & interior walls are often areas for low-light plants. The more windows in a room, the brighter your interior space. It is beneficial to note that plants that are listed to require "low light" generally will tolerate low light, but would grow more vigorously & with more vibrant color with medium light. Low-light plants are "understory" plants naturally, meaning they grow underneath the branches of larger plants. Low lighting is not sufficient for starting seeds indoors. It is also beneficial to note that in environments with less light, plants grow more slowly & use less water. You want to avoid overwatering by feeling the soil before doing so.
NOTE: No living plant will thrive in a very dark corner.
It is also worth noting that while some plants can survive in low light conditions, that does not mean they thrive in them.
THE SUN SHIFTS WITH THE SEASONS
Keep in mind the sun changes places in the sky depending on the time of day & season, which affects how much light your plant will get. Monitor how the light changes throughout the year & adjust your plant's position accordingly.
SOLUTIONS TO TONING DOWN WINDOW SUNLIGHT INTENSITY
Scorched foliage, bleached flower colors, & increased pest problems (such as spider mites) can all be symptoms of unbearable sunlight intensity coming in from your window. Fortunately, there are a few solutions to toning down the sun's strength.
Sheer curtains will protect plants, while also creating a more pleasing light environment for yourself.
Light filtering shades, fabrics, & films for windows
Get creative with natural materials, such as woven fencing
GENERAL TIPS FOR LIGHTING & HOUSEPLANTS
Don't allow plant foliage to touch windows, especially in winter.
Keep plants away from heating vents, air conditions, winter doorways, & anywhere they may receive a draft.
Some plants will drop leaves or otherwise show signs of stress when relocated -- even to a spot with similar conditions. They should adapt & recover.
LIST OF POPULAR HOUSEPLANTS BY THEIR LIGHTING NEEDS
Bright Light with Some Sun | Bright, Indirect Light | Medium Light | Low Light |
Cactus | Anthurium | Dracaenas | Pothos |
Citrus | Alocasia | Ferns | Peace Lily |
Fig, Fiddle Leaf (Ficus lyrata) | Bromeliad | Schefflera | Snake Plant |
Fig, weeping (Ficus benjamina) | Money Tree (Pachira aquatica) | Spider Plant | ZZ Plant |
Hibiscus | Monstera | Hoya (Wax plant & Hindu rope plant) | Aglaonema (Chinese Evergreen) |
Kalanchoe | Orchids | Alocasia | Ponytail Palm |
Succulents | Peperomias | Asparagus Fern | Parlor Plam |
Thyme | Philodendron | Rubber tree (Ficus elastica) | Dracaena |
Basil | Fig, weeping (Ficus benjamina) | Begonias | English Ivy |
Lavender | Hibiscus | Jade plant | Arrowhead Plant (Syngonium) |
| Kalanchoe | Peperomia | Lady Palm |
HOW PLANTS ARE AFFECTED BY TOO LITTLE OR TOO MUCH LIGHT
When plants lack light, they don't produce chlorophyll (the green pigment in plants), resulting in plants turning pale green to yellow to white.
Plant stems become "leggy," meaning stems become long & thin & appear to be reaching toward a source of light.
A lack of sufficient light causes the plant to grow long spaces between leaf nodes on the stems; leaves will grow farther apart from each other than usual
Plants without sufficient light may drop their leaves, especially older ones.
You may find that a variegated plant may revert to being solid green.
Flowering plants may fail to produce flower buds.
Plants exposed to too much light may result in scorched or bleached leaves.
THE SHADOW TEST
The Shadow Test is a popular trick to determine how much light your plant is getting.
Take a piece of paper & hold it up to a light source when the sun is high around midday.
Take your free hand & place it a foot or so above your paper.
A sharp shadow indicates bright light. A softer shadow indicates medium light.
If your plant is a few feet away from a window, even a sunny window, your plant is surviving in low light.
Sources: Gardener's, University of Minnesota, The Sill