All living things require food and water. For plants, light is food. They use it in an essential process known as photosynthesis, where the energy of light is captured by chloroplasts, sparking numerous metabolic reactions-- one of these being creating sugars (food) for plants. Sugars propel plant growth, so the more light a plant is exposed to, the more energy it will make and the faster it will grow.
Know these pairs:
These properties of light are important in understanding how and why a plant will behave in your home.
Quality
Quality is based on the color and type of light. Light is energy that comes in differing wavelengths, each with a corresponding color. Colors we can see with our own eyes may not be useful for plants, because they 'see' light in a different way than we do. Plants appear green because they reflect green light, so green light is useless to plants. Rather, plants need light they can absorb and make use of like yellow, orange, red, blue and violet, along with invisible light like UV light from the sun and some infrared.
Quantity
Quantity of light is based on the intensity or the brightness of light that hits the leaves. The more light photons that hit the leaf = the more energy captured and faster growth. Begonias or Oxalis, for instance, depend on strong light to maintain their fast-paced growth. Any kind of plant that makes flowers or fruits relies on powerful light also. These plants are working with basic components like water, CO2, sugars and nutrients that are chemically built into complex molecules, like flower pigments, but just when the right light intensity conditions are satisfied. Some plants have self-regulating mechanisms and will even refuse to flower or will try to but fail midway through the process if there isn't adequate intense light.
In and Out
Outdoors, even in the shade, light is bouncing from all angles-- from 360 degrees around and from the 180 degree arc above in the sky. When a plant is indoors, light generally only comes from one source, like your sunny window, greatly decreasing the angles light is bouncing off from, and the amount of light and essential photons a plant requires. When we bring a plant inside, we actually invoke something called exponential reduction in photon exposure. The poetic quote above helps us remember this fact a little more easily.
Light Bright
You may have heard the terms "bright light" and "low light" plants, but what does that really mean?
"Bright light" or "full sun" means there is no obstruction (curtains or blinds, a tall tree or building that produces shade) between the plant and the light source (a sunny window). This is where your plant will receive the most bright or direct light while inside. Ficus, succulents and Monstera are sunlight worshipping plants and should be positioned directly in or no more than 2-3 feet from a window. Typically speaking, you would want to place them in the brightest area in the room.
"Medium light" or "filtered sunlight" is light that's been diffused (sheer drapes) between the plant and the source of light (a sunny window). Some refer to it as "dappled sunlight". Anything partially blocking the path between your plant and the light source creates this medium light. Ferns and aroid plants (ZZ and Philodendron) have evolved to survive on the forest floor, so they are used to being shaded from the sun. They have not evolved to manage the strong rays of direct sunlight so they favor medium light conditions.
"Low light" means no direct sunlight will reach your plant. It is most likely a few feet away from your source of light (sunny window) or any area where it can see outdoors but can not see the sky. Low light means less energy and less food. Some plants can survive in low light conditions however they will not thrive.
Keep in mind that the sun changes places in the sky depending on time of day and season, affecting how much light your plant will receive. Observe how the light changes throughout the year and adjust your plants placement accordingly.
