Why Do Plants Need Light?
All living things require food and water. For plants, light is food. They use it in an important process called photosynthesis, where the energy of light is captured by chloroplasts, sparking multiple metabolic responses-- 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 create and the faster it will grow.
Know these pairs:
These characteristics of light are necessary in knowing how and why a plant will behave in your home.
Quality is based on the color and kind 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 beneficial for plants, because they 'see' light in a different way than we do. Plants look green because they reflect green light, so green light is useless to plants. Rather, plants need light they can absorb and utilize like yellow, orange, red, blue and violet, in addition to invisible light like UV light from the sun and some infrared.
Quantity of light is based on the intensity or the brightness of light that hits the leaves. The more light photons that reach the leaf = the more energy captured and faster growth. Begonias or Oxalis, for example, depend on intense light to maintain their fast-paced growth. Any type of plant that produces flowers or fruits depends on intense light too. These plants are working with basic components like water, CO2, sugars and nutrients that are chemically constructed into complex molecules, like flower pigments, but only when the ideal light intensity conditions are met. Some plants have self-regulating systems and will even refuse to flower or will attempt to but fall short midway through the process if there isn't enough 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 inside, light typically only comes from one source, like your sunny window, greatly reducing the angles light is bouncing off from, and the amount of light and essential photons a plant needs. 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.
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 (drapes or blinds, a tall tree or building that produces shade) between the plant and the source of light (a sunny window). This is where your plant will receive the most bright or direct light while indoors. Ficus, succulents and Monstera are sunlight worshipping plants and should be positioned right in or no more than 2-3 feet from a window. Typically speaking, you would want to put them in the brightest spot in the room.
"Medium light" or "filtered sunlight" is light that's been diffused (sheer curtains) between the plant and the source of light (a sunny window). Some refer to it as "dappled sunlight". Anything partly obstructing the path between your plant and the source of light 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 sunlight. They have not evolved to manage the harsh rays of direct sunlight so they prefer 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 kind of area where it can see outside but can not see the sky. Low light means less energy and less food. Some plants can survive in low light environments 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 get. Monitor how the light changes throughout the year and adjust your plants placement accordingly.