You can use a light meter to determine the light intensity in an area of your home, or you can evaluate other characteristics of the light to draw some likely conclusions about the plants you should be keeping and how to supplement, reduce or otherwise modify your natural light.
Why is Houseplant Light Important?
During the day, plants are busy absorbing moisture and carbon dioxide through their leaves and minerals from the water around their roots. Through photosynthesis, they use chlorophyll to convert these raw materials into sugars and starches that they use for food. Without light conditions that approximate a plant's native environment, it starves to death. It is almost impossible to grow houseplants that are healthy and strong without giving them the proper light.
Understanding Sun Exposures (Northern Hemisphere)
The direction your windows face is called your room's exposure. Windows that face the rising sun, or east, are identified as having an eastern exposure. Exposures help identify the strength and probable duration of the natural light in a room. Other factors, like the size and number of windows, the amount of reflected light from other surfaces, the distance of plants from windows and the season of the year are all contributing factors in determining light intensity, but exposure is a good rule of thumb measurement when creating your room's light profile.
Eastern Exposure: Windows that face east receive bright light in the morning when the sun is cool. The quality of the light is excellent, while the risk of scorching plants is very small. The one problem with an eastern facing window is that the cool sunlight doesn't penetrate very far into the room, so plants need to be close to the window to take advantage of the great light.
Southern Exposure: The brightest and most intense light of the day comes from the south. Rooms with a southern exposure have a lot of flexibility. Bright light in the morning makes them ideal for light loving plants, and the intense quality of the light penetrating into the room making more space available for growing plants. The disadvantage of a southern exposure is that the sunlight may be too bright for some plants in the afternoon hours.
Northern Exposures: A northern exposure offers the least light to houseplants. Without putting plants directly in a northern facing window, the light is too weak for plants that require anything but low light.
Western Exposure: Western exposures provide the brightest light during the afternoon. You can compensate for the hotter environment in summer by moving plants back from the window and monitoring them closely. A western exposure has the disadvantages of a southern exposure without the advantage of bright morning light.
Test Your Available Light
You can use the shadow puppet test to evaluate the available light in a room. On a bright day, make a shadow puppet with your hands against a piece of white paper. If you can see a distinct image with a clear outline, you have bright light, a slightly blurred image indicates medium light, and a vague blur is a sign that you have low light.
What Does the Term Foot-Candle Mean?
When you read plant profiles, you will sometimes see light requirements expressed in foot-candles. One foot-candle is equal to the amount of light a candle will cast from one foot away. Light requirements for houseplants will be expressed within a range of foot-candles. A good working rule is that low light plants will adapt to a 25 to 75 foot-candle range, medium light plants will typically need a 75 to 200 foot candles, and plants that need bright light will need 200 or more.
Enhancing Light in a Room
If you decide that the light in your room is too dim for your houseplants, there are some things you can do. Make sure any window treatments block as little of the window as possible. It also helps if the windows are clean and exterior foliage is trimmed. Painting the walls white or another light color, and adding mirrors to the room can help too. If these measures don't help enough, you can always invest in grow lights.
Shading Harsh Light
If the light conditions are too bright in your room, installing some screening can help filter bright sunlight. Other solutions would be to move plants away from windows during the summer months, or to install window treatments, like sheers, that can help filter light and reduce heat.
Knowing what the natural light limitations are in your room will make it easier to choose the types of plants that will do well. Remember, there are many types of plants available. If your room gets even a little light, there is a plant that will like living there.
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