Determining Light Levels

How do you evaluate light?  During prime light time (for plants situated near windows), place a sheet of white paper on the table or floor where the plant will be situated.  Hold your hand about 12 inches above the paper.  If a clearly defined shadow results, the site receives bright light.  If a muted but clearly definable shadow results, the light is medium. If your hand shadow is barely visible, the amount of light is low. 

High Light: 300 — 400 foot-candles

Plants that thrive in high light are those that enjoy more than eight hours of bright light.
Cacti and many succulents are good choices for  very sunny locations.  Croton, Yucca, and Jade are also among the high light plants.  Refer to “Plant Care Guide,” for a complete list of high light plants.
To prevent damage to plants in high light, try moving the plant away from the window; even a couple of feet can make a big difference in the intensity of the light.  Also, replacing the plant with one more suitable for a high light situation may be in order.

Medium Light: 200 —  300 foot-candles

Plants for medium light need about six hours of light per day.  Many of the plants used in offices will thrive in medium light if given appropriate care. 
Ficus, Dracaena, Spathiphyllum, palms, and Pothos are plants that thrive in medium light.

Low Light: 75 — 200 foot-candles

Low light plants are those that require fewer than four hours of light per day. The intensity of the light should be “reading level.” These include Sanserveria, Aspidistra, some ivies, Spathiphyllum and ferns. 

No plant will tolerate a “no light” situation for very long.  Plants placed in windowless conference rooms that are rarely used will soon die.  In this type of situation, a replica plant may be in order.

All plants have a minimum light level requirement.  When a plant is placed in a situation below its natural minimum it will begin to lose more leaves than it grows, become spindly and eventually die.  When you notice a problem like this, talk to your supervisor about possible solutions such as reduced watering, adding more light by leaving lights on adjusting the blinds, or exchanging the plant for one better suited for the conditions.

Seasonal Changes

During the summer months, the sun is directly overhead.  During the winter months, the sun is lower in the sky.  In some situations, a plant will be situated in medium or even low light during the summer months because the sun is overhead; but in the winter months, it will change to high light because the sun is lower in the sky and may extend into an office or home at a lower angle.

Conversion Units for Lighting

Light is most often measured by its level of intensity.  Traditionally, the simplest term of measurement has been foot-candle (ft-c). In the lighting industry, foot-candles are a common unit of measurement used to calculate adequate lighting levels of workspaces in buildings or outdoor spaces.  Instruments (light meters) which measure light intensity in foot-candles will measure visible light falling on the instrument. 

Manufacturers of windows, window film, and light bulbs list measurements of their product’s light output in units lumen and lux. (Read the packaging for light bulbs and you will see these terms.)