Re: How much light do plants need?

Plants need light as the energy source for photosynthesis. 

Most houseplants are shade plants that normally live in forests. Shade plants 
have lower light requirements than plants that live in full sun, or sun plants. 
However, some shade plants are very adaptable to higher light levels. Some 
aren't however. African violet (Saintpaulia spp.) will be harmed by more than 
about 10% of full sunlight. A houseplant's shade tolerance often depends on how 
quickly they are transferred from high to low light. If they are transferred 
gradually to low light, or acclimatized, they survive much better. The reverse 
is true as well. People often damage or kill houseplants if they move them from 
a shady indoor location directly into full sun when they set them outside for 
the summer.

Scientists measure photosynthetically active radiation (PAR). Full sunlight is 
about 2,000 micromoles per square meter per second of PAR or about 10,000 foot 
candles. Some foliage houseplants can survive with less than 1% of full 
sunlight or less than 20 micromoles per square meter per second of PAR or under 
100 footcandles. Some of the low light houseplants are Philodendron, pothos 
(Epipremnum aureum), Chinese evergreens (Aglaonema), ferns and parlor palm 
(Chamaedora elegans). The website cited below has lists of low light 
houseplants.

Houseplant books often group houseplants based on their light requirements. The 
book by Hartmann and others has five levels:
Low 	 	25 to 75 footcandles
Medium 	75 to 200 footcandles
High 		200 to 400 footcandles
High + 	400 to 600 footcandles
High ++   	600 to 1200 footcandles

The above scale ranges from about 1% to 12% of full sunlight.

Other books sometimes use the direction the window faces to group plants by 
light requirements. South windows get more light than north windows for example.

References

Hartmann, H.T. et al. Plant Science: Growth, Development, and Utilization of 
Cultivated Plants. 
Englewood Cliffs, NJ: Prentice Hall.


Light and Houseplants