What do the plants need?

Plants need light for the process of photosynthesis to work.  To put it simply, water and carbon dioxide react in the presence of light in the plant's leaves to produce carbohydrates and release oxygen as a waste product.

This reaction takes place with the help of a complex set of molecules called chlorophyll and its associated enzymes.  Not all the light in the visible spectrum is needed by plants for photosynthesis, just that in the red and blue ends of the spectrum.  Green light is not required and is reflected back, hence the green colour of plant leaves.

Plants from different environments have different requirements for light.  Those that grow in the understorey of a forest are well adapted to low light levels and will have a number of features that make the best use of any light that is available.  On the other hand, plants that grow in deserts have more light than they need and will often have a grey waxy or downy covering to reflect the excess light (and heat) away from them.

The origins of plants in the wild, therefore, give us some clues to their light requirements.

Use the plant selector in the Specifiers' guide to find plants adapted to different light levels.

Light sources

Your building will have a wide variety of different light sources.  There may be natural light from windows, atriums and light guides as well as a whole range of artificial sources.  By and large, plants are unaffected by the type of light that is available, as long as it is bright enough and not green.

Sunlight in many buildings is filtered, often to help keep the building cool.  This is to improve the comfort of the occupants but will significantly reduce the amount of light available to any indoor plants.  However, most office lighting schemes give enough light for all but a few species.

Fluorescent light is very common.  It is economical, generates little heat and gives light of a quality that suits plants quite well.  Full spectrum fluorescent lights are especially good as they mimic daylight very effectively.  They also have some benefits for people. (See details of research into full-spectrum lighting here.)

Other factors to consider

Heat

Incandescent bulbs produce a lot of heat.  This is especially true of halogen spotlights.  Plant leaves can be scorched if placed too close to an incandescent light.

Light distribution

Task lighting and some ambient lighting is focussed or has a well defined distribution pattern.  Light levels can vary considerably within short distances.  This is why it is important to measure light precisely where you intend to place a plant.

Lighting duration

Most plants benefit from a daily period of darkness.  For some plants it is essential.  Many species of plants originating in arid or semi-arid places have an adaptation to prevent moisture loss during the day - they close their leaf pores (stomata).  However, this means that they can't take in carbon dioxide during the day.  When it is dark (and therefore cooler in the desert), the leaf pores open and they take in carbon dioxide and fix it chemically in special cells until it is reacted with light and water the next day.  If these plants are kept in constant light, they will fail to thrive.  Most other species will survive constant light conditions, but they will never be as healthy as those that get at least 6 hours of darkness every day.