Functional lighting: lights to keep plants alive


All plants require light to survive. It provides the energy they need to convert water (taken in through the roots) and carbon dioxide (absorbed by the leaves) into the carbohydrates that make up the structure of a plant. This process is called photosynthesis, which means “assembly by light”.

The conditions inside buildings are very different from those in nature so interior landscapers need to understand how different plants use light and how they adapt to life indoors.

Light spectrum

Any source of electromagnetic radiation, whether it is the sun or an artificial radiator, emits a certain amount of energy consisting of electromagnetic vibrations. This radiant energy spreads out from the source as a wave motion having a constant speed, but varying wavelengths. The principal types of electromagnetic radiation can be arranged, in wavelength order, into what is called the electromagnetic spectrum.

Photosynthesis may take place only in the presence of chlorophyll, a magnesium containing pigment found in the leaves and stems of all green plants. Chlorophyll absorbs light most strongly in the red and violet/blue portions of the spectrum and least strongly in the green portion. Hence, when natural light shines on a plant mainly green light is reflected and the specimen appears green.

Variations in leaf colour occur when there is either a shortage of chlorophyll, e.g. in the white/cream parts of variegated leaves, or the green colour of the chlorophyll is masked by other pigments, such as the yellow and red carotenoids.


Light sources

The light that indoor plants receive comes from many different sources. Both daylight and light from artificial sources are used by the plant for photosynthesis. Natural daylight, if available in sufficient intensity and duration, is the cheapest and best. However, the amount of sunlight getting through to the inside of a building can be surprisingly low. Outside on a bright, sunny day there may be 50,000 lux (1 lux = 0.093 foot candles), but the absorbing or blocking effects of glass, blinds, furniture, walls and the shading from surrounding buildings can reduce this to a few hundred lux inside a room.

For interior landscapes, artificial light often needs to be provided as a substitute or supplement for natural light. From a plant’s point of view, the quality of the artificial light depends on the type of lamp providing it. There are three main types of artificial light used in buildings, incandescent, fluorescent and gas discharge. They are described below.

Incandescent lighting

Incandescent, “filament” type lighting is generally unsuitable for photosynthesis because they produce light high in the orange/red bands, which may cause excessive elongation of the plants. Some tungsten lamps are given a blue filter and called “plant lights” but these tend to be short lived and uneconomic to run as a principal source of illumination. However, when used in spots and/or pendant luminaires they can be useful in providing accent lighting on flower or plant displays.

Halogen lighting

Halogen lights are a type of incandescent lamp that are increasingly popular in buildings. They are small but produce a lot of light and they are often mounted as spot lights. The quality of light produced is satisfactory for plant growth, and there are some very nice 'plant lights' sold as an accessory for indoor plant displays. There are, however, two disadvantages that must be considered.

First, many halogen spotlights are focused on a small area, so if they are to be used for plants, it is essential that correct light measurements are made. Illumination levels drop off rapidly away from the centre of illumination.

Secondly, the bulbs get very hot. They should not be placed too close to the leaves of a plant, otherwise the they will be scorched. A distance of at least 50 cm is recommended.

Fluorescent lighting

Fluorescent lighting is probably the most common in offices, restaurants and other commercial buildings. Most are designed for a maximum output in the 550nm band (green/yellow) and are therefore not ideally suited to plant growth, as the 440nm and 660nm bands are deficient. However, some specialist fluorescent tubes reproduce a much fuller spectrum and are becoming more popular in offices where they have been shown to benefit people as well as plants. Compact fluorescent luminaires that replace standard incandescent light bulbs are becoming increasingly popular as they are economical to use and relatively inexpensive to buy. Some compact fluorescent luminaires are also available with near daylight quality.

High pressure discharge lighting
High pressure discharge lamps based on mercury were, until recently, widely used in horticulture. However, mercury has now been largely displaced by metal halide and sodium. Both have a high radiant efficiency and are probably the best types of lamp to use where high light levels are required. Metal halide lights give the best spectrum for photosynthesis and, as the light appears white, are often suitable for buildings where appearance is important. This type of lighting is unusual in offices, but can be used in atriums and other large spaces in buildings such as factory floors.

Plant light requirements

Whatever the light source, a plant’s first “instinct” when installed in a building is to orientate its leaves towards the strongest light source. This is most easily observed near a bright window, where the leaves will turn rapidly to face the sun. However an interior plant’s long-term chances of survival depend on its ability to adapt to low light levels. It may do this in several ways.

• By rearranging the light-trapping chloroplasts so that they are all on the upper surface of the leaf. This ensures that they are facing the light rather than being randomly distributed throughout the leaf.

• By losing leaf variegation, e.g. in species such as Hedera helix, Epipremnum aureum and Ficus benjamina. The increased amount of chlorophyll in the previously unpigmented parts of the leaf helps to trap more light.

• By stretching new growth towards the light. The spindly stems and small leaves of Ficus benjamina under low light are a good example of this.

• By dropping leaves, so that a smaller “area” of plant is competing for the available light. To get the best out of a plant it is therefore important to know the light levels available and to choose species accordingly.

The light requirements of individual species have been determined after several years of research and experience. It is surprising to think that most of the plants used in interior landscapes evolved in tropical or subtropical environments where light levels are often very high. The Yucca, for example, originally comes from the arid and semi-desert regions of America where light levels are extremely high, yet it can survive in offices where the light is only 1000 lux.
Detailed light requirements of plants can be found by using the plant selector or a - z plant search utility.

Effects of buildings

The light that reaches a plant in a building is made up of a lot of complex interacting components. The total illumination reaching individual plants in a building is the sum of the artificial illumination inside the building (e.g. ceiling lights) and the daylight that reaches the plant from outside.

Different types of building will have different lighting characteristics. Large, open shopping centres, leisure centres or atria may rely on daylight as a major source of illumination. Offices and restaurants may have relatively small windows and solid ceilings, and direct illumination by sunlight may be limited to only an hour or so a day. It should also be remembered that Natural light may fall by more than 50% for every metre that you move away from a window.

Atriums and high glass buildings
Buildings with glass roofs or where large expanses of glass make up the walls on more than one side will experience several hours of direct sunlight. In these cases, the amount of artificial lighting may appear insignificant, or may not exist at all during the summer months. The tracking of the sun across the sky will ensure that at some point in time, almost every part of the building interior will experience direct illumination, although for most of the time, high levels of indirect illumination will occur.

Offices and buildings with smaller areas of window
Locations where there are only small areas of window will not experience much direct illumination from the sun; any direct illumination will be for a very short time. North facing windows will experience no direct sunlight except for a few hours at either end of the day during mid-summer. Most daylight illumination will be indirect.

Aspect of the building
The aspect of a building will also have an effect on the amount of daylight transmitting to the plants inside. In the Northern Hemisphere, North-facing windows will for much of the year overlook a shadow cast by the building. East-facing aspects will be in shade for much of the afternoon, west-facing aspects for much of the morning, and south-facing windows will experience some direct illumination for most of the day. Given that the average position of the sun is its point in the sky at midday, the aspect of the building will not matter if light measurements are taken at midday. Any shadow cast by the building can then be regarded as being part of the building for the purposes of light measurement.

Whilst the absolute quantity of daylight reaching a plant varies over time, experiments have shown that the proportion of daylight transmitted to the plant appears to be constant. This ratio is referred to as the 'Building Effect Coefficient' and is very helpful when making accurate light measurements inside a building.

Light measurement

Light can be measured in several different ways, including the energy of the light, the quantity of light or the brightness of the light.

Solar radiation
The visible wavelengths of light can be measured in terms of the energy received per unit area irradiated. The unit of solar radiation is watts per square metre (Wm^-2).

Photosynthetically active radiation (PAR)
The wavelengths of light that are used in photosynthesis can also be measured in terms of the energy received per unit area irradiated. The unit of measurement is again watts per square metre (Wm^-2) at the specified wavelengths.

Quantity of light
The quantity of light can be measured in terms of photon flux density, and is recorded in units of µmoles.s^-1.m^-2.

Illumination
The brightness of light as experienced by the eye (illumination) is measured in lux (foot candles in the USA, 1 lux = 0.093 foot candles). This is the form of light measurement that most interior landscapers and architects use.

When measuring the light levels in a building for plants, the following points should always be considered.

• Natural light may fall by more than 50% for every metre that you move away from the window. Levels should be measured exactly where the plant is to be installed and not simply in the general area. This should also be borne in mind before moving a plant to a new position.

• The contribution made by artificial lighting should be assessed, for example by taking readings with the lights both on and off. In many situations this is the predominant source of light.

• Allowance must be made for the time of year at which readings are taken. If the light level on a bright summer’s day is just enough to support a particular species it will probably struggle to survive in the winter months.

• With large plants, light levels in both the upper canopy and lower branches should be taken into account.

• Dust and grime on the surface of leaves can greatly reduce their ability to absorb light. Regular cleaning is therefore very important, especially in low light levels.

Once light levels have been assessed, plants can be selected according to their known light requirements and by the application of sound judgement.

Plants are organic and like people they can have differing strengths and weaknesses. In a single batch of plants there may be several resilient individuals able to cope and acclimatise, together with less dynamic plants that are easily stressed and which may never recover. A strong plant will often have the ability to adapt and cope with either higher or lower light levels than its classification.

Plants react to several different factors, i.e. light, heat and water. These factors interact with one another causing a plant to respond differently if any one single factor is changed. As temperature differs, so does a plant’s suitability to a particular light regime.

Click here to download more information on light measurement.

Author: Kenneth Freeman

Additional content: Mike Lothian