Advice
Artificial lighting
Poor
watering and incorrect lighting are two of the commonest causes
of houseplant death. Plants respond to different aspects of
light: its intensity, duration
and quality. The best quality light
is natural daylight and wherever possible plants should be
placed close enough to a natural source of light for healthy
growth.
Artificial lighting can be beneficial in winter when daylength and light intensity decrease. It is used to increase daylength in winter triggering responses such as flowering. The best lights to choose are those that produce the highest light levels.
Artificial light sources
Incandescent lights: Ordinary house light bulbs create large amounts of heat but little light. This tends to be high in red light which can affect photoperiodic responses but does little to increase growth. Close to the plant they can scorch the foliage.
Fluorescent tubes: Fluorescent lighting is more suitable for supplementing natural light. It produces mainly blue light and some green and yellow, but is low in red light. Cool white and warm white coated tubes give the most satisfactory light for plant growth.
Surprisingly little light is emitted from household fluorescent tubes, so that the tubes should be positioned close to the plant. The small fluorescent bulbs emit even less light, and have little use in helping plant growth.
Mercury lamps: Mercury lamps are suitable for greenhouse use suspended above the plants. Mercury fluorescent lamps additionally emit red light.
Metal halide lamps: Metal halide lamps produce more blue light than mercury lamps.
Sodium lamps: Sodium lamps produce yellow light through to the red end of the spectrum.
A mixture of mercury, metal halide and sodium lamps is often used to give a balanced light for maximum plant growth. These lights give a very high light intensity and are very economical to run, but are rather costly to buy.
Intensity
This depends on the type of light and the proximity of the plant to the light source. Light intensity falls away dramatically from windows and during the winter. The human eye compensates for the lack of light and is not a good gauge of light levels. Intensity can be determined with a photographic light meter.
Quality
Light ranges in colour from ultra-violet at one end of the spectrum to infra-red at the other. Colour affects plant growth and responses. Blue light is required for photosynthesis with high levels causing dark green compact growth. Red light affects photoperiodic responses, such as flower production and plant dormancy with high levels causing spindly growth. Yellow light affects chlorophyll production. A wide spectrum of light is required for healthy plant growth.
Duration
Light has an accumulative effect in plants. With shorter daylength in winter the total amount of light received is significantly reduced. Day length also has an effect on functions such as flowering. Plants triggered into flower by less than 12 hours are short day plants e.g. poinsettia. Plants requiring more than 12 hours of light to flower such as Saintpaulia are called long day plants. Tropical plants often require 12-16 hours of daylight for active growth.