A field of sunflowers bathing in the sun.

How Important is Light for Plants

A healthy plant depends on an adequate supply of light to allow the process of photosynthesis to take place. Photosynthesis provides the carbohydrates essential to a plant’s existence. Without it, the plant’s growth and flowering will begin to slow down.

Light is essential to the plant in order for the process of photosynthesis to take place, when chlorophyll within the leaves reacts with light to produce sugars and starches that nourish the plant. Inadequate levels of light mean that the process slows down, and the plant begins to suffer slow growth and a lack of flowers. Prolonged exposure to poor light will result in a pale-colored plant that becomes elongated in its search for light, and is left weak and floppy in the process.

Many indoor plants — especially those with large leaves — are prone to a build-up of dust in the home, and this can interfere with photosynthesis. Plants with smooth leaves should be wiped regu­larly with a damp cloth or sponge, while those with hairy leaves should be brushed gently with a soft paintbrush.

Variegated leaves have no chlorophyll in the white, cream, or yellow parts of the leaf, so the area available for photosynthesis is restricted, and the need for light is correspondingly higher. If there is inadequate light, the plant will adapt by increasing the pigment in the paler part of the leaf, resulting eventually in a total loss of variegation.

Sources of light

As well as the intensity of the available light, the duration of the light is also important. Nearer the equator, light levels are consistent all year round, whereas farther away there is a marked difference between levels in summer and winter. Flowering plants, in particular, demand long periods of light to initiate the formation of the flower buds.

All flowering plants fall into one of two categories: long-day plants, which flower when the light lasts for 12 hours or more over a given period, or short- day plants, which flower when the light lasts for less than 12 hours a day over a given period. It makes no difference to the plant whether the light is natural or artificial (supplementary), which is how growers are able to produce flowers on plants such as poinsettias and azaleas outside their natur­al flowering season. They manipulate the plants’ day using a combination of black plastic sheets to simulate the darkness of night and electric lighting to simulate daylight.

Plant tolerance

In a situation where the light received by a plant is from one side only, the plant’s natural instinct is to grow toward the light source, making the plant lo- sided. This is called phototropism and is best avoid­ed by either moving the plant to a position with a more even supply of light, or by turning it around by a quarter every day (so that over a four- day peri­od, the whole plant has received equal shares of direct light).

“Phototropism – When the light received is from one direction only, the natural tendency of the plant is to grow toward it (phototropism) — making it lopsided. Turn the plant regularly to avoid, this.”