The Importance of Light Intensity for Indoor Plants Growth

Houseplants add beauty, warmth, and personality to any interior. They make it more pleasant, more restful, and give it a note of freshness. One of the essential conditions for the desired aesthetic effect is the light intensity, which determines the plants’ development. Shopping malls, hotels, and beauty centers rely on the colorful and relaxing atmosphere created by plants, either through the fresh green of the leaves or the diverse colors of the flowers.

Offices, benches, and other buildings rely on the natural decor created by indoor plants to give a friendlier touch to the working environment and thus increase productivity. In addition to these functions, there are other functions that are often overlooked – directing traffic, dividing or sub-dividing enclosed spaces into separate areas, reducing the reflection of very bright lights at night, and, last but not least, creating private, protected areas.
True success in growing indoor plants depends primarily on selecting and adapting to environmental conditions. Light intensity, temperature, and humidity are the key aspects to consider. In addition to these, suitable planting soil, proper watering, and the addition of fertilizer will contribute to healthy plant growth.

Light has a major influence on the growth processes of all living organisms, and optimal levels will ensure healthy, long-lived plants. Light intensity is measured in lux (1 lux = 1 lumen/square metre) or footcandles (1 footcandle = 10 764 lux). In summer, for example, near a south-facing window, the light intensity is around 100 000 lux, which is a high value. Most plants, for normal growth, are happy with 5 000-10 000 lux, but this value should be constant for 6-8 hours a day, which in reality is not possible.

It is enough for a cloud to cover the sun to reduce the light by half. The time of day and the seasons cause significant changes in brightness. Moreover, the window itself is a filter that dims the light. Then, as we move away from the window, the values decrease. A simple calculation shows that a plant one meter away from the window receives maximum light, at two meters, four times less, at three meters, nine times less, etc. (i.e., the loss of light is equal to the square of the distance from the window inwards).

Of course, this is an approximate calculation, as light intensity is also influenced by other, less obvious factors – the color of walls, floors, the shape and size of windows, buildings, nearby trees, etc. It is important to remember that below 500 lux, plants stop growing and that above 3000 lux, growth is natural, regardless of species.

In a room, on a sunny day, the sun shines directly in the morning if the window faces east, most of the day if the window faces south, and in the afternoon if the window faces west. Indirect light is that which comes through windows facing north. However, our eyes are not the best assessors of light intensity, tending to overestimate this aspect, which is not to the advantage of the plants, which we must keep under observation and, after symptoms, take the necessary measures.

Houseplants grown in suitable environments will be vigorous, compact, and bushy and have firm branches, brightly colored leaves, and flowers. Houseplants grown at sub-optimal light levels will have small, pale leaves, will grow erratically to capture as little light as possible, and will need frequent pruning to achieve a compact shape. These plants need to be watered much less often than plants grown in a light optimum.

Houseplants kept in light conditions far below their growth and development standards will manage to survive for a few months, after which they will gradually deteriorate. Excessive light intensity is not beneficial either, as it will cause yellowing of the leaves due to the destruction of the green pigment. Over time, brownish spots, i.e., destroyed plant tissue, will appear on the leaves, indicating that light should be reduced.