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Sunlight and growth

Plants need light to grow. Light provides energy. Plants use this energy to make sugars, which are the building blocks for leaves, stems and roots to grow. Without light, there is hardly any growth at all. This is why the amount of light a plant gets makes a big difference.

Amount

Plants grow slowly when there is too little light. With enough light, they produce new leaves and stems. But even more light does not lead to extra growth.

In bright sunlight, leaves can heat up strongly. This causes more water to evaporate from the plant. If the soil is dry and little water is available, the plant slows its growth to keep from drying out.

Seasons

In spring, the amount of light increases quickly. For many plants, this is the signal to start growing. In summer there is plenty of light but also more chance of heat and drought, which can slow down growth.

Day length

Plants respond to day length: the number of hours that it is light. The length of the day changes in a predictable way throughout the year. For many plants, this is a more reliable signal than temperature. Day length helps plants time their growth.

Colours

Light from the sun is made up of different colours, though you don’t usually see them separately. Together, they form white light.

For plants, the mix of colours in light matters, because not every colour is equally useful for growth.

Red and blue

Plants mainly use red and blue light. These colours provide the most energy for growth. Green light is used much less and mostly reflected. This is why leaves look green: this is the light plants don’t use. Which colours are strongest changes over the course of the day and year.

Position of the sun

When the sun is low in the sky, sunlight travels a longer path through the atmosphere. As a result, not every colour reaches the ground the same way. This changes the composition of light that reaches the earth.

When does sunlight contain relatively more blue?

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Sunlight and growth