The colour of light affects the efficiency of photosynthesis, but also growth and development. It is becoming more and more possible to work with these facts.
Many LED lights contain red, blue and far-red LEDs. This easily gives the impression that these are the ideal colours for plant growth and development, but that needs a clear explanation. When it comes to light colour, you need to distinguish between two things: the influence of colour on photosynthesis and its influence on development.
With LEDs, red produces the most light output per kW of electricity. This is why red is the dominant colour in the fixtures. A plant cannot grow with purely red light, so blue and sometimes far-red light are added. But plants also use the other light colours quite well. The McCree curve shows this: it has peaks in red and blue, but photosynthesis is certainly not zero in the other colours.
It is already possible to grow leafy crops and some types of green pot plants under LED light. That is still out of reach for flowering ornamental plants. All crops still grow better under sunlight, which has the full colour spectrum. Trials have shown this time and time again.
Sunlight consists of UV, PAR (photosynthetically active radiation) and infrared. As the name implies, only PAR is important for growth. Half of the solar energy is in the infrared part of the spectrum, and this makes an important contribution to heating the greenhouse. Infrared is not needed for photosynthesis, so shading out infrared (heat radiation) is a good idea. Suitable coatings for this are ReduHeat and ReduFuse IR.
The colour of the light also determines the development of the crop: how much it elongates, how much it branches, when it flowers and more. The intensity of light is very important for photosynthesis, but to steer the development of the crop, much smaller light differences can be enough. For example, a very small amount of red light during a long night is enough to keep chrysanthemums vegetative.
It is the relationship between the colours that gives light its steering effect. One example is the ratio between red and far-red: more red results in more compact plants. The ratio between red and blue is also important. Less blue, and therefore more red, leads to roses with longer stems.
This example shows that it is not that simple. On the one hand, more red results in more compact plants. On the other hand, it results in more elongation. Keep in mind that it is always about the ratio between colours.
In practice, it is difficult to steer with light colours. There is always an ample amount of sunlight available. To change the ratios during the day, you would need to add a lot of the desired light colour. Son-T lamps contain so much red light that, at low levels of natural light, there is a clear red-light effect.
Extended lighting at dusk, to change the ratio of red to far-red, can lead to more compact plants. This follows from research by Wageningen University & Research on fuchsias. Rose growers have had good experiences with the coating ReduFlex Blue, which partly blocks blue light. They can harvest longer roses under this coating. In this case, it is the ratio of red to blue light that causes the effect.
There are many interesting developments in the field of light colours, but practical application, or applying the results to other crops, is still difficult. Wageningen University & Research has achieved spectacular results with tomatoes under extra far-red light. The plants sent more assimilates to the fruits. It is still unclear whether such results can be applied to ornamental plants, such as gerbera or rose.