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What is light spectrum and how does it affect plant growth?

We spend a lot of time talking about grow lights and the light spectrum. But very little time try to explain what the light spectrum actually is. 

Today’s post is going to do just that. By the end, you’ll know way more about what the light spectrum is, and which parts of the spectrum are important for your plants.

What is light spectrum? 

In science at school, you’ll no doubt have learnt about the electromagnetic spectrum. Well, the light spectrum is part of it. The ‘light’ part of the spectrum is the wavelengths that us humans can actually see. Then, there’s radiation – which can’t be seen. This includes ultraviolet (UV), far-red and infra-red.

When talking about light spectrum for plants, plants can see or use our visible light spectrum, as well as UV and far-red radiation.

If you’re growing indoors, your grow light set up will need to provide your plants with the total light spectrum in order for them to thrive. Those able to grow in a greenhouse outdoors, can take advantage of the spectrum of light and radiation provided by the sunshine.

Further still, your plants need a certain amount of each wavelength within the spectrum in order to growth. And how your plants react to light will differ from one species to another.

How light affects plant growth 

As you probably know, plants use light for photosynthesis; a process in which they turn light into chemical energy that they can then use to grow flowers and fruit. There’s also a process called photomorphogenesis, which refers to how plants might modify their growth depending on the light spectrum. If you’ve ever noticed grass or plants in general slightly lean towards the window or away from the shaded areas of the garden, this is an example of photomorphogenesis.

The wavelengths used for photosynthesis typically range between 400-700nm. This is what is often referred to as PAR; Photosynthetically Active Radiation. Within PAR there are red, blue and green wavelengths which is why so many grow lights now offer multiple coloured wavelengths within them.

Photomorphogenesis on the other hand, can occur in a much wider wavelength range from 260nm to 780nm, and it’s this range that includes UV and far-red.

Why is the light spectrum important to know about? 

It’s important to know about the light spectrum in depth because it gives you the chance to almost manipulate your plant’s growing habits.

When activated by photons from different wavelengths, plants will grow in different ways. They might grow taller, fatter, offer higher yields or boost their root development. The light spectrum can even help you enhance the colour of your plants, the flavour of the fruit and the nutritional value.

It’s pretty powerful stuff if you can get your head around it.

The light spectrum for plants: A breakdown 

If you like the sound of manipulating your plant’s growth, here’s a breakdown of how different wavelengths can impact your plants

Bear in mind however, that there are plenty of other things that need to be monitored and controlled at the same time. Plant species, light intensity, temperature and photoperiods are all things that need to be looked at in order to achieve the best results.

UV 100-400nm

Ultra-violet is outside the window of PAR, and its effect on plants is yet to be fully understood.

That said, an easy way to think of UV’s impact on plants would be similar to humans. Heading on a beach holiday and hitting the sunbeds can result in a nice little tan and a top up of that all important vitamin D. But too much sunshine exposes us to UV which can lead to sunburn and skin damage.

The same can be said for plants. Plants thankfully already have a protective layer to prevent UV tissue damage, but they still might turn darker or even more purple – depending on the type of plant you’re working with.

The major bonus of UV is that when used in moderation, it can provide greener, and thicker leaves. It can also help your plants gain a little more resistance towards pests, fungus and other environmental stresses.

Blue light 400-500nm

Blue light is particularly good for enhancing plant growth and flowering. It’s ideal for leafy greens and decorative plants.

In other situations, blue can be combined with other coloured light to boost root development, increase chlorophyll accumulation and regulate healthy growth.

Green light 500-600nm

A lot of people think green light is less important than other wavelengths, as chlorophyll can’t actually absorb this light as well as others. But it’s actually this lack of absorption that gives your plants their zesty green colour.

Green light has also been found to be helpful for photosynthesis in the lower leaves of a plant. Once the plant has had enough of blue and red light, green light can still activate the chloroplast deep within the leaves of the lower canopy. It can also help to increase crop yields.

Red light 600-700nm

Red is an exciting wavelength to use around plants. It’s great at stimulating photosynthesis and when combined with blue, it can create thicker leaves and boosted plant development.

Far-red light 700-850nm

There has been recent research into the potential of far-red light within growing.

It promotes flowering and leaf expansion which provides more surface area of the plants to capture the photons needed for photosynthesis. It’s also thought that far-red could help increase the efficiency of PAR wavebands.