We’ve all seen light bulb manufacturers talking about how the spectrums of their bulbs are better for plant growth and development. What is a light spectrum, and why does this matter for plant growth and development?

The leaves of plants are really just biological solar panels. They capture the light energy from the sun and store this energy for later use to drive growth. This is done through a process known as the Calvin cycle.


The Storage Of Light Energy 

Instead of storing this energy in a battery like our man-made solar panels do, plants store this energy in individual, high-energy molecules we call glucose or sugar.

The energy stored in these high-energy molecules is made available to the plant when it gets broken apart. This energy is then used to structure the plant, and allow it to grow, develop, and resist stress and damage.

Not all light is the same, however, even for plants. As light rays leave the source (the sun, or an artificial light), different wavelengths are present which results in different colors. White light is a combination of all colors. Since our eyes pick up all of this light it appears to be white to us. To plants, however, they see many of the individual spectrums using complex light-reactive proteins on the leaf surface. These proteins measure and react to different spectrums of light. The plant uses this information to then decide what it needs to focus its growth on.


What are color spectrums of light?

Light waves are measured on a spectrum according to the frequency of the waves they produce. The visible spectrum of light is actually just a small portion of a much larger frequency scale which includes infrared, radio waves, UV, and X-rays.

In the visible light spectrum, however, it can be separated into the different wavelengths, which can be seen as going all the way from red to blue. White light through a prism, or more commonly as seen as a rainbow, the light waves are separated out and you can see them in the order from lowest energy to highest.

Violet light has the highest frequency, which means it produces waves that are very close together and have a high energy. Red light is on the opposite side of this spectrum and has the lowest energy.


The Sun's Spectrum

The sun releases an incredible amount of energy in the form of light onto the surface of the earth each day. In fact, the sun provides so much energy that in just one hour, the sun has provided earth with more energy than it uses in an entire year.

With plenty of energy hitting the earth each day from space, plants really have no shortage of usable energy. As mentioned earlier, the spectrums that are released can vary greatly, however, and this too will have an effect on the efficiency and usability of this light by the plants.

The sun, for example, releases all full supply of these color spectrums but depending on the time of year, or even the time of day, different light waves will be filtered out. This is why the sky appears to have a red hue at dawn and dusk, and why the autumn and winter seasons have more of a reddish look than the spring and summer or when the sun is directly overhead at noon.


Artificial Grow Lights

When it comes to artificial grow lights, it is important to consider that these spectrums will no longer be influenced by the atmosphere, and due to the mechanisms controlling how the light is produced the spectrums will vary quite a bit.


What is Full Spectrum?

“Full-spectrum” is not a scientific word used to describe the spectrum, rather it is a marketing term to describe a light source that mimics natural light. This does not say anything as to what stage of plant growth the light will be best suited for.


How Plants See the Color Spectrum

Plants have contained within their leaves a series of proteins called photoreceptors. These receptors act as a sort of “eye” to see the spectrum of light that is reaching the leaves. The plants then use this information to decide how it should respond. Depending on the species of plant, there may be different photoreceptors present, or they may respond differently to the same stimulus.

 As mentioned earlier, in normal daylight, and with most grow lights, a full spectrum of all light waves can be “seen” by the plant. The amounts of each wavelength are what varies, however. For example, in the fall, or during dawn and dusk, the earth's atmosphere filters out much of the higher energy wavelengths, allowing a majority of the red spectrum to reach the earth's surface. The plants register that this is happening when a larger number of their red photoreceptors is signaled compared to their blue photoreceptors.

What to do with this information

Knowing how a plant sees light, and that there is a significant difference in how a plant responds due to different light spectrums, allows you to optimize your grow with the choice of light bulb you buy.

For plants in the vegetative stages, choose a lamp that offers a blue spectrum of light. For those in flower, look for a lamp with more red light waves emitted.

You can even optimize your current grow with the addition of supplemental LED lights to give your spectrum a boost. If you're growing with HPS for example, which emits more of a red spectrum, you can optimize this lamp without having to change it out for a different one during the vegetative cycle by adding a blue LED to add some of this blue spectrum.

No matter what you're growing, the spectrum of light is an important consideration to make, especially for more advanced growers looking to optimize their crop. There are a ton of information on the web about what spectrum of light your particular crop likes best, and this is an area of research that is quickly developing.

The light spectrum is a very interesting topic and one that is still in heavy development. It's fun to experiment with different spectrums and see how your plants respond to them.