How Does Color Spectrum Affect Growing Marijuana Plants?

 Most plants growers have multiple objectives in mind when planning an indoor grow. Drafting scenarios to achieve higher yields, increase THC levels, or simply improve the overall health of a plant is an integral part of their hobby. This element of strategic planning involves the challenge to link knowledge of different scientific fields and to match those findings to a technical solution that helps to achieve predefined goals.

Besides dedication and passion, it is the willingness to learn that differentiates good growers from future experts — so let us try to grow the royal way and learn what it takes to cultivate plants of exceptional quality. Today, we are looking at the fundamentals of physics, and learn how the light spectrum affects the growth of a plants plant.

What Is The Light Spectrum?

The sun emits energy in the form of solar radiation including gamma rays, x-rays, ultraviolet light, visible light, and even radio waves. Life on Earth is only possible because the ozone layer blocks this radiation, and reflects most of it back into space. This filtering process only allows wavelengths between 300nm and 1100nm to reach our plants and an even smaller portion of this light is visible to us. It is often referred to as the light spectrum, color spectrum or visible spectrum, and ranges from 380nm to about 750nm.

180–280nm — UVC: Extremely harmful and luckily almost completely absorbed by the ozone layer
280–315nm — UVB: Cause of sunburn and suspected to increase THC levels (!)
315–400nm — UVA: Not absorbed by the atmosphere, commonly known as black light
380–750nm — The visible light spectrum: Bands of wavelengths represent visible colors
700nm-1mm — Infrared light: Not visible above 750nm but noticeable as heat on our skin

How the light spectrum affects plant growth

Every living organism needs information about what is happening around it in order to react to environmental changes and, in theory, gain a slight advantage over other members of its species. plants receive much of this information in the light to which they are exposed and react almost immediately to the different bands of a wavelength — a very complex subject that would occupy several books, but let’s focus on the basics.

1. Vegetative stage — “blue” light for healthy leaves (range: 400–500nm; ideal: 460nm).
During the vegetative stage, it is recommended to concentrate the light on the leaves as much as possible and ensure that the plants are compact, that they do not stretch too much, and that they develop strong stems. To achieve these goals, indoor growers often use metal halide lamps, fluorescent lights or energy-saving light bulbs (CFL), and blue-band T5 / T8 lights during the first few weeks. When plants grow wild, the angle of the sun in spring lets more “blue” waves pass into the atmosphere, a signal for plants to develop large, strong, and healthy leaves.

2. Flowering Period — “red” light for huge buds (range: 620–780nm; ideal: 660nm).
When plants begin their flowering period, higher yields can be obtained by exposing them to a light spectrum with lots of “red” waves, which stimulate the development of buds. The rate of photosynthesis reaches its peak when plants are subjected to “red” wavelengths of 660nm, although a NASA discovery indicates that “green” waves, which are not closely related to photosynthesis, also can have an effect on how plants grow. Seeing a plant as a simple photosynthesis factory is, therefore, a bit rushed. But for now, the best way to mimic the angle of the sun in late summer / early fall is to choose a light with a high degree of “red” in its spectrum.

What if I can only pick one spectrum?

We’ve learned that the spectrum can change how a plant grows. So it’s definitely a good idea to try to match the spectrum with your goals, but what do you do when you can only get one light and must choose between one spectrum or the other?

If you have to choose between one or the other, it’s generally recommended to use the flowering light spectrum (redder) for the whole grow. This results in bigger yields watt-for-watt. However, you can successfully grow buds in the blue light spectrum for the whole grow, and many growers do this with excellent results. Some growers even claim they like their quality better under blue lights!

In other words, the color spectrum is important, but it won’t make or break your growth.

Note: I’ve used all types of grow lights during all stages of growing plants, and even when you’re not using the “right” color spectrum, you will still produce potent buds with any strong grow light! I’ve even used “bright white” CFL grow lights (which have high levels of green), and you may be surprised to learn that these work great for growing plants, too!

The color spectrum of your grow lights does affect how a plant develops, but the color spectrum won’t make or break your growth!

Light energy is what matters most when it comes to plants' growth rates and yields. Your plant turns light into sugars via the process of photosynthesis, and it’s these sugars that it uses to power the growth of the plant and especially the buds!

When it comes to photosynthesis and yields, as long as you’re using a grow light, the most important thing is the pure power of light. Because of this, one of the best ways to increase your yields is to increase the amount of light your plant gets.

There are two main ways to increase the amount of light for your indoor plants:

Train your plants to use more of the light you have (get-ups to 40% more yields compared to an untrained plant in the same setup)
Upgrade to bigger grow lights

PAR vs Lumens: Which is Best to Measure Grow Light Intensity?

When it comes to measuring how “good” a grow light is, or how much light it’s producing for your plants, it can get confusing because a lot of terms get thrown around willy-nilly.

The truth is that there is no one best way to measure light, but there are several ways and each has its own strengths and weaknesses.

Lumens

One of the most common ways you’ll see light measured is with lumens. Lumens measure “luminous flux” which is a fancy way of saying that it’s how much light you can see as a person. It measures how “bright” something is to human eyes, and it’s weighted so that the light we see counts more than the light we don’t see as well. The amount of light received at a particular point in space is known as lux, or lumens per square meter (1 lumen/m2 = 1 lux).

When it comes to plants, the type of light we can see, and the type of light they can use are pretty close. So even though lumens don’t measure light exactly as a plant sees it, it does give us a good general idea of how bright a light is to plants for many types of grow lights. Not perfect, but a good ballpark figure.

For example, lumens are a pretty great way to compare the amount of light put out by fluorescent lights, CFLs, HIDs, MH and HPS grow lights. However, when it comes to LED grow lights, lumens are not as a good measure anymore since LEDs usually give off light only in very narrow wavebands of light, and the weighted numbers from lumens aren’t as good a way to predict how much usable light is getting sent to the plants.

This takes us to another common way to measure light from grow lights…

Photosynthetically Active Radiation (PAR)

Instead of considering how much light is produced from a light source as humans see it, PAR actually considers only the spectral range of solar radiation from 400 to 700nm, which is the spectra of light that plants can use for photosynthesis. It’s pretty close to the range of light that humans can see, but not quite.

PAR refers to light in the 400–700nm range, as this is the range plants use for photosynthesis

Photosynthetically Active Radiation (known as PAR) is a measure of light in the 400–700nm range.

There is a lot of confusion about this term! PAR is actually just a way to talk about the spectrum of light between 400–700nm. It doesn’t actually measure anything. When people are talking about “the amount of PAR a grow light gives off,” they’re actually talking about how much light in the PAR range that light is giving off, or PPFD (Photosynthetic Photon Flux Density, sometimes abbreviated PPF).

So when most people talk about PAR, they’re actually talking about “PPFD” or how much light is getting sent that can be used for photosynthesis. When scientists and plant biologists are measuring how much light is being produced for a plant in experiments with the light spectrum, they almost always measure in PPFD.

Plants are more efficient at producing energy from light in some parts of the PAR spectrum than others. For example, we know that plants are most efficient at photosynthesis when using light in the red and blue range. This is why you see graphics like this when people are talking about PAR. This measures how well each type of chlorophyll in plants is able to absorb energy from light at different parts of the PAR spectrum.

Optimal absorption ranges for plants at different spectrums on the PAR scale

However, it’s important to note that plants can photosynthesize light from all parts of the spectrum, including in the green section as you can see in the graphic above, even if a plant doesn’t absorb energy from all types of light evenly.

One way green light is helpful is it penetrates further down into the canopy than red and blue light, which get mostly absorbed by the upper leaves. This adds to the amount of photosynthesis happening to leaves further down on the plant.

Additionally, since the spectrum of light also has an effect on how plants grow in ways that are completely separate from photosynthesis (as explained in this article), you probably wouldn’t get the best results focusing only on only photosynthesis anyway! As has been demonstrated by NASA and others, many plants grow more healthfully and faster when they also receive at least a little (not too much!) light from the green section, even though it’s not the most efficient light for photosynthesis.

Conclusion

As you’ve read today, plants use many aspects of grow light to develop into the bud-bearing plants we love! Instead of focusing on any one aspect, it’s important to look at your results as a whole!