How do plants react to red and blue light?

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In this article, we’re going to talk about red light and blue light. How do plants react to a red light and blue light? Should you get a grow light with bloom or veg-specific modes? Or can you grow successfully with one light? Hopefully, by the end of this post, we can answer those questions and have you on your way to producing a nice healthy yield.

As we’ve covered in a different blog entry, plants respond differently to different types of light. In general, plants prefer to use red light for photosynthesis, followed closely by blue light. The reason being, red photons are high enough energy to be used by Photosystem 1 and 2 to perform photosynthesis, but not too high energy so that the plant wastes a lot of energy converting them down to a usable energy level. Blue light is of shorter wavelength, therefore higher energy, and the plant has to use up the energy to lower its energy level to be usable in photosynthesis. There, we just covered the first semester of Introduction to Plant Biochemistry in like, a paragraph.

There’s more to the story, though (of course). Red and blue light have pretty important effects on the life cycle of your plants. In nature, sunlight contains every spectrum of visible light, and they’re all important for your plant. However, the relative proportion of each type of light will vary slightly through the different seasons, and depending on the environment your plant is in, which vastly affects your plants’ growth.

Plants don’t have eyes like us to really know what is going on around them, so they use photoreceptors (hormones that sense light) to “measure” the abundance of each type of light that is present. This then lets your plants know two things:

1) what other plants are around me taking light away from me?

2) what season am I in right now/what should I be doing?

Disclaimer: before you keep reading, obviously there are more things happening here like changes in temperature, humidity,  soil changes etc. but we are just talking about light here, so just relax.

Imagine for a moment that we are a new seedling of some kind of plant chilling on a forest floor. There are lots of other plants around us taking up light that we need. In nature, due to the characteristics of their wavelengths, blue light is absorbed first by plants, then red light, then far-red light. So, if there are other taller plants around us, they will be taking up all the blue light, while letting us have a more of the red light. If we are getting a lot of this red light early in our life, this tells our photoreceptors that there are lots of taller plants around us (or that we’re still in the soil), and we need to grow taller. So, we, the plant, invest growth in our shoots and stems, mostly forgoing growth in leaves (for now) until it can eclipse the plants around us and get to that sweet, sweet blue light in the heavens above.

If you begin growing a seedling and it is getting too tall and wiry for your liking and not producing any leaves, this is likely because it’s getting too much red light and not enough blue light this early in its life.

Once we grow taller than these other hypothetical plants, we can start to bask in the lovely blue light that the sun is shining down upon us. This tells us (the plant) that we can stop growing taller for now and it’s time to create leaves, to increase the surface area of our chlorophyll-containing leaves. The presence of blue light inhibits the hormone auxin which tells us to stop growing up and start growing out. If you begin growing a single, young plant under sunlight, it’s getting a lot of blue light right away. So, it knows there are no other plants around it and it doesn’t have to waste energy growing stems upwards, and can just invest straight away to more growth in outwards (auxiliary) growth in the form of leaves and secondary stems. So because it’s getting lots of blue light it will begin to produce leaves and grow short and squat.

If you begin growing a seedling and it doesn’t grow up at all and just starts growing leaves and spreading out, too short and squat for your liking, it may be getting too much blue light this early in its life.

So, you can see that a proper balance of red and blue light are crucial for a seedling’s early life. Enough red light to grow tall and strong while having lots blue light to invest in its leaves. This is why we generally, want lots of blue light in a vegetative cycle for cannabis. Blue light = bushy growth, red light = tall growth. Remember how I said that the type of light also tells the plant what season we’re in? Well during spring, there is a higher amount of blue light in sunlight. So more blue light tells a plant it’s spring time and it’s time to grow (along with a lot of other factors).

This is also the reason that you will often see LED grow lights marketed as having a blue-heavy vegetation mode, which will promote leafy growth. But, when you’re ready for flowering your plants, however, many grow lights market their lights as having a red-heavy bloom mode. Why is this?

Well, in nature, during summer red light is relatively more abundant than it is in spring. The red light is sensed by photoreceptors and tells the plant: “Hey, it’s summer time! Let’s make flowers so we can pollinate and hopefully spread our genes around the world before winter comes!” Or something along those lines… So the red light (along with the change in the amount of daylight/artificial light it is receiving) induces the plant to first, stretch out, and in the case of cannabis, potentially even double their height in as little as two weeks (remember why from earlier?) and then begin to produce flowers. Once your plants begin to produce budlets, the plant will be less focused on vegetative growth and will focus the majority of its energy on this reproductive cycle. For this reason, many LED grow lights will have a bloom-specific function that is red-heavy during bloom to accentuate this type of growth, almost completely eliminating the blue from its spectrum to avoid vegetative growth, focusing on red for bud development. Having too much blue light present during this stage can waste energy in axial growth and also produce shorter buds (something you may be finding with one-mode LED lights) because of the inhibition of auxin, inhibiting upwards growth (how you want your buds to grow, up).  So remember, red light not only will induce flowering but also upwards growth, leading to big, long beautiful buds. Then, unless you’re growing with auto-flowering plants (a topic for another time) you can just return your plants to a vegetative stage and do it again!

So, that is essentially how your cannabis plants grow and react in response to the presence or absence of red and blue light throughout their life cycle. This was a fairly basic explanation of what’s happening, so make sure you keep reading up to learn more. One of the other most important factors that tells your plant whether to grow or flower is the light cycle it is being exposed to. We skimmed over it in this blog post but it’s also very important to understand what’s going on so make sure you check that out as well. But, we answered the questions we wanted to when we set out at the beginning of this post. We certainly can grow great yields with a well-balanced single mode red/blue LED grow lights, and LEDs with bloom/veg specific modes will simply (hopefully) accelerate the specific type of growth we are targeting.  If you’re not sure what to look for, make sure you check out our buyers guide for LED Grow Lights.

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