How Does Color Spectrum Affect Growing Marijuana Plants?

by Nebula Haze

Table of Contents

What is the Color Spectrum of Light?

How Each Light Spectrum Affects Cannabis Growth

What is the Best Spectrum for Growing Cannabis?

PAR vs Lumens: Which is Best to Measure Grow Lights?

Note: This article is aimed at intermediate to advanced cannabis growers looking for ways to improve their results.


What is the Color Spectrum of Light?

Although natural light might appear white to us, it is actually made up of many different colors. You can see this in action when you put light through a prism, or if you catch the sight of a rainbow. When light is refracted into its individual parts, you can see all the colors that make up that light.

All light is made of many colors, which you can see in a rainbow.

A rainbow shows you all the colors that are hidden in the light we see - these colors hidden in light, and the ratios between them, have a big effect on how a cannabis plant grows!

Over the years, scientists have learned that plants actually make “decisions” about how to grow based on the spectrum of light they receive. A different color spectrum can change how cannabis germinates, grows, and even makes buds. Although we have few options to change the spectrum of sunlight when growing outdoors, we have almost complete control of the spectrum when growing cannabis indoors since we’re using grow lights.

So what’s the “best” light spectrum for growing cannabis? Unfortunately, that’s a big question since each spectrum has its own unique effects and some are more useful to you than others, but let me share everything we know so you can decide what’s best for your goals and situation.

There is no “best” light spectrum for growing cannabis, but you can choose the best one for your goals and grow room.

So why do cannabis plants react to the spectrum of light? When growing outdoors under the light of the sun, the spectrum of light received actually gives a cannabis plant a lot of information about what’s going on in the world around them.

As just one example, during the spring and summer, more of the blue spectrum of light makes it to earth because the sun takes a more direct path through the sky.

Cannabis plants lazing happily in the sun - although the light looks white to us, it's actually made up of many colors and the plant uses this hidden information to grow better!In the summer, a plant responds to this bright direct light (with lots of blue) as a cue to grow vegetatively with lots of big leaves and short stems. The plant tries to spread out as much as it can and increase its leaf mass.

Blue-Heavy light (appears white or bluish) – Grow lights with a higher ratio of blue light are often used in the vegetative stage as they tend to make cannabis plants grow short and squat, with big healthy leaves.

Blue-heavy light tends to keep cannabis plants shorter and bushier in the vegetative stage, but reduces yields if used in the flowering stage. Blue-heavy light typically appears white or tinged with blue.

2 Marijuana plants under a Metal Halide Light in a grow tent - vegetative stage

As fall approaches, the sun starts hanging lower in the sky. As a result, some of the blue light gets filtered out by the atmosphere and a higher percentage of the light that reaches the plant falls within the red spectrum. More light in the red part of the spectrum is one sign to the plant that summer is coming to an end and it’s time to get in gear to make flowers/buds before winter. As a cannabis plant gets exposed to a higher ratio of red light, it reacts by growing longer stems with more space between the leaves, getting as tall as possible for the best position to start making buds and pollinating via the wind.

Red-Heavy light (appears red, yellow, or pink) – Grow lights with higher amounts of red are often used in the flowering stage to encourage plants to grow tall (stretch) and to help promote budding, resulting in bigger yields for the amount of electricity used. Plants may also switch to the flowering stage faster under red-heavy light than under blue-heavy light.

Red-heavy light tends to cause plants to grow tall and stretchy in the vegetative stage, but increases yield when used in the flowering stage.

This is a 600W HPS grow light, which has light that appears a golden yellow due to the lack of blue in the spectrum. HPS grow lights get excellent cannabis yields but run extremely hot and are considered old technology. However, if you have a cool grow space and/or a small starting budget, they can still be a good choice (here’s the full setup).

Typically these days the best flowering grow lights for cannabis are LED grow lights, because they have custom spectrums designed specifically to get the best yields and bud quality from cannabis plants when used in the flowering stage. With high-quality flowering LEDs, the light usually appears mostly white with a tinge of yellow or pink. This is a 600W LED grow light that gives off a pinkish light. Modern LEDs produce some of the best yields per watt of any grow light, and also produce high bud potency.

Note: LED grow lights that appear “blurple” (Blue-purple) are old technology and typically won’t produce as good yields as modern LEDs for the amount of electricity. You want to choose flowering LEDs with a cannabis-specific flowering spectrum.

Avoid “blurple” LED grow lights if possible when growing weed. Their spectrum gets poor yields and lower bud potency compared to modern LEDs.

Avoid "blurple" LED grow lights for growing cannabis. Not recommended! They get lower yields and lower bud potency compared to modern LED grow lights.

In the wild, the ratio of colors in the spectrum of light allows a cannabis plant to “learn” a little bit about what’s going on around them, helping plants grow in the best way possible. As an indoor grower, it’s your job to be the sun and tell your plant what to do. Luckily, other growers have already done the work of figuring out the best spectrum to get the most from your cannabis plants, so that you don’t have to think about it much.

If you’re trying to keep plants short with lots of leafy growth, use grow lights that give off light in more of the blue spectrum during the beginning of your plant’s life, typically marketed as “vegetative” or “all purpose” grow lights. When it’s time to switch the plant to the flowering stage, you might switch the lights to something that has more of the reds and yellow/oranges that cannabis likes during the flowering stage, typically labeled as “flowering” or “bloom” lights.

But honestly, you don’t have to follow the “rules” if you don’t want to. I have used an HPS bulb from seed to harvest, which gives off primarily red/yellow light. Even thought young plants grow stretchy under red light, it can be easier to train the plants when they tend to grow tall and have a little bit more space between nodes. But in a height-limited space, I give lots of blue light to help keep stems naturally shorter. In a tiny space like a micro tent, I might give blue-heavy light from seed to harvest in order to help keep plants shorter, even though that reduces the final bud size overall.

I have grown plants from seed to harvest under red-heavy grow lights. This makes young plants grow tall (less wide and bushy). However, longer stems may not cause problems if you have plenty of height to grow. Notice how tall these plants are. I used the lollipopping technique to remove the lower leaves and ensure the top buds got all the energy during the flowering stage.

I have also grown plants from seed to harvest under blue-heavy light, which reduces yields (a little), but helps keep them short. Great for a mini tent or short grow space.

You can grow your cannabis plant from seed to harvest under any grow light as long as the light is bright enough. Ideally, most growers should try to grow with more blue in the vegetative stage and more red in the flowering stage. But these days with LEDs, you don’t have the option to switch partway through with bulbs. That means you typically have to switch the whole grow light.

So what do you do when you can only choose one light spectrum from seed to harvest?

How big a difference does spectrum make to plant growth?

As long as a cannabis plant is getting bright light with at least some red and blue, it will grow normally, so any reputable plant grow light you get on the market will do the job and get you to harvest with top-quality buds. Even blurples will get the job done and give you good weed at the end.

Although certainly not essential to healthy plant growth it is considered good form to choose lights in the proper light spectrum to encourage the growth you want in the vegetative and flowering stages of the marijuana plant. If you have a tool, why not use it?

However, keep in mind that different strains are affected differently by the color spectrum of the light. Some plants are barely affected at all while others might react much more strongly. It’s important to remember that the changes caused by light spectrum (such as stems tending to grow longer, or slightly smaller yields) are not huge. Many other factors drive some of the same changes.

For example, if your plant isn’t getting enough light, it will grow long and lanky even if the light has lots of blue because it’s “reaching” for more light.

Cannabis seedlings will grow long and lanky if they're not getting enough light, even if that light has a lot of blue in it

On the flip side, if you give your plant ultra-high levels of light with lots of yellow, it tends to stay shorter because it’s getting so much light that there’s enough blue and no need to get taller. Since almost all grow lights give off a spectrum of light that is suitable to healthy cannabis growth, in many ways the amount of light has a much bigger effect on your plant than the spectrum.

The best way to increase yields is to give your plant more light altogether.

In other words, giving your plant more light is going to give you a bigger return on yields than simply changing the spectrum. So if it’s possible to give your plant more light (up to a point), that is going to increase your yields even if you’re adding light in the “wrong” spectrum. Even bright white lights that dip into the green spectrum will produce great plant growth. If it’s a grow light, it will do the job. Learn about different cannabis grow lights.

Ready to choose a grow light for your cannabis plants? Here’s a list of common cannabis grow light setups with yield estimates.

 

In-Depth Look at How Light Spectrum Changes How a Cannabis Plant Grows (Advanced)

Light is part of the electromagnetic spectrum, which includes not just the visible light spectrum, but also includes X-rays, Gamma rays, and infrared light. Plants react to the light spectrum via a process known as photomorphogenesis, which is completely separate from photosynthesis. This section has nothing to do with PAR or lumens or how a plant makes energy from light (which I talk about below).

In this section, we’re only talking about how the plant uses hidden information contained in the spectrum of light to grow better.

Although a plant can’t physically get up and move to a new spot, it is constantly growing and changing its growth patterns to maximize the amount of light it gets. For example, a plant turns its leaf blades to face the light as the sun moves across the sky and then puts all its leaves down at night to save energy and protect itself. Like all plants, a cannabis plant has ways to measure different spectrums of light, each giving the plant information about the best way to grow.

Cannabis plants “smell” the light spectrum to conduct photomorphogenesis

When a young cannabis plant is under the light, it's doing more than eat, it's asking itself, "What does that light smell like?"

Plants “smell” the light spectrum!

Like humans, plants use the signals from their senses to help find the energy to grow. For example, we have a nose to smell something tasty and help us seek out food. But although we use our noses to capture the hidden messages in smells, ultimately a nose can only signal to us about possible food sources. We can’t actually eat food with our noses. Animals need our mouths to get food in our bellies.

Plants have certain light sensors (photoreceptors) found all over the plant that act kind of like the plant’s “nose” to detect information about the light spectrum. Just like how humans use their sense of smell to help find good food, the plant uses the information about light spectrum to help the plant grow in a way that gets the most of the best light available.

However, similar to how we can’t actually eat food with our noses, plants can’t make energy from light using their photoreceptors. (Plants do photosynthesis in their chloroplasts).

Summary: When it comes to photoreceptors and a plant’s response to light color, we’re talking about the “nose” of the plant, not the “mouth”. The reactions to light spectrum we talk about below are completely separate from photosynthesis. If you’re interested in how the plant actually obtains energy from light via photosynthesis, check out the section at the end of this page about PAR, lumens, and light intensity.

Modern LED grow lights use custom color spectrums to get cannabis plants to grow more efficiently.

Jump straight to the section that interests you!

 

UV-A / UV-B / Ultraviolet Light (280nm-400nm)

You may be familiar with the term UV-A/UV-B when it comes to protecting your skin outdoors. We use sunscreen to protect our skin from UV rays from the sun in order to prevent skin damage. Too much UV exposure can give you sunburn now, and even carcinoma/melanoma in extreme cases!

When we’re talking about plants, UV-A (315 to 400 nm) and UV-B (280 to 315 nm) are sometimes classified as part of the “blue” light spectrum, though in the last several years, we’ve discovered the UV spectrum has its own unique effects on plants.

Sunblock is used to protect human skin from UV light. Plants make their own sunblock to protect them from too much UV.

Cannabis plants make their own "sunscreen" to protect themselves from UV-A and UV-B rays from the sunLike humans, plants given too high levels of UV light become damaged, and we’ve learned that plants react to UV rays by making chemicals, antioxidants, and enzymes to help prevent and repair damage. Basically, they’re making their own “sunscreen”. It’s a little bit like how our body gives us a tan after UV exposure to protect us from further skin damage.

In my experiments and lab tests, I’ve found that providing a lot of extra UV-B light to cannabis plants in the flowering stage seems to slightly lower yields while increasing the overall trichomes (“sparkle”) and increasing the ratio of terpenes (strength of weed smell). I’ve heard some growers claim that UV light may increase the THC levels or some other aspect of the potency of cannabis buds, but in my tests, the high-UV lights actually seemed to produce slightly lower overall THC levels, which surprised me. So I personally would only go for extra UV when you want strong smells and sparkle, and I would avoid high UV supplementation if your goal is the biggest yields or most potent buds.

However, I have only tested this a few times and I (so far) haven’t run into any other side-by-side tests from other growers who did lab testing. It is still not well-established exactly how cannabis responds to UV exposure. Please let us know if you have come across any empirical tests on UV with cannabis flower production.

Most grow lights that use bulbs naturally produce some amount of UV rays, though far less than the sun. LED grow lights are an exception; they need special UV diodes to produce any UV light. There are other sources of UV light such as reptile lamps.

Blue Light (400 nm – 500nm)

Blue light is very important. Without at least some amount of blue light, most plants won’t grow normally. The plant has several different ways it measures blue light, and they each have their own effects. For example, when you see a plant growing towards the light, it’s actually only responding to just the blue light. Without any blue, the plant wouldn’t know where to grow.

Blue light also has an effect on how stems and leaves tend to grow. Cannabis seedlings and plants given plenty of blue light tend to stay short, with short stems and squat growth. They tend to grow big leaves and spread them out. For many growers with limited height to grow, high-blue grow lights help keep plants as short as possible in the vegetative stage.

Which grow lights provide a lot of blue? Cannabis growers can use “vegetative” LEDs, fluorescents like T5s using “bright white” bulbs, or metal halide grow lights to give their plants extra levels of blue during the vegetative stage. These lights help keep young plants from growing tall and lanky, encouraging lots of leafy growth.

Vegetative LEDs (also listed as “veg” or “veg and bloom” or “all-purpose” or “full spectrum” or “4100k” or “4000k”) are high in blue to help keep plants short

Metal halide lights are powerful and provide a ton of blue. MH bulbs are typically used in the same fixture as an HPS. In fact, MH/HPS fixtures often come with both kinds of bulbs so you can switch partway through.

Metal Halide grow lights contain a lot of light in the blue spectrum, and are well-suited to vegetative growth

If using fluorescent lights like T5s in the vegetative stage, get “bright white” (also known as “6500k”) bulbs.

A bright white fluorescent light (6400k) contains a lot of blue light, which can be a good choice for the vegetative stage.

Blue light also works together with red light to help the plant “know” whether it’s day or night time. In other words, blue helps set the plant’s circadian rhythms. You may notice that cannabis plants start drooping right before the lights go off each day, and they start perking up right as lights come on. This is a way for the plant to save energy while it’s “sleeping.” Since the blue in the light helps it “know” the time schedule, it will prepare as best it can for lights-out and lights-on.

 

Green Light (500 nm – 600nm)

There has been quite a bit of research on how different spectrums affect plant growth. Especially over the last decade by researchers at NASA trying to find a way to grow plants most efficiently in space with LED grow lights.

We have long known that plants need at least red and blue light to grow normally, and some plants (like lettuce) can grow well under just red and blue light. It used to be thought that green light is not the most efficient spectrum for photosynthesis (making energy from light) and therefore the most efficient light spectrum from plants is a mix of red and blue only. However, discoveries found that green light does produce significant photosynthesis under strong light at least in lettuce plants. On top of that, it’s been discovered that green light has a significant effect on how plants grow. Most importantly to you as a cannabis grower, we’ve discovered the right amount of green light is crucial to the best results when growing cannabis.

The presence of green in the light spectrum is what causes leaves to appear green.

The green color of these cannabis leaves appears to us because the plant is reflecting back the green contained in sunlight

Green light has been shown to be involved in seedling and vegetative development, the initiation of the flowering stage, CO2/water usage, stem growth and overall plant height. Additionally, green light penetrates further down into each leaf, and also penetrates further down into the canopy, than red and blue light (which get mostly absorbed by the upper leaves). That means a good amount of green light adds to amount of photosynthesis happening to leaves further down on the plant, making buds tend to grow longer and deeper into the plant. There’s probably more we don’t know about yet.

But like the other spectrums, plants seem to want just the right amount of green light for the best growth, not too much or not too little. After many experiments with green light, the NASA Biological Sciences research group has reported that light sources consisting of primarily green will cause plants to grow slower. However, combinations of red and blue including up to 24% green enhanced growth for some species of plants. We’re learning that green is really important. In fact, in one experiment NASA did with lettuce, giving plants only blue/green/red light produced higher yields than their version of full spectrum grow lights.Note: There’s plenty of evidence that different types of plants respond to spectrum differently, so make sure you take any light spectrum study with a grain of salt if it wasn’t done on actual cannabis plants.

Cannabis light needs are definitely not the same as lettuce. Luckily for you, LED growers and manufacturers have been experimenting with cannabis plants to discover just right right amount of each color of light to increase your cannabis yields. You’ll notice you you can always see the green color of leaves under modern grow lights because they all contain some amounts of green.

This old-fashioned LED grow light contains no green. You can tell because you can’t see the green color of the leaves except at the front where natural light is coming in from the room. Not ideal!

Cannabis plants growing under LED grow lights - you can see the natural green color of the leaves only in the front, since a little bit of natural light is hitting them from outside the tent. The rest of the leaves look purple!

In addition to the effects on how a plant grows, green light is what make plants appear natural and green to us. Plants under LED grow lights with just red and blue light tend to look dark or purple similar to how plants look very yellow/gold under a HPS. By adding a little bit of green we can actually see the natural color of leaves, which makes it a lot easier to spot problems like nutrient deficiencies.

Leaves appear essentially colorless under LEDs that don’t have any green diodes. In the picture above, leaves appear purple except in the front where a little bit of natural light from outside the tent. You actually see the leaves’ true green color wherever it’s touched by light that contains green.

Neat fact. Although your plant can respond to green light in some ways, it can’t “see” green when it comes to photoperiods and knowing when it’s day or night. So you can actually use green light to look at your plants in the dark and it won’t interrupt their dark period at all.

Use green light to check on plants in the dark so you don’t disturb their beauty sleep. Make sure to get a light made for this purpose, like the one here. Not just any green light will do.

Get a green horticulture headlamp light to check on your cannabis plants at night without disturbing their dark period.

Red & Far Red / Infrared Light (640-740nm, especially 660 nm & 730nm)

We can see red light of course, but generally, humans can’t see far-red (infrared) light, although we can feel it as heat. Plants, on the other hand, can sense both red and far-red light.

Note: Red light is considered to be 640-700nm and far red light is considered to be 700-740nm. However, the ratio specifically of 660nm and 730nm light effect when cannabis plants start flowering, and how fast buds form. Keep reading for the full details.

Red light is extremely important to all plants and cannabis plants are no exception. When it comes to photosynthesis, plants are usually best able to make energy out of red light. Kind of like how cake seems great at helping humans build fat compared to other sources of food. In fact, many plants can actually grow even if they only get pure red light, though they won’t grow as big or as healthy as they do under a more suitable spectrum light. Every type of grow light, even ones that are rich in blue like metal halides, provide quite a bit of red light.

A cannabis plant can see red light and infrared light at sunset - it uses this information as a signal to help it know when it's day or night!

However, when it comes to red and far-red light as far as signals (not photosynthesis), it’s not the amount as much as the ratio of red to far-red that’s important to cannabis plant because this ratio has a profound effect on cannabis development and especially how buds form. It is also what triggers budding through a process known as photoperiodism. For the purposes of a cannabis plant and its flowering response to light spectrum, the most important red light is ~660nm and most important far-red light is ~730nm on the light color spectrum. Let me break it down and explain how red light affects cannabis plants throughout its life cycle.

A germinated seed moves toward red light

After a seed cracks open its shell, but before it makes it to the surface, the seedling senses gravity to aim the root to go down. On the other hand, the seed/seedling grows in the direction of greater levels of red light compared to far red. It likely responds to red because blue light doesn’t make it as far underground. Since the seed can sense red coming from the surface, it “knows” to grow in that direction.

Once the seed reaches the surface and gets exposed to blue light, it stops acting like a root and starts acting more like a seedling, opening its leaves and growing towards the closest source of blue light. If it doesn’t get a good amount of blue light at the surface, it continues to grow its main stem longer and longer without making any leaves, acting more like a root than a plant because it still “thinks” it’s underground, or at least hidden from the sun.

Seeds move toward the light, and the leaves inside don’t open up once the plant hits the surface…it’s looking for the right amount of the right color of light!

This cannabis seedling has started emerging from its shell after being exposed to light at the surface!

If a plant doesn’t get access to enough light at the surface, it stays in “root mode” and keeps growing taller without opening up its leaves

This cannabis seedling is still in "root mode" because it hasn't gotten enough light - it's just trying to grow closer to a light source before opening up its leaves.

Stems tend to grow long and stretchy when plants get a lot of far-red light compared to red light.

In bright sunlight, a cannabis plant tends to grow short and squat. This is because direct sunlight usually has more red than far-red and the plant reacts to this ratio. So if a plant is getting more 660nm than 730nm light, the stems tend to stay short and the plant grows a lot of nodes with shorter stems.

On the flip side, if a plant is getting a more 730nm light than 660nm, it tends to grow tall and stretchy. This is because, in the wild, when a plant is surrounded by a lot of vegetation, the surrounding leaves absorb a lot of the red light, and so whatever light does filter down to the hidden plant or stem has a much higher ratio of far-red light.

In response to higher levels of far-red light, stems will start to elongate and grow taller, as the plant is “stretching” up towards the light until it gets a ratio with more red and “senses” it’s in direct sunlight again.

If a plant is surrounded by greenery, it starts sensing higher ratios of far-red light, and starts “stretching” upwards to grow past the other vegetation and get access to better-quality light

When a cannabis plant is surrounded by vegetation, it reacts by trying to grow taller to get access to better light

What does this mean to you as a grower? Since “flowering” grow lights contain a lot of far-red light, that means if you use flowering grow lights in the vegetative stage that your young cannabis plants will tend to grow more tall and stretchy vs short and squat. That’s not ideal if you have limited height to grow, but luckily stretching can be controlled with simple plant training (bending stems and tying them down with plant twisty tie).

Photoperiod plants like cannabis help keep track of days and nights using the ratio of red to far-red light

You’ve seen red light at sunset. The reason you see red when the sun is on the horizon is because red light has the longest wavelengths, which travel the furthest. At sunset, the light is fading quickly, and when all other light is too weak to see, you can still see the red coming through the horizon before it disappears too. Infrared light has even longer wavelengths, so it hangs on just a little bit longer. That means for the plant, the highest ratio of far-red light occurs at sunrise and sunset.

When it comes to sunlight, the highest ratio of far-red light occurs at sunrise and sunset

At sunset, a cannabis plant can see that there are higher levels of far-red light as the sun gets lower on the horizon. It uses this information to help know when it's day or night.

The information contained in the light is important to a plant because it needs to be able to “know” when it’s the beginning or end of the day. A plant can take advantage of the light spectrum information to keep track of day and night by sensing when there’s the highest ratio of far-red light.

Far-red light is the last thing a plant “sees” at sunset, so when there are high far-red levels followed by a period of darkness the plant “knows” that it’s nighttime. Like a helpful internal clock.

A cannabis plant keeps track of the ratio of red and far red light to help it set its internal "clock"

When a plant starts getting higher levels of red light, especially when combined with blue from the morning sun, it “knows” that the day has started again.  If a plant is exposed to red light (not far red) during the dark period, it will “wake it up” because it thinks day is happening. Blue light also affects the plant at night by messing up its circadian rhythms, though blue light alone likely won’t stop the plant from flowering.

These processes help the plant set itself on an internal clock. Keeping track of the length of nights is the main way a cannabis plant knows when to start flowering (making buds). When nights get long, it initiates the flowering response because the cannabis plant “thinks” winter is coming. So growers are able to force the plant to start flowering at any time just by changing the plant’s light schedules.

Note: Even if the plant doesn’t get far-red light before dark, it will eventually “realize” it’s nighttime, but it starts the clock 1-2 hours later because it didn’t get that far-red “sunset” signal. Because of this, there’s some evidence that if you provide a plant with a burst of far-red light right before it goes to sleep in the flowering stage, you could cut 1-2 hours off your night period and it wouldn’t interrupt the flowering cycle. That being said, it’s always a good idea to give your flowering cannabis plant at least 12 hours of uninterrupted darkness every night to make sure budding goes smoothly. When time schedules get weird, you sometimes end up getting hermies or have your plant revert back to the vegetative stage, which most growers don’t want.

When growing outdoors, cannabis plants react to the ratio of red and far-red in the sunlight to help with a variety of plant processes, including knowing when to start flowering.

Can I use a thermal imaging camera to watch over cannabis plants at night?

Yes. A thermal imaging camera won’t affect your plant’s dark period. I just wanted to include this in the section about far red light because many growers have asked, “Doesn’t the far-red light from a thermal imaging camera interrupt the plant’s dark cycle?”

The way a thermal camera (also called an infrared camera, or thermographic camera) works is it actually absorbs/measures how much infrared light is being given off by objects. It doesn’t give off far red light. It tracks it.

You can use an infrared camera to "spy" on your plants while they're sleeping - it won't interrupt their slumber

A thermal camera works by showing warmer objects as lighter colors. So for example, the cold ground will appear dark, while a plant is a little warmer and will appear lighter. A human is warmer still and appears almost white on such a camera.

A thermal (infrared) camera doesn’t actually give off any light, it only measures it. So although far-red light does have an effect on plants, it won’t be affected by a camera that doesn’t produce any.

However, if your camera has a screen, the light from the screen is definitely an unwanted source of light.

What’s the best way to spy on your cannabis plants at night?

I like the Wyze cams. These are meant to be used as security cameras. You can actually use a phone app to see through them at any time, and they include built-in thermal imaging cameras so you can see your plants at night too. Unless a full thermal camera like above, they’re actually pretty affordable. As one last bonus, you can put a memory card inside and use it to take timelapse videos of your plants to see how they grow over time.

A Wyze camera only costs $50 and can be used to check on your plants 24 hours/day, even at night when the lights are off

Here’s what the app looks like

You can even see your plants at night without disturbing them. Here’s an example of a time-lapse video I took of my last grow that shows how they look day and night under a Wyzecam. If I hung a thermometer in front of the camera I could even track the temperature over time.

Red light seems to influence not just the size and appearance of buds, but also their potency.

I had suspected this from previous tests. For example, whenever I grow with HLG R-spec lights, I seem to get higher levels of THC than growing those strains under some other types of lights. I had this confirmed at a cannabis conference last summer, I had multiple growers confirm light spectrum makes a difference to cannabinoid production. The problem…. it depends on the strain.

For example, some strains tend to produce higher levels of THC but lower yields if they receive a lot of far-red in the light spectrum. But other strains get lower yields without the increase in THC. Part of why this can be such a confusing topic to test is that every strain is different. We will definitely be investigating this in more detail. Check out my side-by-side tests for more experiments about increasing THC.

Can more far-red light during the flowering stage increase cannabis potency? Perhaps.

Looking at the trichomes on the plant through a magnifier. Trichomes are also known as crystals and resin glands depending on where you live.

 

What is the best spectrum for growing cannabis plants?

We’ve learned that spectrum changes how a plant grows through a process known as photomorphogenesis. This is like the plants “smelling” the color of light to help it “decide” how to grow.

So it’s definitely a good idea to try to match the light spectrum with your goals, but what do you do when it comes to buying a grow light? How do you choose between one spectrum or another?

If you have to choose between a “vegetative” or “all purpose” grow light or a “flowering” grow light, it’s generally recommended to use the flowering light spectrum (more red) for the whole grow. This results in slightly stretchier young growth but better bud potency and 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.

In other words, the color spectrum is important, but it won’t make or break your grow. The total amount of light is much more important to your final results than anything else.

The total amount of light usually has a much bigger effect on your results than the spectrum of light. Brighter light = bigger yields (to a point).

Outdoor cannabis bud paradise

Note: I’ve used all types of grow lights during all stages of growing marijuana, and even when you’re not using the “right” color spectrum, you will still produce good cannabis buds with almost any strong grow light.

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

Light energy (total amount of light, which is essentially “food” for your plants) is what matters most when it comes to cannabis 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 up to 40% more yields compared to an untrained plant in the same setup)
  • Upgrade to bigger or more efficient grow lights

Learn more about the different kinds of marijuana grow lights for indoor growers.

 

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 received 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, and can be compared against each other to a point. However, when it comes to LED grow lights, lumens may give unpredictable readings since LEDs usually give off light in relatively narrow wavebands of light. Although a lux meter is still an excellent way to measure how much light is getting to the plants within the tent via a direct comparison, the weighted numbers from lumens aren’t as good a way to predict how much usable light is getting sent to the plants in absolute terms. In other words, you may get unpredictable results if you compare the lux reading of LEDs against each other or against other lights. With lux meters, they’re best for measuring how the light changes within a grow space.

Which 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 what we used to think was 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 a 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 light spectrum, they almost always measure in PPFD.

What about ePar Meters?

An ePAR meter measures light all the way up to 750nm, while a regular PAR meter only goes to 700nm.

Learn more about PAR vs ePAR meters here.

What to focus on for best results when it comes to quality of light:

As you’ve read today, cannabis uses many aspects of 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.

  • Power is more important than spectrum. The total amount of light given is more important to growth than spectrum (as long as you’re using a regular grow light).
  • Give bright light with lots of blue for shorter stems and bigger leaves
  • Give light with lots of red/yellow to promote germination, for longer stems, and to promote flowering
  • Give a wide spectrum of light that includes green (this is pretty much all modern grow lights)
  • As long as you start with a great cannabis grow light, you don’t really need to worry about spectrum. Always train your plants to get the most out of your grow light (up to 40% more yields under the same grow light compared to not training) or if you’re still having trouble reaching your yield goals, upgrade to a bigger light!

I hope that helps you cut through all the conflicting information and gives you the information you need to make the right choices for your cannabis garden.

 


 

Reference

Changes Direction a Plant Grows

  • Phototropism – Plant bends towards light
  • Gravitropism – Plant points new growth upwards, opposite of gravity’s pull

Changes How a Plant Grows

  • Photomorphogenesis – plant’s response to spectrum of light (affects germination, length of stems, and when a cannabis plant starts flowering)

Determines How Much Energy is Produced from Light

  • Photosynthesis – The process of capturing energy from light to produce sugars in the plant to power growth! The amount of light (often measured in lumens or PAR/PPFD) is directly related to how fast a plant grows!

 


 

Jump to….

How Long Does It Take to Grow Weed?

How to Train Your Plants

7 Tips for Growing Top-Shelf Buds

5 Ways to Increase Yields (with any strain)

Where is the best place to buy marijuana seeds online?

 


 

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