The HPS vs LED grow light matchup is a clash of the titans. Is LED more efficient than HPS? Do LED grow lights yield more? Are they equivalent? How do we conduct the ultimate grow light review?
Growers endlessly speculate and debate which is better for plant growth. I know that you have an opinion, I certainly did!
When I stumbled upon the results of what I’m about to show you, I was completely shocked.
It went against everything I was told and read about online, which is why I am eager to share it with you.
You’re about to learn something you’re never going to forget, and your plants will thank you!
If you’re completely new to indoor LED grow lights, you might want to check out How LED Grow Lights Work
The Spectrum Breakdown: HID Bulb vs LED Chip
First of all, it is no question that LEDs are more efficient at creating light than an HPS lamp. LEDs will always beat HPS on a light meter…
But what we haven’t seen is the definitively better growth performance from LED grow lights, even at wattages that are nearly the same. It has confused the growing community for years, and for good reason!
Some days it seems like LEDs are just… a passing and expensive fad. Other days it looks like HPS is outdated after the next bulb swap.
You’ve seen the side-by-side grows on YouTube, however, they are often biased or just done wrong. It is hard to put together a scientific test! There are a lot of factors to consider: temperature, distance, humidity, water, nutrients, strain, grower experience…
If we took the simple approach and put one brand of light against another, that isn’t exactly scientific, and actually, won’t tell us much.
For example, comparing the “GrowBoss 400W LED” vs the “HotDawg DE 1000W HPS” light doesn’t help us understand why or what is causing the plants to grow better.
Don’t get me wrong, comparisons are valuable, but they are quickly outdated as new technology is introduced. Grow lights are always getting brighter and brighter…
What we need to know is what type of light is actually best for plant growth.
Am I right?
LEDs come in all sizes and colors: blue, red, purple, white, and every manufwhiteacturer claims they have created the “perfect spectrum”. With so many conflicting marketing claims, a lot of them must be wrong and only a few right… but who?
HPS bulbs simply come in different wattage sizes. Some HPS bulbs contain enhanced blue or other small spectrum changes, but at the end of the day, an HPS is an HPS.
Let’s face it, this is going to be a hard one to crack, so where do we turn?
Introducing Our Test Subject: The Tomato Plant
The tomato plant is a fantastic test subject. It’s commonly studied and is a good representation of the entire family of flowering & fruiting plants.
When growing a flowering plant, we want them to grow big and strong in the vegetative stage. Plants with many big leaves and thick stems give us the power and structure needed for the flowering stage to achieve massive yields.
Got wimpy and small plants in veg? Maybe on the tight and bushy side? You’re going to have disappointing results come harvest time. Plants need a robust, strong, and large structure to gain momentum for explosive growth in flower. There is no way around it.
In fact, you should be waiting until your plant becomes a certain size and strength before flowering, so if your growth spectrum isn’t helping you achieve that goal, think of the time you are wasting!
We will be presenting the results of an exciting scientific study that puts grow lights on a level playing field. This research equalized technology by testing all the grow lights at the same intensity of light.
We present the results of this study done by a scientific laboratory and used wideband, narrowband, and white LEDs, some with filters to test how plants responded to different % light spectra (blue/yellow/green/red/infrared) which shows us how plants truly respond to colors, which is very exciting.
This means we rule out the many factors that change from brand to brand. We can now focus on how efficient the spectrum of the light is to achieve growth.
A plant’s response to spectrum is timeless. No matter how far technology advances, plants remain constant and react to light the same from day to day.
We can’t beat the sun, but when creating light ourselves with electricity, certain colors are better than others.
Identifying the best performing spectrum is a true test of efficiency that will stand the test of time…
32 tomato plants per light source were tested in this study, and were grafted from clones for consistency. They were scored on dozens of criteria after 4 weeks of growth, and a few of them help us understand growth performance:
- Leaf Area (growing power)
- # of Leaves (growing power)
- Fresh Weight (plant structure)
- Dry Weight (plant structure)
- # of Flower Buds (beginning flowering results)
We want high scores in all of these categories at the end of our vegetative cycle. Bigger leaves, heavier stems, more flower buds = bigger harvest!
What you’ll discover is that colors and their combinations significantly affect how plants grow. Armed with this knowledge we can grow better plants in our own homes using the latest science!
Now…on to our contenders. If you’ve been growing for a while, I’m sure you’ll recognize these familiar faces.
Contestant #1: Purple LED Grow Lights
These “narrowband” LEDs are the oldest LED technology, developed in the 1990’s, and are still in use today. Each LED only outputs a small range of color.
Blurple LED manufacturers claim that plants mainly use red and blue light because that is what Chlorophyll A/B pigments absorb.
These lighting fixtures use many different “LED colors” (aka multi-band) to create a wider range of light, and the spectrum will match as closely as possible the graph of Chlorophyll absorption.
Additionally, these manufacturers claim that the reason why HPS is inefficient is that it pumps out too much green and yellow light. This makes this showdown a bit more interesting. They’re showing a spectrum, and we’re about to see some actual results 🙂
You might also see them mention similar statements about white LED lights — that yellow/green light is not important for plant growth.
This group also holds a belief that blue is for vegetative and red is for flower. If you have switches on your LED light, you will learn if they are doing what you actually want to your plants!
Below are two graphs from a leading purple grow light manufacturer, Kind LED Grow Lights. These claims are echoed by others in the industry.
Are they right? Do plants use only red and blue light? Are yellow and green colors wasted?
Contestant #2: White LED Grow Lights
White LEDs are increasingly popular and are becoming ubiquitous in our society. LEDs are making their way into every corner of our world, from street lamps to flashlights.
Huge advancements in this technology allow for a very efficient LED that can produce a rainbow of colors (unlike narrowband LEDs) by using a “phosphor”.
A phosphor spreads light across many colors like a prism. The shape and curve of the spectrum is designed to affect how our eyes perceive the color. “Kelvin” (Color Temperature) and “CRI” (Color Accuracy) are two metrics that white LEDs will always abide by.
White LEDs are created for human eyes. The LEDs are designed to create a color that is both efficient and conforms to a Kelvin and CRI rating. They are “locked” into certain shapes of the spectrum, so the color options are somewhat limited. If you’re interested in learning how to think beyond white grow lights for plants, see this article.
White LEDs will always try to maximize green and yellow light, because those are the colors that appear brightest to human eyes. That’s the lot in life for most LEDs – serving humans, and not plants!
While these LEDs are not rated nor designed for plants, some growers have found that they perform well because they provide a wide spectrum of light, not only red and blue.
Manufacturers that use these LEDs often say that they are efficient because they match the “McCree PAR graph” or “fit inside the HPS graph”.
Here is a graph commonly used by white LED grow light manufacturers, this one is taken from Nextlight LED Grow Lights. The green “line” is a white LED, the rainbow outline is the spectrum of a High-Pressure Sodium bulb.
Do you think white LEDs can match the growth performance of HPS? Does the graph above even match the HPS spectrum?
Below is the famous “McCree Curve”, the version found in the original scientific paper by Dr. McCree:
The White LED seems to fit in the same shape as this curve and “match” it.
Both purple and white LED manufacturers point to valid scientific graphs relating to plants… who will win?
Contestant #3: High-Pressure Sodium (HPS) Grow Light
Here comes the king! The reigning champion of grow lights. Do you think the HPS can be dethroned?
The High-Pressure Sodium bulb originated in the 1970’s and became ubiquitous in street and warehouse lighting. The “yellow glow” was everywhere.
Someone had the grand idea of trying this bulb for plant growth, and now growers swear by the HPS. It’s got a spectrum with plenty of yellow, red, and far-red, which is great for flowering.
How will it do in vegetative growth? Will it stand up to the science of LED?
HPS is generally shunned for vegetative growth because of a low amount of blue, so plants can get a bit stretchy. However, is it a myth that HPS shouldn’t be used in vegetative growth because of its spectrum? Will plants grow lanky with a weak structure and small leaves?V
Maybe plants really don’t use green or yellow light…
Okay! Here are the spectrum compositions that were used in the laboratory test.
These graphs help clarify exactly the ratio of colors in each spectrum. A specific breakdown is useful because it’s not (always) obvious when looking at a spectrum curve.
When we’re talking about photobiology, it’s important to work off of percentages and color ratios. These are the clues we have to understand what a spectrum is doing to the plant.
What do you think? Is the red and blue spectrum optimized for Chlorophyll absorption going to pull ahead?
Is the green light in the white LED spectrum going to bog it down?
What about the little amount of blue in HPS? Are plants going to grow weak and lanky in veg?
Here we go… prepare to be surprised.
Are you surprised? Remember, all of the lights were scientifically adjusted in a laboratory setting to be the same intensity in the same environment. 32 plants were used for each spectrum, which means this is a very accurate test.
Since HPS won in every category, it scores 100%. For the other two contenders, their score is measured as a percentage of that score. This means purple LEDs are only half as efficient and white LEDs lag by 25%.
This also means that more light from LED is needed to achieve the same results as HPS. If you do a quick search in Google for “LED light burn”, you’ll see horror stories… this is one reason why. More light is needed for equal results — eek!
If you’re a data person, check out the results table below:
|Fresh Weight (g)||Dry Weight (g)||# of leaves||Leaf Area (cm2)||Flower Buds|
To this day, even after analyzing this particular study for over a year, I can’t get over how spectrum caused such a difference in performance!
Different colors of light cause different characteristics to come out in plants. There are multiple layers of biological light receptors within a plant – they are complicated, living beings that sense for light information in their environment! It’s way cool…
Can you figure out what wavelengths were responsible for the growth changes?
And as far as performance goes, I think you will agree that there was a clear winner.
For you HPS growers out there, it remains the king! If you can handle taller plants in your grow setup, the HPS provided the biggest leaves, heaviest structure, and the most flower buds — a surefire winner for the vegetative growth of the tomato leading to a bountiful flowering harvest.
Now, we might want to add a tiny bit more blue to the spectrum to help with stretching. However, we can only do this to a point before yields start to diminish.
Did you think that blue light would have assisted vegetative growth? Notice how that as blue light increased, the smaller the leaves became. Less growing power!
How about that white LED? Green light certainly seemed like it gave a boost in the results. This must mean there is more to know about photosynthesis than just Chlorophyll absorption. If you’re going for LED, at least choose something that has a large amount of green or yellow light.
This spells some trouble for those of us out there who have already purchased LED systems… up to 100% more light from LED may be required to achieve the same results as HPS. How’s that for efficiency?
Is this the end for LED?
Far from it! If you couldn’t tell after reviewing these LED technologies, neither purple nor white was the best for plant growth. There has to be something better out there…
… and there is.
I have a surprise for you… this scientific study also tested new, emerging LED technology.
The results are stunning.
This LED technology allows us to attain the results of HPS at the same intensity.
Check out the results below. In some cases, HPS was outperformed…
Again, remember, we are working with the same intensity, only special colors within the spectrum are causing this massive boost in growth.
Do you have an idea of what could be causing the massive efficiency gain?
Two Men of Plant Science: McCree and Emerson
Two men made breakthrough discoveries in plant science. Their discoveries, together, unlock secrets we can use to maximize growth in our gardens with artificial light.
The first man, Dr. McCree, observed and recorded what is known as the McCree PAR Graph:
Many people look at this graph and draw the conclusion that plants do not use light above 700nm.
Light in this color (above 700nm) is called “infrared light”.
The graph does show little effectiveness above 700nm… That must mean infrared light has little to do with photosynthesis, right?
But get this:
That isn’t the right way to look at this graph. McCree put a leaf in a special growth chamber and gave it a single wavelength, and measured the result.
So — the key thing to note is that this graph shows the effectiveness of a photon once absorbed in a plant, but it is showing the result of how a plant responds to one color at a time.
I’ll repeat this…
One color at a time.
Our collective knowledge has this fact confused, so it’s important for you to understand how to interpret this graph correctly.
May your mind think differently about plants and light after this article!
What this is telling you is that the closer light gets to red (600-700nm) the more effective it it is, which is why red is so good for plant growth.
That brings us to the part where infrared light becomes incredibly effective.
It’s almost like it has a secret power…
Robert Emerson discovered the “Emerson Effect” in 1957.
Can you figure out the special ability that infrared light (700nm) has when it is combined with red light (680nm)?
That’s right, the amount of photosynthesis jumps by 50%. Instead of 1+1 = 2… It’s 1+1 = 3
Okay, so we really want infrared in our spectrum.
How do we do that?
Well, if you’ve read 7 Myths LED Grow Light Companies Tell You, it’s clear that the older “narrowband” LEDs is not very efficient at creating infrared light.
Also, it’s aim is off.
See the graph above? We’re looking for light surrounding 700nm.
Infrared diodes miss the mark at 680-730nm, reducing their effectiveness to activate the Emerson Effect.
So it’s best we avoid older narrowband technology and focus on creating a continuous band of light in the red and infrared range.
This way we can pick up the full benefit of the plant photosystems (I & II) that reside at 680nm and 700nm.
What’s that technology called?
The Wideband LED
Wait a second, I’m sorry, the graph above was actually the old narrowband LED.
This LED will have the following issues, so it’s best we look for another solution for our red and infrared:
- Low canopy penetration
- Oversaturation of the top leaf tissue, leading to light burn
- Limited color output, very small range
So, on to the new technology that is going to start taking over the LED game, the wideband red LED.
I hate to gush, but this type of light technology puts color right where we need it.
It’s dreamy really… Just think of the amazing plant growth you’re going to get.
Just look at all that color in the 680nm – 720nm range… Beautiful!
… sweet sweet yields ahead!