What's the perfect light spectrum for plants?
Full spectrum, white sunlight?
Or would plants grow better under some finely tuned, colored LEDs?
Some sellers claim to have a special sauce, their magical formula of colors.
What does the science show?
And what does that mean for small-scale consumers, like home gardeners?
To start with, we might need to re-frame the way we think of light colors.
Kelvin color temperatures like 2100 K have a "warm" amber hue while "cooler" bulbs have
more blue light.
This gives you a general idea of a light's color profile.
But the system falls flat on its face once you throw LEDs into the mix.
It's true, some LEDs do show a Kelvin rating - but many plant grow lights do not.
Instead, they contain multiple diodes, each emitting unique bands of color.
So it's better to look at a light's color spectrum chart.
A spectrometer can analyze a light source revealing the color ratios.
Light manufacturers may share these spectral graphs which may be portrayed as a curve like
this or perhaps this.
What should we make of these?
use?
What ones actually matter in horticulture?
Plant scientists have long recognized PAR light as the most crucial range for plant
growth.
What is PAR light?
PAR stands for "Photosynthetically Active Radiation."
It includes all photons within the 400 to 700nm color range.
These are the most effective at driving photosynthesis.
A famous physicist, Keith McCree, tested 22 different plants to create this curve.
It shows the relative efficiency of various light colors for plant growth.
This helped to set the standard for PAR as we've known it, and it was the gold standard,
until recently.
(But more on that in a bit.)
The key takeaway about PAR, is that photons within this range count towards photosynthetic
growth.
And all photons are weighted or counted as equally useful.
Note: This range even includes green light.
Wait a second...
What about growers, who push blue and red diodes, claiming that plants don't use green
light?
They may refer to it as being completely wasted.
Is green light "rejected" or do plants really use it?
Where did this misconception come from?
Sure, plants are green because they reflect green light.
In part, that's true.
But in reality, "green leaves of land plants absorb a substantial fraction of green light."
If we look at a cross section of a plant leaf, we find that blue and red photons are quickly
absorbed in the upper surface.
But green light is reflected many times as it scatters into the lower layers, ultimately
driving photosynthesis.
So yes, plant leaves reflect SOME green light.
But even that's good because it allows light to penetrate deeper into plant canopies, reaching
lower leaves.
In the end, most of the green light is harnessed for plant growth.
But what about this chart?
It doesn’t help things when LED marketers misapply this graph of photosynthesis absorption
spectra.
People love using it to make green light APPEAR useless.
Even laying on that popular carotenoid graph still leaves a big gap.
How was this data gathered though?
This is based on testing of thin algal solutions.
Higher plants don't process light in the same way.
They have more complex structures, you know, like leaves.
That's why we refer to McCree's curve instead of these reductionist graphs.
For decades, researchers have shown that green light is useful and drives photosynthesis.
Green photons might work... but wouldn't it work better to use only red and blue photons?
Researchers tested this on lettuce and found green photons were just as effective as reds
and blues, yielding the same amount of growth.
Other research has shown unexpected benefits of green light, as a means of helping with
organic pest control!
The point is, don't prescribe to the oversimplified mentality that plants only need one or two
colors to thrive.
They may live and grow, but that doesn't mean all other colors are useless or even wasted.
And remember, photosynthesis isn't the only way light drives plants.
They also respond through the process of photomorphogenesis.
Various light colors can activate plant receptors which alter shape or growth response.
Take for example UV light.
It does less to drive photosynthesis.
Even so, plants have shown physiological responses to UV with improved disease and pest resistance.
UV radiation can induce the production of secondary metabolites.
Plants may activate genes that boost flavonoid concentrations including those super healthy
anthocyanins.
At the other end of the spectrum, we have Far-red light.
Plants are highly sensitive to it too.
Historically, it was presumed that Far-red doesn't do much to drive photosynthesis.
But it WAS shown to greatly effect plant development or morphology.
Plants have special photoreceptors, known as phytochromes, which allow them to detect
ratios of red to far-red light.
This can tell plants if they’re under a canopy of taller plants, triggering them to
stretch taller in response.
Then there are cryptochromes and phototropins, which are blue light receptors.
Have you ever noticed how plants in a windowsill grow towards the sunlight?
Phototropins use blue light to do this.
Plants also use photoreceptors to track day and night cycles.
That's how long-day plants, like spinach, know to bloom or bolt during long Summer days.
Meanwhile, short-day cannabis strains track 12 hour dark cycles to trigger their reproductive
phase.
All across the light spectrum you can find interesting ways that plants harness light
for energy.
Or they'll use it as a signal telling them what sort of environment they're in.
The more researchers experiment, the more complexities they discover.
One significant shift in the horticulture industry relates to the adoption of ePAR as
a superior spectral range for predicting photosynthesis.
While conventional PAR drops off at 700 nanometers, ePAR extends the photosynthetic range up to
750 nanometers.
This encompasses a large swath of far-red photons.
While these photons were previously discounted, researchers now assert that far-red will also
drive plant growth.
On one hand, this is big deal!
It could explain why some HPS lights seem to have an edge over LEDs, even when PAR photons
are matched.
It adds weight to the assertion that far-red diodes are useful in LED fixtures.
At the same time, it shouldn't come as a big surprise.
More than a decade before McCree's famous curve, Emerson had observed something remarkable.
By combining far-red photons with "white" photons, plants experience a synergistic effect.
Known as the "Emerson Enhancement Effect", this results in even more growth than one
would expect for the number of photons emitted.
As it turns out, plants use far-red photons for Photosystem One.
Think how this can benefit plants.
If you're a small sapling growing under a forest canopy, most of the red and blue photons
are absorbed by tall overhead trees.
But many of those "rejected" green photons bounce around and reach the canopy floor.
The same is true of far-red light.
The plants growing at ground level are finely tuned to harness those green and far-red photons
for photosynthesis!
Understanding the amazing ways plants use light, what do you think of when you see all
of the wavelengths contained in full spectrum sunlight?
Is that a bunch of wasted energy?
Perhaps we shouldn't be quick to dismiss entire color ranges.
It's common to see narrow band LED plant lights.
This one only contains two diode colors.
Is it custom tuned for some "perfect" ratio of colors?
Or is that just creative marketing?
It's true, you can tweak spectral compositions to trigger novel responses in a plant.
Light Quality can have a drastic impact on plant development, effecting the shape and
length of plant leaves.
Adding blue light may yield shortened leaves or shorter node spacing.
This is great if you want shorter plants.
Adding red or far-red can increase leaf surface area which makes lettuce growth explode.
Sometimes professional growers use HIDs with certain spectrum profiles to enhance results.
Bulbs are swapped out at different growth phases.
Metal halide lamps emit a larger amount of blue and UV, good for vegetative growth.
High pressure sodium lights are heavier in yellows, reds and far-reds.
Perfect for mature plants entering their fruiting or reproductive phase.
In large commercial operations, researchers can tune lights to get just the right effect
with a certain cultivar.
But studies have shown time and again that you can't extrapolate from one species to
another.
Besides genetics, there are many variables impacting plant response.
Temperature, light cycle duration, light intensity.
All of these can alter plant development.
Should home growers look for some highly specialized, tweaked spectrum?
How 'bout a well rounded balanced spectrum that's flexible enough to work under any condition?
If you're growing six different species under one light, why not just look for a broad or
full spectrum light source?
Through years of testing, I've observed that white light is a good thing.
It's good for the grower, making it much easier to monitor plant health.
Those nasty blurple LEDs are not fun to work around and they can really mess with your
eyes.
In a vertical farm when every single penny counts, red and blue LEDs yield the best energy
efficiency, which means higher profits.
At the same time, it's good to know that white light grows plants just as well.
Home gardeners have enjoyed working with fluorescent lights like linear T5s.
Those aren't very efficient though.
That's a big reason why LEDs are so appealing.
Electrical efficiency.
Some claim to be "full spectrum".
This can be done with phosphor coated blue diodes.
The coating shifts the energy state of blue photons.
The result is a mixture of wavelengths that produce white light.
This process slightly reduces an LED's efficiency rating.
But production costs are extremely low because these diodes are used at scale for conventional
home lighting.
To further enhance electrical efficiency and plant response, LED grow lights often add
in some highly efficient red diodes and even far-red.
It doesn't perfectly mimic sunlight, but it creates a well rounded spectrum that's likely
to support a broad range of plants.
So what's the best spectrum?
I'm in the camp for sunlight.
For one thing, you can't beat the price.
But even from a horticulture perspective, broad or full spectrum light sources work
well.
They're not wasteful to plant growth.
If you're growing at home and trying to decide, try to capture free sunlight when you can.
When you need more, I recommend some sort of LED grow light system.
Look for white diodes with extra reds to boost efficiency.
If you see a far-red diode, all the better.
For quick tips on how to compare entry level grow lights, check these two videos which
show what specs to look for.
Thanks for watching and supporting my channel.
I hope you have a great growing season.
And as always, I'd like to wish you Happy Gardening!