Why Green Light Still Shows Up Strong in the McCree Curve
When I first started trying to understand how plants use light, I was surprised to see how strongly green light appeared on the McCree curve. For most of my early gardening, I had assumed plants only really used blue and red light. I thought green light might be mostly wasted or reflected.
It took weeks of using a spectrometer and comparing how plants behaved under different light sources before I started to understand why green light still shows up strong in the McCree curve and what it means for everyday gardening.
This article shares how I came to understand the role of green light, what my measurements showed, and how this insight changed the way I interpret light readings in my own garden.
What I Thought at First
When I read about the McCree curve for the first time, I expected blue and red light to dominate plant use, because those are the regions most talked about in grow light marketing.
But then I began testing actual light sources that I used in my garden. When I looked at measured photon distributions, green wavelengths often had a higher count than I would have guessed.
That puzzled me. If chlorophyll absorbs best in blue and red, why would green appear so prominently?
What My Measurements Showed
Over several weeks, I recorded light spectra from full sunlight, LED fixtures, and shaded outdoor spots using a handheld spectrometer. My goal was to understand how different parts of the spectrum contributed to plant growth.
Here’s an example of one day under full natural sunlight:
| Wavelength Range | Photon Count Relative to Other Bands |
|---|---|
| Blue (400–500 nm) | Moderate |
| Green (500–600 nm) | Strong |
| Red (600–700 nm) | High |
| Far Red (700–800 nm) | Present but less useful |
I expected blue and red to dominate, but the green region was consistently strong when measured in real plant light environments.
This did not mean green light drove photosynthesis as efficiently as blue or red. Rather, it meant that sunlight naturally contains a large amount of green photons, and those photons reach the leaves in high numbers simply because of how the sun emits light.
How Plants Use Green Light
When I compared my measurements with plant responses, I noticed something important:
Plants did absorb and use green light, but not in the same way they use blue and red.
Under mixed spectrum light that included plenty of green photons, my plants often had fuller leaf structures and better overall balance. They were not just responding to the blue and red peaks. It seemed that green photons contributed to light absorption deeper in the leaf or helped balance signals that guide leaf orientation and structure.
This matched what I began to see in different garden spots:
In shaded areas where light had passed through leaves above, the remaining light was often richer in green wavelengths. Plants in those spots still grew, although differently than in direct sun.
Why the McCree Curve Shows Green Light Strongly
The McCree curve measures the relative effectiveness of wavelengths for photosynthesis, not just chlorophyll absorption alone.
Green light does not drive photosynthesis as efficiently as the peaks in blue or red. But because green photons are so abundant in natural sunlight and in many artificial light sources, they still register strongly when measuring the distribution of usable photons.
My own measurements confirmed this pattern. Green wavelengths were present in large numbers, even if each green photon did not contribute as strongly as a red or blue photon to the chemical reactions inside the leaf.
That abundance makes green show up strongly on the curve even though the efficiency per photon is lower.
In practical terms, this means that green light is not useless. Plants interact with it, and it plays a role in how light filters through leaf layers and how the entire canopy receives light.
How This Affected My Gardening Decisions
Before understanding this, I tended to favor lights with noticeable peaks in blue and red because I thought those wavelengths alone mattered. After testing and watching plant responses, I changed how I look at light.
Here is how my thinking evolved:
- I no longer ignore green light when I see it in a spectrum chart.
- I consider that green light helps fill in the distribution of usable photons, especially in full sunlight or broad-spectrum fixtures.
- When plants look balanced and healthy under a light source that has strong green content as well as blue and red, I pay attention to that as a positive sign rather than dismissing the green.
In everyday gardening, this meant that when I measure light around my garden, I look at the overall pattern rather than focusing only on a few peaks.
A Simple Takeaway for Everyday Gardeners
If you use a light meter or spectrometer in your garden and see a strong green presence in the spectrum, do not assume it is wasted light.
Green photons are part of the mix that plants experience, especially in sunlight. They help create a more even distribution of usable light and can reach deeper into leaf tissue when sunlight passes through upper leaf layers.
Plants do use blue and red more efficiently, but green plays a supporting role in overall plant light absorption. My own garden results showed that plants under broad spectrum light that included green often looked fuller and more balanced than under narrow spectrum sources that omitted it.
Final Reflection
At first, I underestimated the importance of green light because I focused only on efficiency per photon. Through repeated measurements and observations of real plant growth, I learned that green light still matters because it is abundant in natural sunlight and interacts with other wavelengths in ways that benefit plant structure and canopy light distribution.
Understanding why green light shows up strongly in the McCree curve helped me interpret my light data more realistically and made my gardening decisions more grounded in observation rather than assumptions.
Plants do not live by red and blue alone. They experience the full range of light that is available to them, and green has a role in how they make the most of that light.
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