Why Green Light Still Shows Up Strong in the McCree Curve

Many growers know that chlorophyll a and b absorb mostly in the blue (~430‑470 nm) and red (~640‑680 nm) light ranges. You may also have heard claims like “green light is largely useless for plant growth.” But when you look at the McCree Curve—showing the quantum yield of different wavelengths for photosynthesis—you’ll notice green light still has a significant response. How can that be? Let’s dig in.

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Chlorophyll Absorption: Red, Blue, and the Green Gap

• Chlorophyll a has absorption peaks around ~430‑450 nm (blue) and ~662‑680 nm (red).
• Chlorophyll b helps by absorbing light mainly in regions ~453 nm and ~642 nm.
• Green light (roughly 500‑600 nm) is absorbed much less strongly by these pigments — it’s more “weakly absorbed” than red or blue.

That’s why red and blue are often emphasized in grow light design.

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Why McCree Response to Green Light Is Still Relatively High

Here are the scientific reasons why, even with weaker absorption by chlorophyll, green light shows up in McCree’s photosynthetic response curves:

1. Leaf Penetration & Canopy Scattering
   Green wavelengths penetrate deeper into layers of leaves because blue and red are absorbed more in outer layers. Thus, green light can reach inner or lower leaves that blue/red cannot.
2. Spectral Redistribution
   Light scattering inside the leaf tissue and among leaf layers means some of the green light is indirectly utilized even if it’s not the most efficient wavelength at the surface.
3. Accessory Pigments & Secondary Absorption
   Other pigments (carotenoids, etc.) can absorb some green light and transfer that energy to chlorophyll, contributing to overall photosynthesis.
4. Balance in Full Spectrum Light
   Natural sunlight includes plenty of green light, so plants have evolved to make use of it. Artificial lighting that tries to mimic sunlight must include green for a more natural, balanced spectrum—not just for efficiency but for other plant responses (leaf morphology, photoprotection, etc.).

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When Green Light May Not Be Enough

While green light is useful, its role has limits:

• Green is less effective for driving photosynthesis compared to blue or red when measured per photon.
• If your light source is low in red/blue and high only in green, you might see slower growth or weaker flowering.
• In environments with intense direct sunlight, red and blue peaks still dominate plant response.

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What Growers Should Do

• Choose light sources that include red, blue, and some green, rather than red/blue only, for more even growth and better color rendering.
• Use a PAR meter/spectrum meter that is accurate across the full PAR range (400‑700 nm), not just red/blue.
• If you’re comparing lights, pay attention to how green light contributes—in plant tests under a full canopy or across a growing season.

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Bottom Line

Green light isn’t wasted — it’s part of the full picture. Though chlorophyll’s peaks are in red and blue, green light plays key roles in leaf penetration, canopy health, and natural growth responses. If you’re designing lighting or choosing PAR sensors, the McCree curve reminds us: include green, value accuracy, and strive for a spectrum that plants can really use.