What your plant actually does with light

Understanding Spectral Quality and Plant Light Absorption

What You Need to Know

Here’s the foundation: plants don’t absorb light the way your eyes perceive it. Most growers operate on the assumption that “full spectrum” marketing descriptions mean their plants are getting optimal light. The research tells you exactly what’s actually happening at the leaf level — which wavelengths matter most, why green light isn’t wasted, and what the evidence says about UV supplementation. Get this right and your light investment pays dividends through the entire grow.

The Science

Here’s the thing most growers get backwards: plants don’t see light the way you do. Your eyes are tuned to green light — that’s why green things look bright to you. Plants are the opposite. They absorb red and blue light like a sponge and reflect most of the green back at you. That’s literally why they look green.

Back in 1972, a researcher called McCree measured 22 different crop species and mapped exactly which wavelengths of light plants actually use for photosynthesis. The result is called the PAR curve — Photosynthetically Active Radiation — and it covers 400 to 700 nanometres. Two peaks: one around 440 nm (blue) and another at 620 nm (red). That’s where the magic happens.

But here’s where Eichhorn Bilodeau’s review flips the script on the “red and blue is all you need” crowd. Green light — the one everyone ignored because the plant “reflects it” — actually gets absorbed too. Not by the surface layer of chlorophyll, but deeper in the leaf. Green penetrates leaf tissue better than red or blue. In a canopy, that matters. The top leaves might be drowning in red and blue, but the lower leaves are starving. Green light reaches them. Eichhorn Bilodeau’s team found that a low percentage of green light (up to 24%) actually enhanced overall plant growth. The problem starts when green overwhelms the mix — above 50%, it begins antagonising blue-light responses and can reduce THC levels.

And then there’s the light you can’t see at all. Below 400 nm, you’re in UV territory. Your plant has a photoreceptor called UVR8 that detects UV-B radiation (290–320 nm). UV-B is essentially a stress signal — the plant interprets it as damage risk and responds by producing protective compounds. Some of those compounds happen to be flavonoids and, potentially, cannabinoids. Eichhorn Bilodeau’s review notes that UV-B has been reported to increase THC accumulation in leaves and buds. But — and this is important — the evidence was thin in 2019 and has since been seriously challenged. File this under “emerging, not proven.” We’ll revisit it in Module 2.1c when we look at Llewellyn’s UV trial.

The paper also clarifies something about photoreceptors that changes how you think about light schedules. Cannabis has five classes of photoreceptor: phytochromes (red/far-red sensing), cryptochromes and phototropins (blue/UV-A sensing), zeitlupes (circadian clock regulation), and UVR8 (UV-B). Phytochrome is the one that triggers flowering — it exists in two forms that flip back and forth depending on whether the plant is getting red or far-red light. That’s why 12/12 works: the long dark period lets the flowering form (Pfr) accumulate. But Eichhorn Bilodeau’s team noted that certain genotypes (like G-170) don’t respond to changes in the red-to-far-red ratio at all. The assumption that every cannabis plant flowers the same way under the same light recipe is wrong.

How To Apply This

• Verify your LED’s actual spectral output using the manufacturer’s SPD (Spectral Power Distribution) chart, not marketing language. A fixture delivering two narrow spikes at 450 nm and 660 nm with a gap in between is leaving your lower canopy in shadow. You want to see smooth spectral coverage, or at least meaningful output across the 400–700 nm range.
• Use PPFD (Photosynthetic Photon Flux Density, µmol/m²/s) as your light measurement standard. Lux and lumens are weighted for human vision, not plant photosynthesis. A quantum sensor is the tool that tells you what your plants actually see. Everything else is marketing noise.
• When comparing fixture types: HPS concentrates output in the yellow-orange range (560–600 nm), which is photosynthetically marginal and generates significant heat. LED fixtures can be engineered to deliver photons where plants use them most, and they run cooler. Eichhorn Bilodeau’s team quantified this — LED conversion efficiency is approximately 50%, versus HPS at about 30% of input energy reaching the useful PAR range. The capital cost difference becomes a return-on-investment question.
• On UV supplementation for now: the evidence isn’t strong enough to justify the expense or the tissue damage risk. Module 2.1c will dig into this properly with Rodriguez-Morrison’s controlled trial.

Watch Out For

• “Full spectrum” as a marketing term. Broad-spectrum white LEDs are legitimately useful; narrow red-blue fixtures aren’t. Always check the SPD chart. Marketing departments don’t engineer light — engineers do.
• Assuming every plant flowers the same way. Eichhorn Bilodeau’s team found that certain genotypes (like G-170) don’t respond to changes in the red-to-far-red ratio. Your light schedule works for most strains, but not all.
• UV-B enthusiasm without evidence. The early research on UV-B and cannabinoid production is thin. It has been seriously challenged since 2019. Don’t spend money chasing a signal that might not be there.
• Confusing leaf-level photosynthesis with canopy-level productivity. A single leaf has a light saturation point. A whole plant doesn’t. This distinction will matter in the next module.

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