Mastering Light Spectrum for Cannabis Production

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.

Quiz

1. According to McCree’s research and Eichhorn Bilodeau’s review, which of the following is true about green light in a cannabis canopy?

a) Plants reflect all green light and don’t use it for photosynthesis b) Green light is absorbed only at the leaf surface by chlorophyll c) Green light penetrates deeper into leaf tissue and is more useful in lower canopy layers d) Green light has no photosynthetic function but improves visual appeal *

Answer: c — Green penetrates leaf tissue better than red or blue, reaching lower leaves in a dense canopy. However, the optimal percentage is 20-24% of total output; above 50%, green begins antagonising blue-light responses.

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2. True or False: HPS fixtures deliver photons more efficiently into the PAR range than LED fixtures.

Answer: False — LED conversion efficiency is approximately 50% into the PAR range, versus HPS at about 30%. HPS concentrates output in the yellow-orange range (560–600 nm), which is photosynthetically marginal.

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3. What does UVR8 detect, and what is the plant’s primary response to it?

a) Far-red light; increases chlorophyll production b) UV-B radiation; a stress signal triggering protective compound production c) Blue light; controls daily circadian rhythms d) Red light; initiates flowering *

Answer: b — UVR8 is the plant’s photoreceptor for UV-B (290–320 nm). The plant interprets UV-B as damage risk and responds by producing protective compounds, including flavonoids and potentially cannabinoids. The evidence for cannabinoid increases is emerging and contested.

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4. A grower assumes their 12-hour dark period triggers flowering because darkness is “dark.” Based on this module, what’s actually happening?

The long dark period allows the Pfr form (red-light-activated phytochrome) to accumulate. Phytochrome flips between forms depending on red and far-red light exposure; the Pfr form triggers flowering. The dark period is critical because it prevents far-red light (which converts Pfr back to the inactive form) from interfering with the flowering signal. It’s not darkness itself — it’s light chemistry.

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5. Your LED’s SPD chart shows two sharp spikes: one at 450 nm and one at 660 nm, with minimal output between them. Based on what you’ve learned about green light and canopy penetration, what’s the consequence for plants growing under this light?

Top leaves receive high blue and red intensity and may be adequately served. However, lower canopy leaves receive reduced red and blue light (filtered by the upper layers), and receive no benefit from green-light penetration to compensate. Those lower leaves are photosynthetically starved. The result is reduced whole-canopy productivity compared to a fixture with more continuous spectral distribution.

Seb’s Corner (Level 2+)

The McCree curve was conducted under low light intensity — below 150 µmol/m²/s. Whether the spectral quality weightings hold at the intensities cannabis growers operate at (300–2,000 µmol) remains an open question. Eichhorn Bilodeau’s team flagged this gap. What we know is that cannabis leaf photosynthesis has been measured up to 2,000 µmol/m²/s with no decline at 25°C, though rates drop roughly 20% between 1,500 and 2,000 µmol at 30°C. The light saturation point for individual cannabis leaves is high — but that’s a leaf, not a canopy. The distinction between leaf-level photosynthesis and whole-plant yield response is critical, and we’ll tear that apart in Module 2.1b with Rodriguez-Morrison’s data.

FAQ

Are there any “full spectrum” LEDs that actually deliver continuous spectral output?

Yes. Broad-spectrum white LEDs (like the Samsung LM301 series) use a blue diode with a yellow phosphor coating, producing a smooth, continuous spectrum from approximately 400–700 nm. It’s not identical to sunlight, but it’s substantially more useful than a narrow red-blue bar. The way to verify: check the SPD chart. A smooth curve means you’re in a functional range. Two spikes with a canyon in between means you’re delivering light like a disco, not a grow room.

If my LED is broad-spectrum white, do I need to add green LEDs separately?

No. Broad-spectrum white LEDs already contain a significant green component. Research suggests roughly 20-24% green in the total output mix is beneficial for canopy depth. More than 50% begins antagonising blue-light responses and reducing THC. Most white LED fixtures naturally land in that sweet spot without any intervention.

When does it make sense to switch from HPS to LED?

From an ROI perspective: if you’re running high-wattage HPS (600W, 1000W+) and electricity costs are eating into margins, the efficiency gain is real. LEDs deliver roughly 50% of input energy to the PAR range; HPS delivers about 30%. The reduced heat load also means lower cooling costs. If your HPS is performing adequately and you’re not thermally constrained, the switch isn’t mandatory — but the numbers usually favour LED for new builds.

What about far-red light (700–740 nm)? Does supplementing it increase yield?

Far-red does several interesting things: it can increase stem elongation, influence flowering timing, and potentially extend the effective photosynthetic range beyond the traditional 400–700 nm window. The research is emerging and the evidence is mixed. The practical risk is stretchy, structurally weak plants. Your priority should be optimising PAR delivery first. Far-red supplementation is an edge-case refinement, not a foundational strategy.

Source

Eichhorn Bilodeau S, Wu B-S, Rufyikiri A-S, MacPherson S and Lefsrud M (2019). “An Update on Plant Photobiology and Implications for Cannabis Production.” Front. Plant Sci. 10:296. doi: 10.3389/fpls.2019.00296. CC-BY 4.0.