Spectrum Engineering

Somewhere out there is a grower who spent four hundred euro on a UV bar because a forum told him it would push his THC up. He ran it for a full flower cycle. His yield dropped. His electric bill didn’t. And his potency? Same as the tent next door without the bar. This lesson is about what spectrum actually does — and about one of the most expensive myths in the hobby.

What You Need to Know

Spectrum shapes the plant; intensity feeds it

Hold these apart. Intensity (PPFD) is how many photons hit the canopy — that’s the yield driver you learned in Level 3. Spectrum is the colour mix of those photons, and it changes the plant’s shape and, more modestly, her chemistry. Magagnini’s review is clear that the two are independent variables. You optimise both; you don’t trade one for the other.

Blue keeps her tight; red lets her run

The headline morphology rule from the review:

• More blue → shorter, more compact plants with tighter internodes. Useful in veg and in low tents.
• More red → stretchier, more vegetative-looking growth; red-heavy spectra tend to favour flowering biomass.

But there’s a cost to leaning too hard on blue. The review found a consistent, roughly linear 12% drop in yield as the blue photon fraction climbed from 4% to 20%. So blue is a structural tool, not a free one — every extra slice of blue you add to chase compactness is costing you flower weight.

Seb’s Corner. This is the trade you’re actually making. Blue buys you a shorter plant; it sells you yield. In a tight tent where height is the binding constraint, that can be worth it. In a tall tent, it’s a tax you don’t need to pay. Don’t add blue for its own sake — add it when height is the problem you’re solving.

Red-to-far-red is a morphology lever

The ratio of red to far-red light is a signal the plant reads through phytochrome. A low red:far-red ratio during a long photoperiod pushes vegetative stretch — the plant thinks it’s being shaded by neighbours and reaches. A high red:far-red ratio during the short photoperiod favours compact, flowering-focused growth. A practical flowering spectrum from the review sits around 60–70% red, 20–25% blue, 5–15% far-red.

Far-red: promising, but read the fine print

This is where honesty matters. Some recent data suggests adding far-red — at the end of the photoperiod or during darkness — can raise total cannabinoid yield substantially in some cultivars (the review cites figures around 70% in Northern Lights). That’s a striking number.

Seb’s Corner. Dave’s note here, and he’s right to make it: one eye-catching figure in one cultivar is a lead, not a law. “Up to 70% in Northern Lights” is not “70% in your plant.” Cultivar response to far-red is variable, and the strongest results are cultivar-specific. Treat far-red as a worthwhile experiment to run on a control versus a test plant in your own room — not as a guaranteed return. We’re flagging this as promising-but-cultivar-dependent rather than banking it.

UV-B: the myth that won’t die

Now the one to put to bed. The belief that UV-B “boosts THC” has circulated for years on the logic that THC is a natural sunscreen, so more UV should mean more THC. It sounds scientific. It is wrong in the way that matters.

The 2021 controlled study tested UV-B across two cultivars at commercial-range intensities. Findings:

• No increase in inflorescence cannabinoid concentration from UV-B. None worth having.
• Reduced yield in at least one cultivar.
• Reduced inflorescence quality in both cultivars.
• The only THC bump appeared in sugar leaves (about 30% higher under UVA+UVB) — and it did not carry into the buds you’d actually harvest.

The authors’ recommendation is blunt: UV supplementation is not recommended for production. You’re adding equipment cost, electricity, and a safety hazard for no flower benefit and a likely penalty.

Seb’s Corner. The sugar-leaf detail is the trap. A stressed plant does throw a little extra THC into its leaf tissue as a defence response — that’s real, and it’s how the myth got its foothold. But the buds don’t follow, and the buds are the harvest. “True in the leaf, false in the bud” is exactly the kind of half-truth that survives on forums for a decade.

How To Apply This

• Use blue as a height tool, deliberately. Lean blue in veg or in a short tent to keep her compact, knowing you’re paying yield for it. Don’t run heavy blue in flower without a height reason.
• Run a red-dominant flowering spectrum in the 60–70% red, 20–25% blue, 5–15% far-red range. Most quality full-spectrum LEDs already land near this.
• Test far-red on a control, not on faith. If your fixture offers far-red, prove it in your own room — one plant with, one without, same everything else — before you believe a headline number.
• Spend the UV money on PPFD instead. A proven intensity uplift beats an unproven spectrum trick every time.

Watch Out For

• “UV raises THC.” The controlled data says no — no bud benefit, a yield penalty, and a quality drop. The sugar-leaf bump doesn’t reach the flower.
• Banking the 70% far-red figure. It’s cultivar-specific and not guaranteed in your plant. Promising, not certain.
• Buying lights on watts. Spectrum and intensity are what move the plant; wattage is just the bill.
• Chasing compactness with blue and then wondering where the yield went. That 12% is the receipt.

Quiz

Sources

Magagnini, G., Grassi, G., & Kotiranta, S. (2018). The effect of light spectrum on the morphology and cannabinoid content of Cannabis sativa L. Medical Cannabis and Cannabinoids, 1(1), 19–27. https://doi.org/10.1159/000489030. Open Access / CC-BY.

Rodriguez-Morrison, V., Llewellyn, D., & Zheng, Y. (2021). Cannabis inflorescence yield and cannabinoid concentration are not increased with exposure to short-wavelength ultraviolet-B radiation. Frontiers in Plant Science, 12, 725078. https://doi.org/10.3389/fpls.2021.725078. CC-BY 4.0.

Next: Lesson 4 — Stress as a tool, and stress as a cult.