“Full spectrum” is the most-used and least-defined term in grow lighting. Two fixtures both labeled “full spectrum” can produce wildly different spectral power distributions, and those differences matter — especially across growth stages. This guide breaks down the wavelengths that actually drive plant response, why they matter at each stage, and how to read a spectrum graph.
The Five Wavelength Bands That Matter
| Band | Range | Primary Plant Response |
|---|---|---|
| UV-B | 280-315 nm | Stress response; secondary metabolite (terpene, cannabinoid) production |
| UV-A | 315-400 nm | Compact growth; pigmentation; mild stress signaling |
| Blue | 400-500 nm | Compact morphology; stomatal opening; chlorophyll synthesis |
| Green | 500-600 nm | Canopy penetration; photosynthesis (yes, plants use green) |
| Red | 600-700 nm | Photosynthesis (peak efficiency); flowering trigger |
| Far-Red | 700-780 nm | Stem elongation; flowering signal; Emerson Effect |
The PAR Curve (and Why It’s Outdated)
For decades, photosynthesis efficiency was modeled by the McCree curve (1972), which shows the relative photosynthetic response of plants across the 400-700 nm range. The McCree curve has two peaks — one in blue (~440 nm), one in red (~620-660 nm) — with a dip in green.
This led to a generation of “blurple” LED grow lights heavy on red and blue, light on green, because green was thought to be wasted.
Modern research (Bugbee, Folta, Kusuma, and others) has revised this view. Green light penetrates deeper into the canopy than red or blue, driving photosynthesis in lower leaves that direct red/blue light never reaches. Full-spectrum white LEDs typically outperform pure red+blue setups for whole-canopy yield, even though the McCree curve technically rates green as less efficient at the leaf surface.
This is why premium LED manufacturers shifted from blurple to full-spectrum white over the last decade.
Spectrum by Growth Stage
Propagation (Seedlings, Clones)
Target spectrum: Blue-rich (15-25% blue), moderate red, low far-red, low UV.
Young plants benefit from blue-heavy spectrum because it promotes compact growth — strong stems, tight internodes, robust leaf development. Excess far-red at the seedling stage causes stretching (etiolation). UV at this stage is harmful: young plants haven’t developed the protective compounds (anthocyanins, flavonoids) needed to handle UV stress.
PPFD target: 100-300 μmol/m²/s.
Vegetative Stage
Target spectrum: Balanced full spectrum, with slightly higher blue (12-20%) for compact growth, or full-spectrum 3000K-4000K white if using LEDs.
During veg, you want robust leaf area and strong stem structure without excessive height. Full-spectrum white with some red and a moderate blue ratio works well. Pure blue lights (older “veg-only” lights) can over-compact growth and limit photosynthetic productivity.
CMH (Ceramic Metal Halide) is excellent during veg because of its broad spectrum with some UV. HPS works but tends to stretch plants without supplemental blue.
PPFD target: 300-600 μmol/m²/s.
Flowering / Fruiting
Target spectrum: Red-shifted full spectrum, with red:blue ratio around 4:1 to 6:1. Far-red supplementation (5-10% of total PPF) accelerates flowering. UV exposure during late flower (last 2-3 weeks) increases secondary metabolite production in some crops.
The peak red wavelengths (~660 nm) drive photosynthesis at the highest efficiency per photon. Far-red at ~730 nm activates the Emerson Effect, where the combination of red + far-red produces more photosynthesis than either alone — but only when both are present together.
HPS dominated flowering for decades because of its yellow-red spectral peak. Modern LED full-spectrum delivers comparable or better results with significantly better efficacy, and most premium flower LEDs are now red-shifted (3000K white + supplemental 660 nm reds).
PPFD target: 600-1,000 μmol/m²/s (ambient CO₂); 1,000-1,500 with supplemental CO₂.
Late Flower / Finishing (Optional)
Target spectrum: Maintain flowering spectrum; consider adding UV-B during the final 2-3 weeks.
For light-loving crops where secondary metabolites matter (cannabis trichome density, basil essential oils, tomato lycopene), UV-B exposure during finishing can increase concentrations of these compounds. UV-B is a stress signal — the plant responds by producing protective compounds, many of which we value as flavors, scents, or active ingredients.
UV-B requires safety protocols. Operators should not be in the room during UV-B exposure without UV-blocking eyewear.
The “Full Spectrum” Trap
Anyone can label a fixture “full spectrum.” What you actually need to look at is the spectral power distribution (SPD) graph — a chart showing relative intensity at each wavelength.
A genuine modern full-spectrum LED SPD shows:
- A continuous curve across 400-700 nm (no zero or near-zero gaps)
- Visible peaks corresponding to the underlying LED diode chemistry (typically a blue pump LED converted by phosphor — visible as a blue peak ~450 nm and a broad orange-red mound)
- Sometimes additional discrete peaks from supplemental diodes (660 nm red, 730 nm far-red, UV)
A “full spectrum” that isn’t full:
- Two sharp peaks at 450 nm (blue) and 660 nm (red) with deep valleys in green and yellow — this is a “blurple” spectrum mislabeled as full
- A spectrum that drops to zero outside 380-650 nm — missing far-red entirely
- SPD that doesn’t include any UV-A — fine for most stages but not for late-flower UV protocols
R:B Ratio (Red-to-Blue Ratio)
R:B ratio is calculated as the total red photons (600-700 nm) divided by total blue photons (400-500 nm). Common ratios:
- Veg: 2:1 to 4:1 (R:B). Balanced for compact growth.
- Flower: 4:1 to 6:1. Red-shifted to drive flowering.
- Outdoor sunlight at midday: ~1.2:1 (much more balanced than typical horticultural LEDs).
R:B isn’t a magic number — variations within these ranges have small effects compared to total PPF and total DLI. But extreme ratios (e.g., 10:1 red-dominant) can cause stretching, weak stems, and reduced leaf development.
Far-Red and the Emerson Effect
Far-red (700-780 nm) is technically outside the PAR range but plays an important role. When combined with red (~660 nm), far-red activates a synergistic photosynthetic response (the Emerson Effect) that increases total carbon fixation.
However, far-red also drives stem elongation. Too much can cause stretching and shade-avoidance behavior. The right amount (typically 5-10% of total photon output) is beneficial; too much is counterproductive.
Some LED fixtures include dedicated far-red diodes. Others have minimal far-red and rely on the small amount of far-red present in white phosphor LEDs.
Quick Reference
| Stage | Spectrum Priority | PPFD Target |
|---|---|---|
| Seedling | Blue-rich, low UV, low FR | 100-300 μmol/m²/s |
| Veg | Balanced full-spectrum, mild blue | 300-600 μmol/m²/s |
| Early flower | Red-shifted full-spectrum | 600-800 μmol/m²/s |
| Late flower | Red-shifted + optional UV-B | 800-1,000+ μmol/m²/s |
Bottom Line
Spectrum matters, but it matters less than total PPF and PPFD for raw biomass production. Don’t pay a 30% premium for an exotic spectrum if it costs you efficacy and total photon output. For most growers, a modern full-spectrum white LED with supplemental red and far-red delivers excellent results across all stages — and dimming + supplemental diode bars handle stage-specific tuning.
When evaluating a fixture, ask for the SPD graph alongside the IES file. The two together tell you everything about what the fixture actually delivers.