Light Spectrum Guide: Veg vs Flower

Stop wasting money on blurple. Learn what wavelengths actually do, PPFD targets, and DLI by stage.

• Spectrum Basics
• Veg Spectrum
• Flower Spectrum
• DLI Calculator
• PPFD Targets
• Myths

Spectrum Basics: What Light Does What

Cannabis uses light 400-700nm for photosynthesis = PAR. But specific wavelengths trigger specific responses.

UV 280-400

Blue 400-500

Green 500-600

Red 600-700

Far-Red 700-750

280-400nm UV-A/UV-B
Trichomes, THC

400-500nm Blue
Compact, Stocky

500-600nm Green
Penetration

600-700nm Red
Stretch, Flower

700-750nm Far-Red
Emerson Effect

Key terms:
PAR: 400-700nm light plants use
PPFD: μmol/m²/s hitting your canopy. This is what matters.
DLI: Daily Light Integral. Total photons per day. PPFD × hours × 0.0036
PPE: μmol/J efficiency. Higher = less heat/watt

Veg Spectrum: Blue-Heavy for Structure

Veg Goals

Short internodes Thick stems Leaf growth Root mass

Blue light 400-500nm triggers phototropins. Plant stays compact, doesn't stretch. Cryptochromes control circadian rhythm.

Ideal Veg Spectrum

Blue: 20-30%
Green: 30-40%
Red: 30-40%
Far-Red: <5%

MH bulbs or 5000K-6500K LEDs. “Full spectrum” white LEDs work great.

PPFD & DLI for Veg

Week	PPFD Target	Hours	DLI	Notes
Seedling	200-300	18-24	13-26	Low light prevents burn
Early Veg	300-450	18	19-29	Build structure
Late Veg	450-600	18	29-39	Push before flip

Veg mistake: Too much red/far-red = stretchy plants. If your “veg” light is 3500K with heavy red, you’re flowering plants slowly. Switch to 5000K+.

Flower Spectrum: Red-Heavy for Buds

Flower Goals

Bud set Stretch control Resin Density

Red light 600-700nm drives photosynthesis hardest. Phytochromes detect red:far-red ratio. More red = less stretch.

Ideal Flower Spectrum

Blue: 5-15%
Green: 25-35%
Red: 50-65%
Far-Red: 5-10% early, 0% late

HPS or 3000K-3500K LEDs. Far-red weeks 1-3 for stretch, then cut it.

PPFD & DLI for Flower

Week	PPFD Target	Hours	DLI	Notes
Week 1-2	600-750	12	26-32	Transition, add far-red
Week 3-5	750-950	12	32-41	Bulk phase, max light
Week 6-8	850-1000	12	37-43	CO2 required >800 PPFD
Week 9+	600-750	12	26-32	Fade, reduce to prevent foxtail

Far-Red Trick: Run 730nm far-red for 10min AFTER lights out weeks 1-3. Triggers faster flower onset, 5-7 days earlier. Turn off after week 3 or you get stretch. This is the Emerson Effect.

Flower mistake: Too much blue = leafy buds, low yield. Too much far-red late flower = airy buds, foxtailing. UV-B weeks 6-8 only = +10-15% THC but risks burn.

DLI Calculator

Calculate Your Daily Light Integral

PPFD at Canopy 600 μmol

Hours of Light 18 hrs

Growth Stage Seedling 13-26 DLI Veg 19-39 DLI Flower 26-43 DLI

Your DLI

38.9

Optimal for Veg

Formula: DLI = PPFD × hours × 0.0036 × 3600 / 1,000,000 = PPFD × hours × 0.0036

PPFD Targets by Light Type

Light Type	Veg PPFD	Flower PPFD	Spectrum	Notes
T5 Fluorescent	200-350	300-450	6500K blue / 3000K red	Low heat, close canopy
MH 600W	400-600	—	Blue-heavy	Veg only, 18" height
HPS 600W	—	700-900	Red-heavy	Flower only, 18-24" height
CMH 315W	500-700	600-800	Full spectrum + UV	Best HID, 24" height
LED 3000K	400-550	700-950	Red-heavy	Flower optimized
LED 5000K	500-700	600-800	Blue-heavy	Veg optimized
LED Full Spec	450-650	750-1000	3500K-4000K	Seed to harvest

How to measure PPFD: Use a PAR meter like Apogee MQ-500. Phone apps are ±30% off. Measure at canopy center + corners. Average them. If you can't afford a meter, use manufacturer PPFD maps at your hang height.

Light Spectrum Myths Busted

Myth 1: "Blurple is best for cannabis"

False. Blurple = cheap red+blue diodes. Missing green/yellow 500-600nm that penetrates canopy. White full-spectrum LEDs outperform blurple by 20-30% yield. Blurple was 2015 tech.

Myth 2: "Green light is useless"

False. Green penetrates leaves deeper than blue/red. Reaches lower bud sites. 30% green = better canopy penetration. You need it.

Myth 3: "UV-B increases THC 30%"

Half-true. UV-B stress increases trichomes 10-15% max, but only if used weeks 6-8, 2hrs/day. More = burn, herms, yield loss. Not worth it for most grows.

Myth 4: "More PPFD always = more yield"

False. Diminishing returns after 900 PPFD without CO2. 1200 PPFD with no CO2 = bleaching, foxtails, stress. Cap at 1000 PPFD unless you run 1200ppm CO2.

Myth 5: "You need different lights for veg and flower"

False. Modern 3500K-4000K full-spectrum LEDs do both. You can optimize with 5000K veg + 3000K flower, but not required. One good light works.

PPFD beats spectrum. Get intensity right first, then dial spectrum. Measure your light.

© 2026 Light Spectrum Guide