What PPFD does cannabis need during flowering?

Cannabis plants in the flowering stage perform best with a PPFD (Photosynthetic Photon Flux Density) of 600-900 micromoles per square meter per second (µmol/m²/s) at canopy level when growing at ambient CO2 levels (~400 ppm). The light saturation point for cannabis without CO2 enrichment is approximately 900-1000 µmol/m²/s — exceeding this without elevated CO2 wastes energy and can cause light stress. With CO2 supplementation at 1200-1500 ppm, the saturation point rises to 1200-1500 µmol/m²/s, justifying higher-intensity fixtures.

What is DLI and why does it matter for cannabis?

DLI (Daily Light Integral) is the total amount of photosynthetically active light delivered to the canopy over a full photoperiod, measured in mol/m²/day. It accounts for both intensity (PPFD) and duration (hours of light). For cannabis, vegetative growth targets a DLI of 25-38 mol/m²/day, while flowering targets 38-55 mol/m²/day. DLI is more useful than PPFD alone because it captures the relationship between light intensity and hours on. A lower-intensity light running longer can achieve the same DLI as a higher-intensity light running fewer hours.

Are LED grow lights better than HPS for cannabis?

Modern quantum board LEDs with Samsung LM301B or LM301H diodes are widely considered superior to HPS for cannabis cultivation in most indoor environments. Quality LEDs achieve 2.5-3.0+ µmol/J efficiency versus 1.7-2.1 µmol/J for double-ended HPS, translating to 30-40% electricity savings for equivalent light output. LEDs also run significantly cooler (reducing HVAC load), have a 50,000-hour rated lifespan versus 10,000-20,000 hours for HPS bulbs, and produce fuller spectrum light including relevant wavelengths that HPS lacks. The upfront cost premium of LED pays back in electricity savings within 12-24 months in most grow scenarios.

What is the Samsung LM301B diode and why is it important?

The Samsung LM301B (and its successor LM301H) is the white LED diode that forms the basis of most high-quality quantum board grow lights. It operates at a color temperature of approximately 3000-3500K, produces a broad-spectrum white light that closely resembles the solar spectrum, and achieves photon efficiency of 3.0+ µmol/J when operating at low to moderate drive currents. Manufacturers like HLG, Spider Farmer, Mars Hydro, and Gavita use these diodes in their premium fixtures. Lights marketed as using LM301B/H diodes are a reliable quality signal, though the full fixture efficiency depends on driver quality, thermal management, and board design.

How far should LED grow lights be from cannabis plants?

The optimal distance between an LED grow light and cannabis canopy depends on the fixture wattage and the growth stage. As a general rule: 600-800W-equivalent LEDs should be positioned 18-24 inches above canopy during vegetative growth and 16-20 inches during flowering for optimal PPFD. Lower-intensity fixtures (200-400W equivalent) can be positioned closer: 12-18 inches in veg, 10-16 inches in flower. Always measure PPFD at canopy level with a quantum sensor rather than relying solely on manufacturer recommendations. Hanging height adjustments are the easiest way to tune PPFD without changing dimmer settings.

What light schedule should cannabis be on?

Photoperiod cannabis strains require 18 hours of light and 6 hours of darkness during vegetative growth, and a switch to 12 hours light and 12 hours darkness to trigger and maintain flowering. Some growers use 20/4 or even 24 hours of light during vegetative growth to maximize speed; research suggests minimal benefit beyond 18/6 for most strains. Autoflowering strains flower regardless of photoperiod and perform well on 18/6 or 20/4 throughout their entire life cycle. Never interrupt the dark period during flowering — light leaks can cause re-vegging or hermaphroditism.

How much does it cost to run LED grow lights?

The electricity cost to run LED grow lights depends on wattage, hours of operation, and local electricity rate. A 600W LED running an 18/6 schedule draws approximately 10.8 kWh per day. At $0.13/kWh (US average), that is $1.40/day or $42/month per light. A single light running 12/12 for a 9-week flower cycle costs approximately $88-110 in electricity. For comparison, a 1000W HPS running the same schedule costs approximately $165-185 for the same flower cycle, plus replacement bulb costs. The LED advantage grows with scale: commercial operations running 50+ fixtures see electricity savings of $30,000-100,000+ annually.

Cannabis LED Grow Lights: Complete Buying Guide

By Jordan Price · Growing Guide · Updated May 2026

2.5–3.0+

µmol/J (Quality LED)

600–900

PPFD Flower (µmol/m²/s)

50,000 hrs

LED Rated Lifespan

30–40%

Energy Savings vs. HPS

KEY FINDINGS

PPFD is the real metric: Wattage numbers on grow lights are nearly useless for comparison. PPFD (photons delivered to the canopy per second) and efficiency (µmol/J) are the only metrics that matter when evaluating LED performance.
Quantum boards dominate: Samsung LM301B and LM301H diodes set the efficiency benchmark. Fixtures using these diodes from reputable manufacturers consistently outperform blurple LED panels and cheap COB designs.
DLI bridges intensity and duration: A light delivering 700 µmol/m²/s for 18 hours achieves a DLI of 45.4 mol/m²/day — well into the flowering target range of 38–55. DLI thinking prevents both underlit and overlit canopies.
LEDs run cooler: A 600W LED fixture produces roughly 40–60% less heat than an equivalent-output HPS lamp. This directly reduces air conditioning requirements and allows lights to hang closer to the canopy.
Light schedule is non-negotiable: 18/6 for vegetative growth, 12/12 to trigger and sustain flowering in photoperiod strains. Any light leak during the 12-hour dark period risks revegging or hermaphroditism.
ROI timeline: 12–24 months: At US average electricity rates, the electricity savings from LED versus HPS recoup the higher LED purchase price within 1–2 years for a typical home grower running one to four lights.
CO2 unlocks higher PPFD: Without CO2 supplementation, the photosynthesis saturation point for cannabis is approximately 900–1000 µmol/m²/s. Running lights above this without elevated CO2 wastes electricity and can cause light stress.

LED Technology Evolution: From HPS to Quantum Boards

The indoor cannabis lighting market has undergone three generations of technology in the past two decades. First-generation blurple LED panels (red and blue diodes only) promised HPS replacement but underdelivered on canopy penetration and spectral quality. Second-generation COB (chip-on-board) LEDs improved intensity but struggled with efficiency at high drive currents and heat management.

The current generation — quantum board LEDs using Samsung LM301B or LM301H diodes — represents a genuine step change. These fixtures arrange hundreds of small, highly efficient white-light diodes across a large board, running each diode at low drive current where efficiency is highest. The result is broad-spectrum, high-efficiency light that closely approximates the solar spectrum while running cool enough to touch. Manufacturers like Horticulture Lighting Group (HLG) pioneered the quantum board format; Spider Farmer, Mars Hydro, Gavita, and dozens of others now produce competitive options.

For growers still running double-ended HPS or CMH fixtures, the LED transition represents the most impactful single upgrade available. See the full comparison table below and the grow tent setup guide for integration recommendations.

LED Brand Comparison: Top 8 Fixtures

Brand / Model	True Wattage	Efficiency	Coverage (Flower)	PPFD at 18" (center)	Price Range	Diodes
HLG 650R	650W	3.0 µmol/J	4×4 ft	~1200	$800–$950	Samsung LM301H
Gavita Pro 1700e LED	645W	2.9 µmol/J	4×4 ft	~1150	$950–$1,100	Samsung LM301H
Spider Farmer SE7000	730W	2.85 µmol/J	4×5 ft	~1050	$600–$750	Samsung LM301H
Mars Hydro FC-E8000	800W	2.8 µmol/J	5×5 ft	~1000	$550–$700	Samsung LM301H
HLG 300L Rspec	300W	2.9 µmol/J	3×3 ft	~900	$350–$420	Samsung LM301B
Spider Farmer SF-4000	450W	2.7 µmol/J	4×4 ft	~800	$350–$430	Samsung LM301B
Mars Hydro TS-3000	450W	2.6 µmol/J	4×4 ft	~780	$280–$360	Samsung LM301B
Migro ARAY 4	200W	3.1 µmol/J	2×4 ft	~850	$400–$480	Samsung LM301H

HPS vs. LED vs. CMH: Technology Comparison

Attribute	LED (Quantum Board)	HPS (Double-Ended)	CMH / LEC
Efficiency	2.5–3.1 µmol/J	1.7–2.1 µmol/J	1.9–2.2 µmol/J
Heat Output	Low–Moderate	Very High	Moderate
Spectrum	Full spectrum (broad white)	Narrow (red/yellow heavy)	Good (includes UV)
Lifespan	50,000 hrs	10,000–20,000 hrs	20,000–30,000 hrs
Replacement Cost	None (no consumables)	$50–$150/bulb (annual)	$80–$200/bulb (1–2 yrs)
Upfront Cost	High ($2–$4/watt)	Low ($0.50–$1/watt)	Moderate ($1.50–$2.50/watt)
ROI vs. HPS	12–24 months	Baseline	18–36 months

PPFD Requirements by Growth Stage

Growth Stage	PPFD Target	DLI Target	Light Schedule	Distance (Avg)
Seedling	100–250 µmol/m²/s	6–12 mol/m²/day	18/6	24–36 in
Early Veg	250–400 µmol/m²/s	16–25 mol/m²/day	18/6	20–28 in
Late Veg	400–600 µmol/m²/s	25–38 mol/m²/day	18/6	18–24 in
Early Flower	600–800 µmol/m²/s	38–46 mol/m²/day	12/12	16–22 in
Peak Flower	800–900 µmol/m²/s	38–55 mol/m²/day	12/12	14–20 in
With CO2 (1200 ppm)	1000–1500 µmol/m²/s	43–65 mol/m²/day	12/12	10–18 in

ROI Calculation: LED vs. HPS Over Two Years

Cost Category	600W LED (HLG Class)	1000W DE-HPS	LED Advantage
Purchase Price	$800	$400	−$400
Annual Electricity (18/6 veg + 12/12 flower ×4 cycles)	~$350	~$550	+$200/yr
Annual Bulb Replacement	$0	$120	+$120/yr
Cooling Cost Reduction	—	—	+$80–$150/yr
Total 2-Year Net	—	—	+$400–$590

At US average electricity rates, a grower switching from a 1000W HPS to a 600W LED recoups the price premium within 14–20 months and continues saving $400–500 per year thereafter. At higher electricity rates ($0.20+/kWh, common in California, New York, and most of Europe), the ROI timeline shortens to 8–12 months.