Understanding Grow Lights for Cannabis: A Beginner’s GuideUpdated 2 days ago
Lighting is one of the most important factors in successful cannabis cultivation. Whether you’re growing indoors or supplementing natural sunlight, the right type of grow light directly affects plant growth, yield, potency, and overall health.
In this article, we’ll explore the main types of grow lights — LED, HPS, and fluorescent — and how they differ in efficiency, spectrum, and cost. We’ll also give you a high-level overview of light cycles for both autoflower and photoperiod plants.
Why Light Matters for Cannabis Growth
Light is the driving force behind photosynthesis, the process that allows your plants to convert energy into sugars for growth. In short:
More quality light = more energy.
More energy = stronger plants, higher yields, and denser buds.
The intensity, spectrum, and duration of light all determine how efficiently your cannabis plants can grow — from seedling to harvest.
The Main Types of Grow Lights
There’s no one-size-fits-all solution for lighting. Each type of grow light has its own advantages, drawbacks, and ideal uses.
Let’s break them down:
LED Grow Lights (Light Emitting Diodes)
Overview:
LEDs are now the most popular choice among indoor cannabis growers. They use semiconductor chips to emit light across customizable spectrums — often full-spectrum, mimicking natural sunlight.
Pros:
✅ Energy efficient (40–60% less power than HPS)
✅ Low heat output — reduces risk of heat stress
✅ Long lifespan (up to 50,000 hours)
✅ Full-spectrum coverage for all growth stages
✅ Compatible with small or large grows
Cons:
❌ Higher upfront cost
❌ Cheap or off-brand LEDs may have poor spectrum balance
Best For:
Growers looking for long-term efficiency, lower power bills, and clean light control. Modern full-spectrum LEDs are ideal for both home and commercial grows.
HPS Grow Lights (High-Pressure Sodium)
Overview:
HPS lights have been the gold standard for decades, known for their bright orange-red glow that promotes vigorous flowering.
Pros:
✅ Proven high yields
✅ Excellent for flowering stages
✅ Widely available and affordable
Cons:
❌ High heat output — requires strong ventilation
❌ Less energy efficient
❌ Shorter bulb lifespan (~10,000 hours)
❌ Limited spectrum — not ideal for seedlings or veg growth
Best For:
Experienced growers or those in cooler climates who need strong light intensity during flowering.
Fluorescent Lights (CFL / T5 Tubes)
Overview:
Compact fluorescent lamps (CFLs) and T5 fluorescent tubes are great for seedlings, clones, or small-scale grows.
Pros:
✅ Gentle light intensity for young plants
✅ Low cost and easy setup
✅ Cool operating temperature
Cons:
❌ Not strong enough for large or full-cycle grows
❌ Lower yield potential
Best For:
Seedling trays, cloning setups, or small grow tents focused on early growth stages.
Understanding Light Cycles
Different cannabis types have different light needs depending on how they flower.
Photoperiod Plants
Photoperiod cannabis relies on light cycle changes to trigger flowering.
Vegetative Stage: 18 hours light / 6 hours dark (commonly “18/6”)
Flowering Stage: 12 hours light / 12 hours dark (commonly “12/12”)
Reducing light exposure signals to the plant that autumn is coming — which tells it to start producing flowers.
Autoflower Plants
Autoflowers don’t rely on light cycles to flower. They bloom automatically based on age (usually 3–4 weeks from seed).
Most growers use:
18 hours light / 6 hours dark — balanced for strong growth and energy savings
Some run 20/4 or even 24/0 (constant light), but beyond 20 hours often offers minimal yield gain
Autoflowers are more flexible, making them perfect for newer growers or those who want faster harvests.
Light Spectrum Basics
Cannabis doesn’t treat “light” as one thing — it responds differently to distinct wavelength bands. Here’s the high-level, horticulturally accurate view of what each band does and how to use it.
Action Bands & What They Do:
Band | Wavelength (nm) | Primary Plant Responses | Practical Use Notes |
|---|---|---|---|
UV-B | 280–315 | Stress signaling; can upregulate protective compounds; excess damages tissues | Use sparingly, short exposures late flower only; optional, not required. |
UV-A | 315–400 | Secondary metabolite cues; modest morphology effects | 0–3% of total output max; optional for aroma/resin nuance. |
Blue | 400–500 | Tighter internodes, leaf thickness, stomatal density, phototropism; drives photosynthesis (Chl a/b peak ≈ 430–460 nm) | More blue = squat, compact veg; too much can slow biomass. Great in veg. |
Green | 500–600 | Deeper canopy penetration; modulates stomata/signaling; contributes to photosynthesis in lower leaves | 10–20% green improves uniformity under dense canopies; helps you see true plant color. |
Red | 600–700 | High photosynthetic efficiency; flowering support (Chl a/b peak ≈ 640–670 nm) | Core engine for biomass and bloom. Keep robust in veg and flower. |
Far-Red | 700–750 | Phytochrome switching (Pr/Pfr); shade-avoidance (stretch); can speed flowering transitions; “Emerson effect” with 660 nm | Use carefully: small doses at lights-off or as a short “end-of-day” pulse to influence morphology/flower timing. |
Near IR/Heat | >750 | Thermal load only (no photosynthesis) | Manage canopy temp/VPD; don’t chase this as “light.” |
Key photoreceptors (why this matters)
Chlorophyll a/b: absorb strongly in blue (~430–460 nm) and red (~640–670 nm).
Phytochrome: toggles between red (660 nm) and far-red (730 nm) states to signal shade/flower cues.
Cryptochrome/UVR8: sense blue/UV for morphology and stress responses.
Practical Spectrum Recipes (High-Level)
These aren’t rigid rules — they’re safe, modern LED targets that work well for most indoor grows.
Vegetative focus
Blue: 18–25%
Green: 10–20%
Red (600–700): 55–65%
Far-red (730): 0–2% (optional/micro-dose)
UV-A: 0–1% (optional)
Flower focus
Blue: 10–15% (enough for leaf health, not too much to slow biomass)
Green: 10–20%
Red (600–700): 60–70% (keep 660 nm strong)
Far-red (730): 0–3% (optional; brief end-of-day pulse or low constant)
UV-A: 0–2% (optional, late flower only if used)
Tip: If your fixture is “full-spectrum” with added 660 nm diodes, you’re already close. Add tiny amounts of 730 nm only if you’re intentionally steering stretch or flower timing.
Intensity & Duration (just enough to be useful)
Even with the right spectrum, crops fail if intensity is off. Aim for these PPFD targets at canopy (ambient CO₂):
Seedlings/clones: 100–300 μmol·m⁻²·s⁻¹
Veg: 300–500 μmol·m⁻²·s⁻¹
Early flower: 600–800 μmol·m⁻²·s⁻¹
Late flower (peak): 800–1,000 μmol·m⁻²·s⁻¹
(>1,000 typically needs supplemental CO₂ to pay off)
And remember light cycle basics (you’ll deep-dive in the dedicated articles):
Photoperiods: 18/6 for veg → 12/12 to flower.
Autoflowers: 18/6 (or 20/4) from seed to finish; they flower by age, not daylength.
Quick Do/Don’t
Do prioritize strong red + some blue for photosynthesis; keep green in the mix for canopy penetration.
Do treat far-red as a tool, not a floodlight.
Do keep UV modest and purposeful; think “seasoning,” not “main course.”
Don’t chase spectrum tweaks before you’ve nailed PPFD, uniformity, and VPD.
Final Thoughts
Understanding the light spectrum and how different wavelengths influence cannabis growth is the foundation of every successful indoor garden. While technologies like LEDs, HPS, and fluorescents all have their place, what matters most is delivering the right intensity, spectrum, and light cycle for your plant’s stage of life.
Think of your lighting setup as the sun of your grow room — it’s not just about brightness, but quality, balance, and control. By combining a full-spectrum light with proper environment management and nutrient support, you give your plants the energy they need to reach their full genetic potential.
Whether you’re growing autoflowers or photoperiods, mastering lighting basics sets the stage for every great harvest — one lumen at a time.