What's the difference between LED, HPS, and CMH grow lights?

Full-spectrum LED (quantum board) is now the best choice for most home growers — low heat, low running cost, and full lifecycle use from seed to harvest. HPS produces excellent flowering results with lower upfront cost at 600W+, but generates significant heat and needs annual bulb replacement. CMH (Ceramic Metal Halide) gives a broader spectrum than HPS, including UV, with lower heat — a middle ground between the two.

What light schedule should I use for hydroponic plants?

18 hours on / 6 hours off keeps plants in vegetative growth. 12 hours on / 12 hours off triggers flowering in photoperiod-sensitive plants — any light leak during the dark period can delay or prevent flowering. Seedlings and propagation often use an extended 20 hours on / 4 hours off schedule.

How close should grow lights be to plants?

Light intensity follows the inverse square law — doubling the distance quarters the intensity. General starting points: 50–60cm for seedlings/clones under LED, 30–45cm for vegetative growth, and 20–35cm for flowering, always checked against your specific fixture's manufacturer PPFD chart.

What happens if grow lights are hung too close or too far from plants?

Too close causes light burn — bleached white or yellow patches at the top of the canopy closest to the light. Too far causes insufficient intensity, leading to stretchy, weak plants (etiolation) as stems elongate reaching for light.

Lighting

Hydroponic Grow Lights: LED, HPS & CMH Compared

By HiGroPonics Team · Updated June 2026

Light is the engine of plant growth. Through photosynthesis, plants convert light energy into sugars that fuel every biological process — growth, root development, flowering, and fruiting. Without adequate light, no amount of perfect nutrients, water quality, or system design will produce a good crop.

Understanding light means understanding spectrum, intensity, duration, and how each interacts with the growth stage of your plants. This guide covers all of it, from the physics to the practical choices.

Part 1

Light Spectrum — What Plants Actually Use

Plants do not use all wavelengths of light equally. Chlorophyll absorbs primarily in two ranges, and the ratio between them signals the plant to grow differently.

380nm450nm500nm550nm600nm660nm730nm

🔵

Blue

400–500nm

Vegetative growth, compact structure

Blue light drives chlorophyll-a production and regulates stem elongation. Under predominantly blue light, plants grow compact with thick stems and dense leaves — ideal for the vegetative stage. Outdoor summer sun is rich in blue light.

🔴

Red

600–700nm

Flowering, fruiting, photosynthesis efficiency

Red light (especially 660nm) is the most efficient wavelength for photosynthesis. It triggers flowering in photoperiod plants and drives fruiting. A high red ratio mimics autumn sun, signalling the plant to flower and set fruit.

🌅

Far-Red

700–750nm

Canopy penetration, flowering trigger

Far-red light penetrates the canopy to reach lower leaves and can be used to extend the photoperiod effect at lights-off (the Emerson effect). Some modern LED fixtures include far-red diodes for enhanced flowering. Not essential but increasingly used in advanced grows.

Full-spectrum LEDs

Modern quantum board LEDs output a broad, balanced spectrum covering both blue and red peaks with white fill across the mid-range. This replicates sunlight closely enough that a single fixture works well across the entire grow cycle — no need to swap lights between veg and flower.

Part 2

PPFD and DLI — Measuring Light Properly

Wattage tells you how much electricity a light uses. PPFD and DLI tell you how much light your plants actually receive.

PPFD — Intensity at the Canopy

Photosynthetic Photon Flux Density measures how many light photons (in the 400–700nm range) land on a square metre of canopy per second, expressed in μmol/m²/s. It is the most useful single number for comparing grow lights and dialling in the correct distance from your canopy.

Seedlings / clones100–250 μmol/m²/s

Leafy greens & herbs200–400 μmol/m²/s

Vegetative fruiting crops400–600 μmol/m²/s

Flowering / fruiting600–1,000+ μmol/m²/s

Reputable LED manufacturers publish PPFD charts showing intensity at different hanging heights. Use these to position your light correctly — not guesswork.

DLI — Total Daily Light Dose

Daily Light Integral is the total amount of photosynthetically active light received over 24 hours, expressed in mol/m²/day. DLI = PPFD × photoperiod hours × 0.0036. It accounts for both intensity and duration — a lower-intensity light run for more hours can achieve the same DLI as a brighter light for fewer hours.

Leafy greens & herbs12–17 mol/m²/day

Vegetative crops20–30 mol/m²/day

Fruiting crops (flowering)30–45 mol/m²/day

High-light crops40–65 mol/m²/day

DLI thinking is useful when you cannot change light intensity — extending the photoperiod slightly achieves more total light without buying a stronger fixture.

Part 3

Types of Grow Light

🟦

Full-Spectrum LED (Quantum Board)

Recommended

The current best choice for most home growers. Quantum board LEDs use high-efficacy diodes spread across a large panel, producing even light distribution and low heat output. Full-spectrum output works from seed to harvest without changing fixtures. Efficiency (μmol/J) has improved dramatically — top boards now rival or exceed HPS at a fraction of the running cost.

Pros

✓Lowest electricity cost per μmol
✓Low heat — simpler cooling
✓Full lifecycle use (veg and flower)
✓Long lifespan (50,000+ hours)
✓Dimmable on quality units

Cons

✗Higher upfront cost than HPS
✗Quality varies enormously — cheap LEDs underperform badly
✗Blue-heavy cheaper boards less ideal for flowering

Best for: All crops, all growth stages, home and commercial

🟡

HPS (High Pressure Sodium)

Proven technology

The industry standard for decades. HPS produces an intense red/orange spectrum that drives excellent flowering and fruiting. At the 600W level, it remains cost-competitive with LED upfront and is proven across countless commercial operations. The main disadvantage is heat output — HPS generates significant heat that must be managed with strong extraction.

Pros

✓Excellent flowering performance
✓Lower upfront cost at 600W+
✓Very high output at distance
✓Widely available parts and replacements

Cons

✗High heat output — requires larger extraction
✗Higher running costs than LED
✗Bulbs degrade and need annual replacement
✗Less efficient than modern LED

Best for: Fruiting crops, larger grows, growers with good cooling

🟣

CMH / LEC (Ceramic Metal Halide)

Full spectrum HID

Ceramic Metal Halide (also called Light Emitting Ceramic) produces a broader, more balanced spectrum than HPS including UV output, which improves terpene and flavour development in some crops. Lower heat output than HPS at equivalent wattage. Available in 315W and 630W configurations. A good middle ground between HPS output and LED spectrum quality.

Pros

✓Better spectrum than HPS — includes UV
✓Lower heat than equivalent HPS
✓Good for veg and flower
✓Improves terpene production in some crops

Cons

✗Specific ballasts and bulbs required
✗Bulbs expensive and need annual replacement
✗Less efficient than top-tier LED
✗Less widely available

Best for: Growers wanting HID reliability with better spectrum

⬜

CFL & T5 Fluorescent

Propagation & seedlings

Compact fluorescent and T5 strip lights produce soft, cool light suitable for seedlings, cuttings, and leafy greens in small spaces. Very low heat, cheap, and safe to hang close to young plants. Not suitable as the primary light for fruiting crops — insufficient intensity. Best used in propagation stations, seed-starting trays, or as supplemental side lighting.

Pros

✓Very low heat
✓Safe for seedlings and clones
✓Cheap upfront
✓No risk of light burn on young plants

Cons

✗Insufficient intensity for fruiting crops
✗Low efficiency vs LED
✗Large physical footprint for the output
✗Limited penetration into canopy

Best for: Propagation, seedlings, leafy greens in small spaces

Part 4

Light Cycles & Photoperiod

Standard Light Schedules

18h on / 6h offVegetative

Simulates long summer days. Used to keep plants in vegetative growth. Most crops and all non-photoperiod plants can run on this schedule indefinitely.

12h on / 12h offFlowering trigger

Simulates shortening autumn days. Triggers flowering in photoperiod-sensitive plants. Maintain strict darkness during the off period — any light leak delays or prevents flowering.

20h on / 4h offSeedlings & propagation

Extended photoperiod for seedlings and clones. Maximises DLI at lower intensities while keeping temperatures stable overnight.

24h onAutoflowering crops only

Some growers run continuous light for autoflowering varieties which do not require darkness to flower. Not recommended for photoperiod plants — most need a dark period for full recovery processes.

Photoperiod vs Autoflowering

Factor	Photoperiod	Autoflowering
Flowering trigger	Light cycle change	Age (automatic)
Veg period	Grower-controlled	Fixed (~3–4 weeks)
Total grow time	Longer, flexible	Faster (8–12 weeks)
Light schedule	18/6 veg, 12/12 flower	18–20h throughout
Yield potential	Higher (larger plants)	Lower per plant
Cloning	Yes	Not practical
Beginner-friendly	Moderate	Yes

Lights-out consistency matters

Use a reliable timer. Inconsistent light schedules stress plants and can cause photoperiod varieties to re-vegetate (return to veg growth) mid-flower, wasting weeks of development.

Part 5

Light Distance & Common Mistakes

Hanging Height Guidelines

Light intensity follows the inverse square law — double the distance = quarter the intensity. This means hanging height has an enormous impact on PPFD. Always refer to your manufacturer's PPFD chart for your specific fixture. General starting points:

50–60 cm

Seedlings / clones (LED)

Low intensity — delicate tissue

30–45 cm

Vegetative (LED)

Moderate intensity for growth

20–35 cm

Flowering (LED)

Higher intensity for bud development

40–60 cm

HPS 600W

Heat buffer distance needed

Common Lighting Mistakes

!Buying cheap LEDs: Cheap LEDs understate true wattage and overstate PPFD. Look for brands with third-party PAR measurements and realistic specs.
!Hanging too close: Light burn — bleached white or yellow patches at the top of the canopy closest to the light. Raise the fixture and check PPFD.
!Hanging too far: Insufficient intensity causes stretchy, weak plants (etiolation). Stems elongate reaching for light. Lower the fixture.
!Inconsistent timer: Missed or irregular cycles stress plants. Use a quality mechanical or digital timer; test it before the grow begins.
!Ignoring heat from HPS: HPS lights raise tent temperature significantly. If ambient + HPS pushes canopy above 28°C, yield and quality suffer.

⚡

Electrical Safety

Grow lights operate at high wattage in a warm, potentially humid environment. Always use a GFCI (Ground Fault Circuit Interrupter) outlet for your lighting circuit, keep all connections dry and elevated off the floor, and never handle electrical equipment with wet hands. For full guidance see the Essential Equipment safety section.

FAQ

Frequently Asked Questions

What's the difference between LED, HPS, and CMH grow lights?: Full-spectrum LED (quantum board) is now the best choice for most home growers — low heat, low running cost, and full lifecycle use from seed to harvest. HPS produces excellent flowering results with lower upfront cost at 600W+, but generates significant heat and needs annual bulb replacement. CMH (Ceramic Metal Halide) gives a broader spectrum than HPS, including UV, with lower heat — a middle ground between the two.
What is PPFD and why does it matter?: PPFD (Photosynthetic Photon Flux Density) measures how many light photons land on a square metre of canopy per second, in μmol/m²/s. It is the most useful single number for comparing grow lights and setting the correct hanging height — leafy greens need roughly 200–400 μmol/m²/s, while flowering or fruiting crops need 600–1,000+.
What is DLI in hydroponic lighting?: DLI (Daily Light Integral) is the total light a plant receives over 24 hours, in mol/m²/day — calculated as PPFD × photoperiod hours × 0.0036. It accounts for both intensity and duration: a lower-intensity light run for more hours can deliver the same DLI as a brighter light run for fewer hours.
What light schedule should I use for hydroponic plants?: 18 hours on / 6 hours off keeps plants in vegetative growth. 12 hours on / 12 hours off triggers flowering in photoperiod-sensitive plants — any light leak during the dark period can delay or prevent flowering. Seedlings and propagation often use an extended 20 hours on / 4 hours off schedule.
How close should grow lights be to plants?: Light intensity follows the inverse square law — doubling the distance quarters the intensity. General starting points: 50–60cm for seedlings/clones under LED, 30–45cm for vegetative growth, and 20–35cm for flowering, always checked against your specific fixture's manufacturer PPFD chart.
What happens if grow lights are hung too close or too far from plants?: Too close causes light burn — bleached white or yellow patches at the top of the canopy closest to the light. Too far causes insufficient intensity, leading to stretchy, weak plants (etiolation) as stems elongate reaching for light.

Ready to set up your grow? Open the System Builder → for a checklist of what you need, indicative prices, and links to find everything.