Cannabis Grow Lights: LED vs HPS & PPFD Explained

Understanding PPFD: The Most Important Metric

Forget "watts"—the real measure of grow light quality is PPFD (Photosynthetic Photon Flux Density). PPFD tells you how much usable light actually reaches your plants.

PPFD is measured in μmol/m²/s (micromoles of photons per square meter per second). Higher PPFD = more light energy = faster growth and bigger yields (up to a saturation point).

Target PPFD by Growth Stage

• Seedlings/Clones: 200-400 PPFD — Too much light stresses young plants
• Vegetative Stage: 400-600 PPFD — Supports rapid leaf/stem growth
• Flowering Stage: 600-900 PPFD — Maximizes bud production
• With CO2 supplementation: 900-1500 PPFD — Plants can use more light with extra CO2

DLI: Daily Light Integral

DLI (Daily Light Integral) is the total light received over 24 hours, measured in mol/m²/day. It's calculated by multiplying PPFD × hours of light × 0.0036.

Target DLI for cannabis:

• Seedlings: 13-19 mol/m²/day
• Vegetative: 20-40 mol/m²/day
• Flowering: 40-65 mol/m²/day

Types of Cannabis Grow Lights

LED (Light Emitting Diode)

Best overall choice for most growers. Modern LEDs offer full spectrum light, high efficiency (2.5-3.0 μmol/J), low heat, and long lifespan (50,000+ hours). Higher upfront cost is offset by lower electricity bills.

• ✅ Most energy efficient
• ✅ Low heat output
• ✅ Full spectrum available
• ✅ Dimmable
• ❌ Higher initial cost

HPS (High Pressure Sodium)

The traditional choice for flowering, HPS produces intense light in the red/orange spectrum. Still popular due to lower upfront cost and proven results, but generates significant heat and uses more electricity.

• ✅ Lower upfront cost
• ✅ Proven results over decades
• ✅ Good light penetration
• ❌ High heat output (needs cooling)
• ❌ Less efficient (1.5-1.7 μmol/J)
• ❌ Bulbs need replacing yearly

CMH/LEC (Ceramic Metal Halide)

A middle ground between HPS and LED. CMH provides full spectrum light with better efficiency than HPS (1.8-2.1 μmol/J) and excellent light quality. Good for growers wanting HID simplicity with better spectrum.

Fluorescent (T5, CFL)

Low-intensity lights best suited for seedlings and clones. Not powerful enough for flowering. Very low heat makes them safe to place close to plants.

Sizing Your Grow Light

The right light size depends on your grow space and plants' needs. Here are general guidelines:

Watts Per Square Foot

• LED: 30-50W per sq ft (efficient LEDs achieve target PPFD with less wattage)
• HPS: 50-75W per sq ft

Example: A 4×4 tent (16 sq ft) needs 480-800W of LED, or 800-1200W of HPS.

Watts Per Plant

• Small plants (autoflowers): 100-200W LED per plant
• Medium plants: 200-300W LED per plant
• Large plants (SCROG): 300-500W LED per plant

Light Distance from Plants

Light intensity follows the inverse square law—doubling distance quarters intensity. Finding the right distance is crucial for maximizing PPFD without causing light stress.

Signs of Light Stress

• Light burn/bleaching: Top leaves turn white or pale yellow while veins stay green
• Tacoing: Leaves curl upward like a taco to reduce light exposure
• Foxtailing: Buds grow tall, spiry, and loose instead of dense

If you see these signs, raise your light 2-4 inches or reduce intensity by 10-20%.

Light Schedules

Light schedule is crucial for controlling when cannabis flowers:

• 18/6 (Veg): 18 hours light, 6 hours dark. Standard for vegetative growth.
• 20/4 (Veg): More light = faster growth. Popular for autoflowers.
• 24/0 (Veg): Continuous light. Maximum growth but higher costs. Debated if rest period helps.
• 12/12 (Flower): Triggers and maintains flowering in photoperiod strains.