Cannabis Grow Lights: PPFD, DLI, Types and Setup

Short answer

Cannabis grow lights supply the photons a plant uses to make sugar. Indoor plants need 12 to 18 hours of light per day, set by stage. Photoperiod plants need 12 hours of dark to flower. A fixture that delivers 600 to 900 µmol/m²/s of PPFD across the canopy covers most flower rooms. Light type matters less than photon delivery at the canopy.

Light numbers that decide a crop

Four metrics control what a light does for the plant.

• PPF (photosynthetic photon flux): total photons a fixture emits per second, in µmol/s.
• PPFD (photosynthetic photon flux density): photons that land on one square meter per second, in µmol/m²/s. This is the reading at the canopy.
• DLI (daily light integral): total photons in one day. DLI = PPFD × hours of light × 0.0036. A 12-hour cycle at 700 µmol/m²/s gives DLI 30.
• Efficacy: µmol of PAR per joule of electricity. Higher numbers mean less power for the same light.

PPFD targets by stage: clones 100 to 200, vegetative growth 300 to 600, flowering 600 to 900. DLI targets run 15 to 25 mol/m²/day in veg and 30 to 45 in flower. Rooms with added CO2 at 1200 to 1500 ppm can run 1000 µmol/m²/s or more.

Light types

LED

Bar-style and board LED fixtures run 2.5 to 3.5 µmol/J. They spread light over a wide footprint and give off less radiant heat than HID. Check the stated PPF, a PPFD map, and a photon efficacy figure from a test report. White diodes plus 660 nm red diodes cover the spectrum. A 480W to 720W LED replaces a 1000W HPS in a 4x4 ft tent.

HPS (high pressure sodium)

Double-ended 1000W HPS runs about 1.9 to 2.1 µmol/J. Single-ended lamps run lower. A 1000W HPS covers 4x4 to 5x5 ft. Lamps lose output with age, so replace every 3 to 4 grows. Air-cooled hoods allow the lamp 18 to 24 in above the canopy. Bare bulbs need 24 to 36 in.

CMH / LEC (ceramic metal halide)

315W CMH fixtures run about 1.5 to 1.7 µmol/J and put out a broad spectrum. A 315W lamp covers 3x3 ft. Suited to veg and small flower rooms.

Fluorescent T5

T5HO tubes run about 1.0 µmol/J. Use them for clones, seedlings, and mother plants. They lack the intensity for a full flower room.

Color temperature

Veg: 4000K to 6500K. Flower: 2700K to 3500K, or a 3000K plus deep red mix. Spectrum shifts change plant shape and stretch more than yield. Photon quantity and canopy coverage drive weight.

Photoperiod by stage

• Vegetative: 18 hours light, 6 hours dark. Some growers run 24/0 or 20/4.
• Flowering: 12 hours light, 12 hours dark. The dark block must be total.
• Autoflowering: 18/6 to 20/4 for the whole cycle.

A light leak in the dark block can stall flowering or produce male flowers. A green bulb is not safe. Use a timer and seal the tent zipper.

Distance, coverage, and heat

Hang height sets PPFD. A 1000W HPS at 24 in reads different from the same lamp at 36 in. Move the fixture up as plants grow. Leaf surface temperature above 85°F slows growth. Airflow at the canopy moves heat and thins the boundary layer. Measure PPFD at the corners of the footprint, not only the center.

Electricity cost math

Watts ÷ 1000 × hours × price per kWh. A 600W light on 12/12 for 30 days uses 216 kWh. At $0.16 per kWh that is $34.56 for the fixture alone.

Common mistakes

• Buying watts instead of PPF. A 1000W lamp with low efficacy can deliver less light than a 600W LED.
• Starting at full power. Raise intensity over 7 to 10 days.
• Running the lamp at 12 in without a leaf temperature check.
• Reusing an old HPS lamp. Output drops before the lamp fails.
• Ignoring the edges of the canopy, where PPFD falls 30 percent or more.