Cannabis Grow Lights: PPFD, Spectrum, and Distance

Short answer

Cannabis needs two things from a grow light: enough total light per day, and the right spectrum. Intensity is measured as PPFD (photosynthetic photon flux density) in µmol/m²/s. Daily totals are measured as DLI (daily light integral) in mol/m²/day. A home grow in flower needs 30 to 45 mol/m²/day. That equals about 600 to 1000 µmol/m²/s across a 12-hour photoperiod.

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How much light cannabis needs

Light needs change with stage. Seedlings and clones use little. Flowering plants use the most.

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• Seedlings and clones: 100 to 300 µmol/m²/s, DLI 6 to 12 mol/m²/day
• Vegetative: 400 to 600 µmol/m²/s, DLI 20 to 30 mol/m²/day
• Flowering: 600 to 1000 µmol/m²/s, DLI 30 to 45 mol/m²/day

Plants under added CO2 at 1200 to 1500 ppm can take 1000 to 1200 µmol/m²/s. Plants without CO2 that go past 1000 µmol/m²/s often show stress.

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Light types compared

LED

Modern LED bar fixtures reach 2.5 to 3.0 µmol per watt. A 480-watt bar light covers a 4 ft by 4 ft tent. LEDs run cooler at the canopy than HPS and hold output for about 50,000 hours before it drops 10 percent.

HPS

High pressure sodium lamps produce about 1.3 to 1.7 µmol per watt. A 1000-watt HPS covers about 4 ft by 4 ft. It puts out more heat and needs an air-cooled hood or a tent exhaust. Bulbs lose output after about 10,000 hours and need replacement.

CMH

Ceramic metal halide runs at about 1.5 µmol per watt. The spectrum carries more blue than HPS, so it works for the full cycle. A 315-watt CMH covers about 3 ft by 3 ft.

Fluorescent

T5 and T8 tubes put out low PPFD. They work for seedlings and clones, not for flowering.

Distance to the canopy

Light drops with the square of distance. Move a lamp from 12 inches to 24 inches and the canopy gets one quarter of the light. Start LEDs 12 to 18 inches above the canopy and HPS 18 to 24 inches above with air cooling. Raise the lamp if leaf tips bleach or curl.

Spectrum

Cannabis uses light from 400 to 700 nm for photosynthesis. Blue at 400 to 500 nm drives leaf and stem growth. Red at 600 to 700 nm drives flower mass. Full-spectrum white LEDs cover both bands. Far-red at 730 nm can shift the phytochrome response and shorten flower time, though the effect varies by cultivar.

Photoperiod

Photoperiod cannabis needs 18 hours of light and 6 hours of dark for vegetative growth, then 12 hours light and 12 hours dark for flower. Light leaks during the dark period can delay flowering or cause hermaphrodites. Autoflowering cultivars flower on age, not on light cycle. They run under 18/6 or 20/4 for the whole grow.

Heat and airflow

HPS sends about 30 to 40 percent of its wattage into the room as heat. A 1000-watt lamp adds about 3,400 BTU per hour. LEDs trim that to about 1,900 BTU per hour for the same output. Size the exhaust fan to move that heat out.

Measuring light

A quantum sensor reads PPFD in µmol/m²/s. Phone apps exist, but calibration drifts across models. A watt-based rule of thumb is 30 to 50 watts of LED per square foot of canopy for flower.

Common problems

• Stretch: too little light at the top of the canopy, or too much distance
• Bleaching: PPFD above the tolerance of the cultivar
• Slow flower set: light leak during the dark period
• Heat stress: lamp too close, or exhaust too small