How far should an LED grow light be from cannabis plants?

Use the fixture maker's hanging range as a starting point, then verify the canopy rather than treating distance as the dose. Record the distance in inches or centimeters, dimmer setting, plant stage, and a PPFD reading or clearly labeled estimate at several canopy positions. A center reading can hide dim edges or a hot spot. Change height or output gradually and compare new growth before changing something else.

What is the difference between PPFD and DLI for cannabis?

PPFD is the photosynthetic light reaching a square meter each second at the measurement point, written as micromoles per square meter per second. DLI adds the photoperiod and describes the total delivered across a day, in moles per square meter per day. For a constant PPFD, calculate DLI as PPFD multiplied by light-hours and 3,600, divided by 1,000,000. Both are comparison tools, not universal prescriptions.

Do autoflowers need a different light schedule?

Autoflowers do not require a 12/12 switch to initiate flowering, but that does not make one long schedule correct for every plant or room. Compare any schedule alongside measured or clearly labeled estimated PPFD, calculated DLI, temperature and humidity, plant stage, energy and heat limits, and the plant's response. Keep the schedule stable long enough to read the result and change one variable at a time.

Should I copy a PPFD chart from another cannabis grow?

Treat an outside chart as context, not a prescription. Fixture spectrum and geometry, meter or app method, canopy shape, cultivar, stage, carbon dioxide, temperature, irrigation, and nutrition all change the response. Make a small grid at your own canopy, label the source of every value, and keep the same measurement method when you compare a later change.

Grower Guide

Cannabis grow light distance, PPFD, DLI, and schedules: what to measure before changing anything

Cannabis grow light distance is a starting measurement, not a dose. Two fixtures hung at the same height can deliver different intensity and coverage, and the same fixture changes as the canopy rises. A useful lighting decision connects distance, dimmer setting, canopy PPFD, photoperiod, calculated DLI, temperature and humidity, stage, and the plant's response — then changes one variable at a time.

Published 2026-07-30 · Reviewed 2026-07-30

Start with a repeatable baseline, not a large adjustment

Record the fixture and model, dimmer setting, distance from the light to the highest canopy point in both inches and centimeters, photoperiod, plant stage, and a photo. Use the manufacturer's range as a safe starting boundary, then verify your canopy. If there is an electrical or unsafe-heat concern, address that first through appropriate local expertise rather than waiting for a plant comparison.

Map PPFD across the canopy instead of trusting the center

PPFD is the photosynthetic photon flux density arriving at a square meter each second, written as micromoles per square meter per second. Make a small, repeatable grid at canopy height and include the center, edges, corners, and any visibly high or low area. Record whether each value came from a PAR meter, manufacturer map, phone estimate, or another method. A center reading alone can hide a hot spot or dim edge, and values collected by different methods are not a clean trend.

Use DLI to connect intensity with the light schedule

DLI is the daily light integral: PPFD multiplied by light-hours and 3,600, divided by 1,000,000, reported as moles per square meter per day. A hypothetical 500 micromoles per square meter per second for 18 hours calculates to 32.4 moles per square meter per day; that illustrates the math, not a target. Autoflowers do not need a 12/12 switch to initiate flowering, but schedule choices still change DLI, heat, water demand, and recovery time. Keep the timer stable and compare the full context rather than treating 18/6, 20/4, or another schedule as universally best.

Let the plant response limit the next change

Controlled cannabis studies show that responses depend on cultivar, stage, local canopy intensity, photoperiod, temperature, irrigation, nutrition, carbon dioxide, and the production setup. They do not create one safe PPFD or DLI target for every home grow. If exposed growth becomes pale, curled, dry, or less vigorous after a documented light change, pause escalation and compare light, heat, airflow, watering, feeding, pests, and root-zone context before deciding what the symptom means.

Log one change, its time, and the next observation window

Write the old and new height, dimmer, or schedule and the exact change time. Keep temperature and humidity source-labeled, photograph affected and unaffected growth from repeatable angles, and check again during the next light period and over the next few days. Do not change lighting, feeding, watering, and airflow together; an honest unchanged variable is part of the evidence.

Common questions

Evidence and scope

These sources support the measurement concepts and study-specific observations in this guide. They do not establish a universal fixture setting or diagnose a plant.

Rodriguez-Morrison et al. — cannabis response to increasing light intensity
A controlled indoor flowering study across a wide PPFD range. It supports measuring local intensity and respecting production context; it is not a universal home-grow target table.
Ahrens et al. — cultivar-specific flowering response to photoperiod
A controlled study of ten photoperiod-sensitive cultivars. It demonstrates cultivar variation and strict light-period controls; it does not establish an autoflower schedule.
Peterswald et al. — 12-hour versus 13-hour flowering photoperiod
A two-cultivar trial connecting photoperiod and DLI. Its narrow treatments are context for comparison, not permission to generalize one schedule to every cultivar or room.

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