Hanging LED Lights Correctly

What it is

Hanging LED lights correctly means setting the fixture at a distance from the canopy that delivers the target photosynthetic photon flux density (PPFD) for the plant's current growth stage, with even coverage across the footprint. PPFD is measured in micromoles of photons per square meter per second (μmol/m²/s) Strong evidence^[1]. Because LEDs can produce very high intensities at close range, hang height is the simplest tool a grower has to control light dose without dimming the driver.

Why growers care

Cannabis is a high-light crop. Photosynthesis increases roughly linearly with PPFD up to around 1500 μmol/m²/s when CO2 is supplemented, and saturates lower (around 700-1000 μmol/m²/s) at ambient CO2 Strong evidence^[2][3]. Hang the light too high and you waste capacity and grow stretchy, airy buds. Hang it too low and you can bleach the canopy, induce leaf curl, or trigger heat stress even with 'cool' LEDs because near-infrared and direct radiant load still heat leaf tissue Strong evidence^[4]. Correct hang height is also how you compensate for footprint: most fixtures are rated for a specific coverage area at a specific height, and moving outside that window creates hot spots and dark corners.

Target PPFD by stage

These are widely used working ranges, not absolutes. Actual tolerance depends on cultivar, VPD, nutrition, and CO2.

• Seedlings / clones: 100-300 μmol/m²/s Strong evidence^[2]
• Early veg: 300-600 μmol/m²/s
• Late veg: 600-900 μmol/m²/s
• Flower (ambient CO2): 700-1000 μmol/m²/s Strong evidence^[3]
• Flower (CO2 supplemented to ~1200 ppm): 1000-1500 μmol/m²/s Strong evidence^[2][3]

Daily Light Integral (DLI) — PPFD multiplied by photoperiod hours — matters more than instantaneous intensity in the long run Strong evidence^[1]. A common flowering DLI target is 35-65 mol/m²/day.

How to hang the light, step by step

1. Read the manufacturer's PPFD map. Reputable LED makers publish a chart showing average and peak PPFD at various heights and footprints. Start there. If your fixture has no chart, treat that as a yellow flag and rely on a meter.
2. Install adjustable hangers. Use rope ratchets or geared hangers rated for at least 2x the fixture's weight. Hang from a sturdy support — not from ductwork.
3. Set initial height for the current stage. For seedlings under a typical 600-1000W class LED, that's usually 24-36 inches above the canopy. For flowering, 18-24 inches is a common starting point. Always verify with a meter.
4. Measure PPFD at canopy level. Take readings at the center and at four corners of the footprint. Aim for less than ~20% variation between center and edge. If the edges are too dim, you may need a wider fixture or supplemental bars rather than just lowering the light.
5. Adjust gradually. Lower in 2-3 inch increments over several days when increasing intensity, especially when moving plants from veg to flower. Sudden jumps in PPFD cause photobleaching and leaf curl Weak / limited^[4].
6. Re-measure weekly. Plants stretch — particularly in the first two weeks of flower. Raise the light to maintain your target PPFD, or commit to a fixed light height and let the canopy grow into it (works well with screen-of-green training).
7. Level the fixture. A tilted light creates uneven PPFD across the canopy. Use a small bubble level on the housing.

If you don't own a PAR meter, a calibrated smartphone lux app pointed at canopy level is a rough proxy. The lux-to-PPFD conversion for white LEDs is approximately 0.015 (multiply lux by 0.015 to estimate μmol/m²/s) Weak / limited^[5]. This is not accurate enough for serious tuning but will catch gross errors.

Common mistakes

• Trusting rules of thumb across different fixtures. A 'hang at 18 inches' rule was written for a specific light. A modern 1000W bar fixture at 18 inches over a seedling will roast it.
• Ignoring dimming. Most quality LEDs have a dimmer. Dimming to 50% and hanging closer can give more uniform coverage than running at 100% and hanging high. Use both controls together.
• Measuring PPFD at the floor, not the canopy. The number that matters is at the top of the leaves.
• Forgetting to re-hang after defoliation or training. Removing the upper canopy raises your effective hang distance.
• Mixing fixtures at different heights. Each one creates its own PPFD field. If you must mix, model the footprint with a meter rather than guessing.
• Believing 'LEDs don't produce heat.' They produce less radiant heat than HPS, but a 1000W LED still dumps close to 1000W of heat into the room. Leaf-surface temperature under a close LED can exceed air temperature by several degrees Strong evidence^[4].

Related techniques

• VPD management: higher PPFD demands higher transpiration capacity, which means tuning temperature and humidity together.
• CO2 supplementation: lets you exploit higher PPFD without diminishing returns.
• SCROG training: flattening the canopy makes uniform hang height far easier to achieve.
• Light burn vs nutrient burn: how to tell whether yellow tops are from hang height or feed.

Sources

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