Light Height by Stage

What it is

Light height is the vertical distance between your grow light's emitting surface and the top of the plant canopy. It is the simplest lever a grower has for controlling light intensity, measured in micromoles of photosynthetically active photons per square meter per second (PPFD, μmol/m²/s) Strong evidence^[1].

Intensity falls roughly with the inverse square of distance from a point source, though modern LED panels with wide diodes behave more like area sources, so the falloff is gentler than a bare bulb Strong evidence^[2]. In practice this means small changes in hang height (5-10 cm) produce meaningful changes in PPFD at the canopy, especially with HID lamps.

Why growers adjust it

Plants have a stage-dependent sweet spot for light intensity. Too little light produces stretchy, weak growth and low yields. Too much light at a given CO₂ level and leaf temperature causes photoinhibition, bleaching, and curled or 'tacoed' leaves Strong evidence^[3][4].

Target PPFD ranges that have held up in cannabis research and commercial practice:

• Seedlings / clones: 100-300 μmol/m²/s Strong evidence^[3]
• Vegetative: 400-600 μmol/m²/s Strong evidence^[3]
• Flower (ambient CO₂, ~400 ppm): 600-900 μmol/m²/s Strong evidence^[3][4]
• Flower (supplemented CO₂, 1000-1200 ppm): up to ~1500 μmol/m²/s before diminishing returns Weak / limited^[4]

Rodriguez-Morrison et al. (2021) found cannabis yield increased nearly linearly with light intensity up to 1800 μmol/m²/s under enriched CO₂, but most home grows without CO₂ supplementation hit diminishing returns and stress damage well below that Strong evidence^[4].

When to start

From day one. Even a freshly germinated seedling under a too-close LED will stall or bleach. Set height before plants go under the light, then re-check every few days through veg and weekly in flower as the canopy rises.

Most modern LED fixtures publish a PPFD map for the recommended hang heights (often 18", 24", 36"). Start there. If your light didn't come with one, the manufacturer's spec sheet usually lists PPFD at center at a known distance and dimming level.

How to do it: step by step

1. Identify your light type. Heat output and intensity per watt differ:

• HID (HPS/MH): large radiant heat load. Start far.
• Quantum board / bar LEDs: moderate heat, high efficiency.
• CFL / T5 fluorescent: very low intensity and heat. Hang close.

2. Use the manufacturer's chart as a starting point. Typical starting hang heights at full power:

• 1000W HPS: 50-60 cm (20-24") above canopy
• 600W HPS: 40-50 cm (16-20")
• 240W LED quantum board: 45-60 cm (18-24") in veg, 30-45 cm (12-18") in flower
• 4-tube T5: 15-25 cm (6-10")

These are starting points, not gospel — every fixture is different.

3. Measure PPFD at canopy. A handheld PAR meter (Apogee, Photone-calibrated phone app, or similar) gives you the actual number. Take readings at center and corners. Aim for the stage range above.

4. Adjust by height first, dimmer second. If your light has a dimmer, you can hang higher (more even spread across the canopy) and dim down, rather than hanging close and creating hotspots. This is generally preferable for uniformity Weak / limited^[2].

5. Re-check weekly. As plants grow, the canopy moves toward the light. Raise the light to maintain target distance. Stretch in early flower (weeks 1-3) is when most growers get caught flat-footed and bleach their colas.

6. Read the plants. Signs the light is too close: bleached white tops, curled-up 'taco' leaves, brown edges on top fan leaves, stalled vertical growth. Signs it's too far: long internodes, thin stems, pale color, plants leaning toward the light.

The phone-app shortcut

If you don't own a PAR meter, the Photone app (or Korona, or similar) uses your phone's light sensor with a diffuser to estimate PPFD. Accuracy varies by phone model and spectrum — they tend to be within ±10-15% for white-light LEDs and worse for HPS or heavy red/blue spectra Weak / limited^[5]. Good enough for hobby grows. For commercial work, buy a real quantum sensor.

Common mistakes

• Trusting the back-of-the-hand temperature test. 'If my hand is comfortable, the plants are fine' works okay for HPS heat but tells you nothing about LED PPFD. LEDs can bleach plants while feeling cool to the touch Anecdote.
• Running a new LED at 100% from day one. Most modern bar LEDs at 30 cm and full power deliver 1200+ μmol/m²/s — too much for seedlings or non-CO₂ rooms. Dim to 50-70% for the first two weeks.
• Not raising the light during flower stretch. Plants can grow 5-10 cm per week early in flower. A light that was perfect on day 1 is too close by day 14.
• Uneven canopy + low hang. Tall colas get bleached, short ones get nothing. Either raise the light or use Plant Training to even out the canopy.
• Ignoring VPD and leaf temperature. Light tolerance depends on environment. A plant at 28°C leaf temp and proper VPD handles more light than one at 32°C with dry air Strong evidence^[4].

Related techniques

Light height is one input to canopy management. It pairs with:

• Topping and FIMming — flattens the canopy so a single hang height works for the whole plant.
• ScrOG (Screen of Green) — same idea, via a horizontal net.
• Defoliation — removes shade leaves so lower bud sites get usable light without needing to drop the lamp closer.
• VPD Management — determines how much light a plant can actually use.
• CO2 Supplementation — raises the ceiling on usable PPFD.

Sources

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