Light Height by Stage
Vertical light distance control for stage-appropriate intensity.
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Light height is the simplest, most direct way to control intensity at your canopy. Unlike buying new equipment, you adjust it free every few days. PPFD targets exist because plants genuinely have intensity sweet spots—too little creates weak growth, too much causes bleaching and wasted electricity. The trade-off: you need at least a rough intensity measurement (phone app or meter) to stay on target. Guessing by feel or Internet hearsay usually leads to either stretched plants or bleached colas.
What It Is
Light height is the vertical distance between a grow light's emitting surface and the top of the plant canopy. It is the primary control for light intensity at the plants, measured in photosynthetically active photon flux density (PPFD, μmol/m²/s) Strong evidence[1].
Light intensity falls with distance according to the inverse square law when the source is small (like a bare HPS bulb) Strong evidence[2]. Modern LED panels, which have wide emitting surfaces, spread more evenly and fall off more gently than point sources Strong evidence[2]. In practice, small changes in hang height—5 to 10 cm—produce meaningful intensity shifts, especially with HID lamps.
Why Growers Adjust It
Cannabis plants have a stage-dependent intensity optimum. Too little light causes weak, stretchy growth and poor yields. Too much light at a given CO₂ level and leaf temperature causes photoinhibition, bleaching, and curled or 'tacoed' leaves Strong evidence[3][4].
Research and commercial practice support these PPFD target ranges by stage:
- 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 indoor grows without CO₂ supplementation plateau and experience stress damage well below that level Strong evidence[4].
When to Start
From day one. A freshly germinated seedling under a too-close LED will stall or bleach. Set the light height before plants go underneath it, then check every few days through vegetative growth and weekly during flower as the canopy rises toward the fixture.
Most modern LED fixtures publish a PPFD map showing recommended hang heights (often 18", 24", 36"). Start with the manufacturer's chart. If your fixture lacks one, the spec sheet usually lists PPFD at center at a reference distance and dimming level.
How to Do It: Step by Step
1. Identify your light type. Heat output and intensity per watt vary:
- HID (HPS/MH): large radiant heat load; needs more distance initially.
- Quantum board / bar LEDs: moderate heat, high efficiency; moderate distance required.
- CFL / T5 fluorescent: very low intensity and heat; can hang closer.
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 based on typical fixture designs; your actual optimal height may differ.
3. Measure PPFD at canopy. Use a handheld PAR/PPFD meter (Apogee, Photone-calibrated phone app, or equivalent) to read actual intensity Weak / limited. Take readings at the center and at least two corners of the canopy. Aim for your target stage range.
4. Adjust by height first, dimmer second. If your light has a dimmer, hang higher (to spread light more evenly across the canopy) and dim down, rather than hanging close and creating hotspots. This approach generally yields better uniformity Weak / limited[2].
5. Re-check weekly during flower. As plants grow, the canopy moves upward toward the light. Raise the fixture to maintain the target distance. Flower stretch in weeks 1–3 is when most growers miss adjustments and bleach the top cola.
6. Read the plants for feedback. Signs light is too close: white or bleached leaf tips, curled-up 'taco' leaves, brown or scorched upper leaf edges, stalled vertical growth. Signs light is too far: long internodes, thin stems, pale foliage, plants leaning or reaching toward the light.
Measuring Without a Professional Meter
Phone-based PPFD apps (Photone, Korona, and similar) use your phone's built-in light sensor with a diffuser to estimate intensity. Accuracy varies by phone model and light spectrum—they typically fall within ±10–15% accuracy for white-light LEDs and perform worse under HPS or heavy red/blue spectrum fixtures Weak / limited. Adequate for hobby grows. For commercial or competitive use, invest in a calibrated quantum sensor.
The hand-warmth test ('if my hand is comfortable, the plants are fine') works poorly for LEDs because they emit little radiant heat while still producing excessive PPFD Anecdote.
Common Mistakes
- Running new LEDs at 100% from day one. Most modern bar LEDs at 30 cm and full power deliver 1200+ μmol/m²/s—excessive for seedlings or rooms without CO₂ supplementation. Start at 50–70% brightness for the first two weeks.
- Not raising the light during flower stretch. Plants can grow 5–10 cm per week in early flower. A fixture that was ideal on day 1 will be too close by day 14 unless raised regularly.
- Uneven canopy plus low hang. When plant height varies widely, tall colas get bleached while short ones receive insufficient light. Either raise the fixture higher or use canopy training to level the crop.
- Ignoring environment. Light tolerance depends on leaf temperature and humidity. A plant at 28°C leaf temperature with correct VPD (Vapor Pressure Deficit Basics) handles more light than one at 32°C with dry air Strong evidence[4].
- Trusting reddit or forums without measurement. Anecdotes vary wildly by fixture, room, and interpretation. PPFD measurement removes guesswork.
Related Techniques
Light height is one input to canopy management. It integrates with:
- Canopy training (topping, FIMming, supercropping)—flattens plant height so a single hang height works for the entire canopy.
- Horizontal nets (ScrOG)—creates a level canopy so uniform light distribution is achievable.
- Defoliation—removes shade leaves to let lower bud sites receive usable light without lowering the fixture.
- VPD (Vapor Pressure Deficit Basics)—determines how much light a plant can physiologically use.
- CO₂ Supplementation—increases the PPFD ceiling before light saturation and stress kick in.
- PPFD Targets by Stage—detailed reference for intensity targets across growth phases.
- Flowering Stage—covers timing and environment changes during bloom.
- Light Bleaching—diagnosis and prevention of light burn.
Sources
- Peer-reviewed Chandra, S., Lata, H., Khan, I. A., & ElSohly, M. A. (2008). Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO₂ conditions. Physiology and Molecular Biology of Plants, 14(4), 299–306.
- Book Cervantes, J. (2015). The Cannabis Encyclopedia: The Definitive Guide to Cultivation & Consumption of Medical Marijuana. Van Patten Publishing.
- Peer-reviewed Eaves, J., Eaves, S., Morphy, C., & Murray, C. (2020). The relationship between light intensity, cannabis yields, and profitability. Agronomy Journal, 112(2), 1466–1470.
- Peer-reviewed Rodriguez-Morrison, V., Llewellyn, D., & Zheng, Y. (2021). Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science, 12, 646020.
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