Light Burn on Cannabis Plants: Fix It and Maximize Your Yield
Too much light causes bleaching and nutrient lockout — here is how to find your plants' sweet spot
LED & HPSDLI CalculatorPPFD Targets by StageHanging Height Guide
Light burn is increasingly common as growers upgrade to high-power LEDs. Plants can only absorb so much light — beyond their saturation point, excess light bleaches buds and leaves and causes nutrient-lockout symptoms that are easily mistaken for deficiencies. The fix is usually as simple as raising the light or dimming it.
💡 Light Burn vs Heat Stress vs Bleaching Light burn: upper leaves curl upward, turn pale yellow-white at tips. Heat stress: leaves cup downward like tacos. Bleaching: buds turn white from excessive PPFD (usually 1200+ μmol). Same cause (too much light/heat), different manifestations.
Symptoms of Light Burn
🌿 Upward Leaf Curl
Upper leaves directly under the light curl upward and inward. Classic sign of excessive PPFD combined with heat stress.
💛 Yellowing Top Leaves
Pale yellow to white coloration on the upper canopy closest to the light. Lower leaves remain green — this distinguishes light burn from nitrogen deficiency.
⚪ Bud Bleaching
White or yellow-white buds in flower. Caused by sustained PPFD above 1,200 μmol/m²/s. Bleached buds lose terpene and cannabinoid content.
📈 Nutrient Lockout Signs
Multiple "deficiency" symptoms without pH problems. Light saturation causes photoinhibition that mimics magnesium and iron deficiencies.
Ideal PPFD Targets by Growth Stage
| Stage | PPFD Target | DLI Target | Light Hours |
|---|
| Seedling | 200–400 μmol/m²/s | 12–20 mol/m²/day | 18 hrs |
| Veg | 400–600 μmol/m²/s | 25–35 mol/m²/day | 18 hrs |
| Early Flower | 600–900 μmol/m²/s | 35–45 mol/m²/day | 12 hrs |
| Peak Flower | 800–1,100 μmol/m²/s | 40–55 mol/m²/day | 12 hrs |
Hanging Heights by Light Model
- comparable full-spectrum LED (500W): Veg 24–30", Flower 18–24"
- AC Infinity CLOUDRAY S6 (240W): Veg 24–28", Flower 18–22"
- Gavita Pro 1700e LED: Veg 36", Flower 24–30" (very powerful — keep high)
- HPS 600W: Veg 24", Flower 18–24"
AC Infinity CLOUDRAY S6 (240W LED)
Built-in PPFD controller — dial in exactly the right intensity per stage
From $189Shop Now
Frequently Asked Questions
How do I know if it is light burn or nutrient deficiency?
Light burn affects only the leaves closest to the light source — upper canopy. Nutrient deficiencies typically start at lower, older leaves (N, P, K, Mg) or at new growth (Ca, Fe, Mn). If your yellowing is only on the top of the plant and your pH is correct, raise your light first before adding nutrients.
Can bleached buds still be used?
Technically yes, but bleached buds have significantly reduced terpene and cannabinoid content. The photodegradation that causes bleaching also breaks down active compounds. Prevent bleaching by keeping PPFD below 1,100 μmol/m²/s in flower.
What is DLI and how does it relate to light burn?
DLI (Daily Light Integral) is the total amount of light a plant receives in a day, measured in mol/m²/day. It is calculated as PPFD × hours × 0.0036. Cannabis in flower performs well at DLI 40–55. Going above 60 risks light saturation. Spreading light over more hours at lower intensity is safer than short periods of extremely high PPFD.
Do LED grow lights burn plants more than HPS?
Modern quantum board LEDs produce very high PPFD per watt compared to HPS. Because LEDs produce less heat radiation, growers often hang them too close, not realizing the PPFD intensity. Always use the manufacturer's recommended hanging height and use a PAR meter to verify PPFD at canopy level.
