Quick answer
Nitrogen burn means the plant is getting more nitrogen than it can use. The classic signs are leaves that turn very dark green, look glossy or waxy, and hook downward at the tips like a claw. Tips and margins may crisp to brown. Stop feeding nitrogen, water with plain pH-adjusted water until you see runoff, and measure the strength of your next batch before it goes in. Healthy new growth usually shows up in one to two weeks. Leaves that already clawed or burned will not straighten out or turn green again.
revive cannabis plants from nutrient toxicity
What nitrogen burn looks like
- Deep, almost blue-green color across the canopy
- Leaf tips curling down and under, a shape growers call the claw
- Crisp brown or bronze tips that spread along the serrated edges
- Shiny, stiff leaves with a leathery feel
- Slow vertical growth and thin, weak stems
- Late or weak flower set, with leafy, airy buds
Because nitrogen moves through the plant, the oldest and lowest fan leaves usually show it first. New growth sits at the top and gets fed last.
Why it happens
Most cases come from feeding a full-strength vegetative formula after stretch, or from topping up a reservoir without checking the reading. A plant in early flower still wants some nitrogen, just not the level it wanted in week three of veg. Other causes are a slow release amendment still breaking down in the pot, compacted media that holds salts, or heavy runoff being recycled into the next feed. Ammonium-heavy feeds can also drop root-zone pH and make the problem worse.
Telling it apart from other problems
Clawed, dark leaves point to nitrogen. Pale yellow leaves starting at the bottom point the other way, to a shortage. Tip burn that appears evenly across the whole canopy, top to bottom, is more often general salt burn from a high electrical conductivity reading than nitrogen alone. Heat stress curls leaf margins upward, not down. Overwatering gives you droop and soft stems, but the color stays normal. If you are not sure, test the runoff before you change anything.
revive cannabis plants from nutrient toxicity
How to fix it
- Stop all nitrogen inputs, including calmag products that carry it.
- Check the pH and EC or ppm of your runoff.
- Run plain water through the pot until runoff reads close to your input.
- In coco or hydro, mix the next reservoir at half strength or less.
- In living soil, skip the heavy flush. Water normally and let the microbes catch up.
- Hold the reduced feed for one to two weeks, then ramp back slowly.
- Keep temperature, humidity, and light intensity steady so the plant is not fighting two stressors.
Recovery and prevention
Recovery is judged by new growth, not old growth. Watch the top two nodes. If new leaflets come in flat and a normal green, you caught it in time. Do not strip the damaged fan leaves during flower unless they are fully dead, since the plant still pulls stored sugars from them. For prevention, I write the EC of every feed on the reservoir lid and log it. Feed strength should follow plant stage, not plant size. Cut nitrogen back when the stretch ends, and never top up a reservoir just because the level dropped. Water volume and nutrient concentration are two separate dials.