Yellow leaves during flowering are normal in the final two to three weeks, when the plant moves nitrogen out of fan leaves and into the buds. Yellowing that starts before week 5, climbs the plant fast, or arrives with brown spots, curled tips, or crispy edges points to a fixable problem: pH drift, a nitrogen or magnesium shortage, overwatering, root rot, or light stress.
Normal Fade vs. a Real Problem
The fade that ends a grow starts low, moves up, and leaves the bud sites green and swelling. The leaves go pale, then yellow, then drop off with a light tug. A problem looks different. It hits one side of the plant, jumps to the top canopy, or comes with spots, clawing, and stalled bud growth.
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Prerequisites
- pH meter, calibrated the same day you test
- EC or TDS meter for feed strength
- Plain water at room temperature for a runoff test
- A notebook or phone for tracking week, feed, and pH
Diagnose the Cause in Five Steps
- Note the flower week. Yellowing after week 6 with firm, fat buds is usually the natural fade.
- Mark where the yellow starts. Bottom-up yellowing points to mobile nutrients: nitrogen, magnesium, potassium. Top-down yellowing points to light, heat, iron, or calcium.
- Measure the runoff. Collect the first runoff from a slow watering and compare its pH and EC to what you poured in.
- Lift the pot and smell the medium. A heavy pot with a sour smell means soggy roots, not a nutrient shortage.
- Read the leaf pattern. Pale leaves from the tip inward mean nitrogen. Yellow between green veins means magnesium. Brown blotches with curled edges mean potassium or calcium.
Fix What You Find
- Set the pH of every feed and watering to 6.0 to 6.5 in soil, or 5.8 to 6.2 in coco and hydro.
- Flush the medium with pH-correct water when runoff EC runs 300 or more points above the input.
- Resume feeding at half strength and step back up over two weeks.
- Add a magnesium source such as one teaspoon of Epsom salt per gallon when the veins stay green and the leaf between them yellows.
- Raise the light 6 to 12 inches or cut intensity when only the top canopy bleaches.
- Let the medium dry back between waterings and add beneficial microbes if roots look brown.
- Pull leaves that come off with no resistance so rot does not spread.
What to Avoid
- Heavy nitrogen feeds in late flower, which slow bud development and stall ripening
- Blind calmag doses without a pH reading first
- Stripping healthy green leaves to "fix" the look
- Flushing the whole medium every time one leaf turns yellow
One clean change at a time tells you what worked. Log the week, the feed, and the pH, then check the plant again in three days.