Cannabis Flowering Stage Problems: Fix Bud Rot, Nute Burn, Light Leaks

Cannabis Flowering Stage Problems: Short Answer

Most flowering problems come from four sources: environment, feeding strength, light leaks, and airflow. Match the symptom to the cause, change one thing, then wait three to five days before you change another. Chasing several fixes at once hides which one worked.

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Symptom Check

• Nutrient burn: dark green leaves, clawed tips, crisp brown edges.
• Calcium or magnesium shortage: rust spots, brown margins, yellow between veins on middle leaves.
• Powdery mildew: white powder on leaf tops.
• Bud rot: brown, mushy patches with gray fuzz inside buds.
• Light leak: single bananas (male flowers) on female buds, seeds later.
• Spider mites: pale stippling, fine webbing under leaves.
• Heat stress: curled leaf edges, foxtail buds, dry feel.

Fix Nutrient Burn

1. Measure the pH and EC of your runoff.
2. Flush the pot with pH 6.0 to 6.5 water until runoff EC drops below your feed strength.
3. Mix the next feeding at half strength.
4. Repeat the runoff test three days later before you raise the dose.

Fix Calcium and Magnesium Shortage

1. Check pH: 6.0 to 6.5 in soil, 5.8 to 6.2 in coco.
2. Add a cal-mag supplement at the label rate.
3. Keep irrigation water above 65 degrees F.
4. Watch new growth for five to seven days. Old leaves will not recover.

Stop Powdery Mildew and Bud Rot

1. Raise airflow with an oscillating fan below the canopy and an exhaust fan running at low speed.
2. Keep humidity between 40 and 50 percent.
3. Remove the bottom third of leaves that block air.
4. Cut infected buds with sterile scissors and seal them in a bag for trash.
5. Spray potassium bicarbonate on leaves before week four of flower, then stop all sprays.

Environment Targets for Flower

• Light: 12 hours on, 12 hours off, no interruptions.
• Temperature: 65 to 80 degrees F lights on, 60 to 70 degrees F lights off.
• Humidity: 45 to 55 percent weeks 1 to 3, 40 to 45 percent weeks 4 to 6, 35 to 40 percent weeks 7 to harvest.
• Air: one fan moving air across the canopy, exhaust pulling stale air out.
• Water: let the top inch of soil dry before the next irrigation.

Light Leaks and Hermaphrodites

A single pinprick of light during the dark period can push a female plant to grow male flowers. Check the tent at night, then fix every gap.

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1. Stand inside the closed grow space and turn off all room lights.
2. Cover zipper runs, port holes, and seams with black tape.
3. Move the tent away from window light and standby LEDs.
4. Do all work during the light period, not the dark one.

Spider Mites in Flower

1. Inspect the underside of leaves with a hand lens.
2. Remove the worst leaves and bag them.
3. Spray insecticidal soap or horticultural oil before week three, covering leaf undersides.
4. Repeat every three days for three rounds.
5. Stop all sprays after week five and rely on airflow and removal.

When to Reset Instead of Repair

If bud rot covers more than a few sites, if male flowers show across the canopy, or if mites return after three spray rounds, harvest what is clean and sanitize the room. A stressed plant in late flower does not finish well, and the next run starts cleaner.

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