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Every grow presents problems. The key to successful troubleshooting is systematic diagnosis: observe symptoms, narrow down possible causes, and apply targeted corrections. This guide organizes common problems by symptom category with diagnostic criteria and corrective actions.
Golden Rule: Change one variable at a time. If you adjust pH, nutrients, and watering simultaneously, you cannot determine which change resolved (or worsened) the problem.
- Identify the symptom — What does the plant look like? Where on the plant is the symptom appearing?
- Match to the symptom table — Use the tables below to narrow down possible causes
- Confirm the diagnosis — Check the diagnostic criteria to distinguish between similar symptoms
- Apply the correction — Follow the corrective action; monitor for improvement over 3-5 days
- Reference detail pages — For pests and diseases, follow links to dedicated identification pages
| Symptom |
Location |
Likely Cause |
Urgency |
Detail |
| Lower leaves turn yellow |
Bottom of plant |
Nitrogen deficiency or natural senescence (late flower) |
Low-Moderate |
See Nutrient Deficiencies |
| Lower leaves develop brown spots |
Bottom of plant |
Potassium or magnesium deficiency |
Moderate |
See Nutrient Deficiencies |
| Upper leaves turn yellow |
Top of plant |
Iron, sulfur, or calcium deficiency; or pH lockout |
High |
See Nutrient Deficiencies |
| Leaves develop purple/red stems or petioles |
Anywhere |
Genetics, phosphorus deficiency, or cold stress |
Low |
Usually genetic; verify if other deficiency symptoms present |
| Leaf tips turn brown and crispy |
Tips of leaves |
Nutrient burn (excess) or light stress |
Low-Moderate |
Reduce nutrient concentration by 25% |
| Leaves develop interveinal chlorosis (yellowing between veins) |
New growth |
Iron or magnesium deficiency |
Moderate |
See Nutrient Deficiencies |
| Leaves develop bronze/copper sheen |
Upper canopy |
Phosphorus excess or potassium deficiency |
Moderate |
Verify nutrient ratios |
| Leaves appear glossy, waxy, or blistered |
Newest growth |
Broad mite damage |
High |
See Pest Identification |
| Leaves curl upward ("taco" shape) |
Lower to upper progressively |
Russet mite damage |
High |
See Pest Identification |
| White powdery coating on leaves |
Leaf surfaces |
Powdery mildew |
URGENT |
See Disease Identification |
¶ Leaf Shape and Structure
| Symptom |
Likely Cause |
Diagnostic Criteria |
Correction |
| Leaves curl upward at edges |
Overwatering, russet mites, or heat stress |
Overwatering: soil stays wet; roots: pale. Russet mites: 60x+ visible mites, shiny stems. Heat: leaf temperature >86°F |
Adjust watering; check for mites; lower canopy temperature |
| Leaves curl downward ("canoe" shape) |
Overwatering, wind burn, or excess humidity |
Overwatering: heavy pot, wet medium. Wind burn: fan too close. Humidity: RH >70% |
Allow drying; move fans; lower RH |
| Leaves are thin, papery, or drooping |
Underwatering or severe nutrient deficiency |
Underwatering: medium dry; pot light. Deficiency: multiple nutrient symptoms present |
Water thoroughly; check EC/PPM of feed |
| New leaves emerge twisted, cupped, or deformed |
Broad mites, herbicide drift, or heat stress |
Mites: 60x magnification confirms. Herbicide: nearby outdoor spraying. Heat: >86°F at canopy |
See Pest Identification |
| Leaf edges turn up and become crispy |
Nutrient burn (nitrogen excess) |
Tips and edges brown; rest of leaf green; often after increasing feed |
Reduce nutrients; flush if severe |
| Leaves develop claw shape (curved downward like a talon) |
Nitrogen excess |
Dark green leaves; stems may be purple; claws point downward |
Reduce nitrogen; increase runoff |
| Leaves develop cupped, "E"-shaped deformation |
Phosphorus toxicity |
Dark metallic sheen on leaves; stems purple; usually from overfeeding bloom nutrients |
Reduce phosphorus; verify nutrient ratios |
| Symptom |
Likely Cause |
Diagnostic Criteria |
Correction |
| Plant is stunted / not growing |
pH lockout, root rot, overwatering, or broad mites |
pH lockout: runoff pH far from target. Root rot: brown/smelly roots. Mites: distorted new growth |
Correct pH; check roots; inspect for mites |
| Plant stretching excessively (long internodes) |
Insufficient light or too-close lights |
Stretching occurs early in flower; internodes >4 inches; stems are thin |
Increase light intensity (PPFD); lower lights if too far |
| Plant will not transition to flower |
Light leak (photoperiod plants) or wrong light cycle |
Light timer incorrect; ambient light during dark period; plants remain in vegetative growth |
Fix light leak; verify 12/12 schedule |
| Bud sites are small / airy |
Insufficient light, poor genetics, or HpLVD |
Light: measure PPFD at canopy. Genetics: strain naturally low-yielding. HpLVD: PCR test positive |
Increase light; source better genetics; test for HpLVD |
| Flowering takes longer than stated |
Strain genetics or environmental stress |
Check breeder timeline; low temps can slow flower development |
Allow extra time; maintain 65-78°F in flower |
¶ Root and Medium Problems
| Symptom |
Likely Cause |
Diagnostic Criteria |
Correction |
| Medium stays wet for days |
Overwatering or poor drainage |
Pot is heavy; medium is saturated; roots may be rotting |
Allow to dry; improve drainage; check for root rot |
| Medium surface develops white/green crust |
Mineral buildup (salt accumulation) |
White crystals on surface or pot edges; high EC/PPM of runoff |
Flush medium with pH-adjusted water; reduce nutrient concentration |
| Roots are brown, slimy, or smell foul |
Pythium or Fusarium root rot |
Healthy roots are white/cream; affected roots are brown and mushy |
See Disease Identification |
| Algae growth on medium surface |
Light reaching the medium |
Green film on top of medium or on pot sides |
Cover medium; block light; reduce surface moisture |
| Symptom |
Likely Cause |
Diagnostic Criteria |
Correction |
| Leaf edges curl up and feel hot |
Light burn / heat stress |
Leaves closest to light; PPFD >900 at canopy; leaf surface temp >86°F |
Raise lights; reduce intensity; increase exhaust |
| Leaves appear bleached or white |
Light burn (severe) |
White/bleached patches on uppermost leaves; tissue may be crispy |
Raise lights immediately; remove severely burned tissue |
| Leaves develop crispy brown edges with green centers |
Light burn |
Upper canopy only; progressive from top down |
Raise lights; increase distance to manufacturer recommendation |
| Plants dry out rapidly |
Low humidity or high temperature |
RH <35%; VPD too high; medium dries in <1 day |
Increase RH; lower temperature; water more frequently |
| Leades develop wind burn (curled, claw-like, stiff) |
Fan too close or too strong |
Leaves closest to fan; symptoms worsen over time |
Move fans further away; reduce speed; point fans at walls |
| Symptom |
Likely Cause |
Diagnostic Criteria |
Correction |
| Plant produces flowers during 18/6 light cycle |
Ruderalis genetics (autoflower) or light stress |
Autoflower: genetics indicate auto. Light stress: 18/6 schedule maintained but stress triggers flower |
Normal for autos; verify light schedule for photoperiod plants |
| Plant develops both pollen sacs and pistils (hermaphrodism) |
Genetic predisposition or environmental stress |
Stress causes: light leaks, heat, root problems, training during late veg/early flower |
Remove pollen sacs; identify and correct stress source |
| Plant produces pollen sacs |
Male plant or hermaphrodite |
Check for pistils (female hairs) — if absent, the plant is male |
Remove male immediately if growing for flower; isolate if breeding |
| Buds are small with many seeds |
Pollination from hermaphrodite or male |
Seeds visible in buds; male or herm plant was present |
Remove pollen source; sinsemilla (seedless) buds are higher quality |
| Flowering does not initiate after switching to 12/12 |
Light leak or insufficient dark period |
Check for light leaks; verify timer; ensure 12 uninterrupted hours of darkness |
Seal light leaks; test timer; ensure true darkness |
Nutrient deficiencies are the most frequently misdiagnosed grow problems. Before treating a deficiency, always check root zone pH first — a plant with perfect nutrients in the reservoir but incorrect root zone pH will show deficiency symptoms because the roots cannot absorb the nutrients (pH lockout).
| Parameter |
Soil |
Coco Coir |
Hydroponics (DWC) |
| Target pH |
6.0-7.0 |
5.5-6.5 |
5.5-6.2 |
| Below Range |
Iron, manganese, boron, zinc, copper become unavailable; copper and zinc are most affected |
Similar to soil but more sensitive; all micronutrients affected below 5.0 |
All micronutrients locked out below 5.0 |
| Above Range |
Iron, manganese, boron, zinc, copper become unavailable; phosphorus and calcium locked out above 7.0 |
Iron and manganese most affected above 6.5; phosphorus above 7.0 |
Phosphorus, calcium, magnesium locked out above 6.5 |
Rule: If deficiency symptoms appear suddenly on a previously healthy plant, check runoff pH before changing the nutrient feed. pH lockout is more common than true deficiency.
| Deficiency |
Symptom |
Location |
Correction |
| Nitrogen (N) |
Older leaves turn uniform yellow; leaf drop; overall pale appearance |
Lower/older leaves first |
Increase nitrogen; apply 50-100 ppm supplemental N |
| Phosphorus (P) |
Dark green to blue-green leaves; purple/red stems; reduced flower development |
Lower leaves first |
Verify pH is in range; increase phosphorus during flowering |
| Potassium (K) |
Brown/burnt leaf edges; brown spots on older leaves; weak stems |
Lower/older leaves |
Increase potassium; verify no excess sodium or calcium antagonism |
| Calcium (Ca) |
New growth distorted; brown spots on young leaves; weak cell walls |
New/upper growth |
Add Cal-Mag supplement; verify pH is in range |
| Magnesium (Mg) |
Interveinal chlorosis (yellowing between green veins) on older leaves; veins remain green |
Lower/older leaves first |
Add Epsom salts (magnesium sulfate) at 1-2 mL/L; or Cal-Mag |
| Iron (Fe) |
Interveinal chlorosis on new growth; veins remain green; severe cases turn almost white |
New/upper growth first |
Lower pH slightly (increases iron availability); add chelated iron |
| Sulfur (S) |
Uniform yellowing of new growth (similar to nitrogen deficiency but on new leaves) |
New/upper growth |
Rare; usually corrected by general nutrient increase |
| Zinc (Zn) |
Stunted new growth; small, crinkled leaves; interveinal chlorosis |
New/upper growth |
Lower pH; add zinc sulfate or micronutrient supplement |
| Manganese (Mn) |
Interveinal chlorosis with small necrotic spots; similar to iron but with dead spots |
New/upper growth |
Lower pH; add manganese supplement |
| Boron (B) |
Growing tip dies; thick, brittle new growth; brown corky spots on stems |
Growing tips |
Rare; add borax at very low rate (0.5 ppm); do not overapply |
| Toxicity |
Symptom |
Correction |
| Nitrogen excess |
Dark green leaves; leaf tips curl downward ("claw"); burnt leaf edges |
Reduce nitrogen feed; flush medium |
| Phosphorus excess |
Metallic sheen on leaves; interveinal chlorosis; reduced zinc and iron uptake |
Reduce phosphorus; verify nutrient ratios |
| Potassium excess |
Stunted growth; reduced calcium and magnesium uptake; salt buildup |
Flush medium; reduce potassium feed |
| Calcium excess |
pH rises in root zone; micronutrient lockout (iron, zinc, manganese, boron) |
Flush medium; correct pH; reduce calcium |
| General salt buildup |
White crust on medium surface; burnt leaf tips; slow growth |
Flush with pH-adjusted water at 2-3x pot volume; reduce overall feed EC |
| Parameter |
Seedling |
Vegetative |
Flowering |
| Optimal Range |
72-80°F (22-27°C) |
70-85°F (21-29°C) |
65-80°F (18-27°C) |
| Stress Below |
<65°F (18°C) |
<65°F (18°C) |
<60°F (15°C) |
| Stress Above |
>85°F (29°C) |
>88°F (31°C) |
>82°F (28°C) |
| Parameter |
Seedling |
Vegetative |
Early Flower |
Late Flower |
| Optimal Range |
65-70% |
50-70% |
45-55% |
40-45% |
| Danger Zone |
<50% (drying) |
>75% (mold risk) |
>60% (PM risk) |
>55% (botrytis risk) |
| Stage |
Target VPD |
Temperature |
RH |
| Seedling / Clone |
0.4-0.8 kPa |
75°F (24°C) |
65-70% |
| Vegetative |
0.8-1.2 kPa |
78°F (26°C) |
55-65% |
| Early Flower |
1.0-1.4 kPa |
76°F (25°C) |
50-55% |
| Late Flower |
1.2-1.6 kPa |
72°F (22°C) |
40-50% |
Note: VPD is the relationship between temperature and humidity that drives plant transpiration. Correct VPD is more important than temperature or humidity alone. See /cultivation/indoor/environment for VPD chart details.
Start: Plant shows symptoms
1. Where are the symptoms?
├── Lower/older leaves only → Nutrient deficiency (N, P, K, Mg) or natural senescence
├── Upper/new leaves only → Nutrient deficiency (Ca, Fe, S, Zn) or pH lockout
├── Roots → Root rot, overwatering, or low oxygen
├── Growing tips → Broad mites, calcium deficiency, or boron deficiency
├── Buds/flowers → Botrytis, powdery mildew, or caterpillar damage
└── Entire plant → pH lockout, root rot, light stress, or temperature stress
2. When did symptoms appear?
├── Suddenly (within 1-2 days) → pH lockout, light burn, or acute stress
├── Gradually (over 1-2 weeks) → Nutrient deficiency or pest infestation
└── Throughout grow → Genetics, chronic pH issue, or environmental mismatch
3. Has anything changed recently?
├── Nutrient change → Possible toxicity or deficiency
├── pH adjustment → Possible lockout or correction
├── Light move → Light burn or insufficient light
├── New plant introduced → Pest or disease introduction
└── Nothing changed → Progressive pest or disease development
4. Check root zone pH → Is it in range for your medium?
├── YES → Problem is likely a true nutrient deficiency or environmental stress
└── NO → Correct pH first; symptoms often resolve within 3-5 days
5. Inspect with magnification (30-60x) → Are pests visible?
├── YES → See [Pest Identification](/cultivation/pest-identification)
└── NO → Continue diagnosis with nutrient and environmental factors
When multiple problems are present, address them in this order:
- Life-threatening diseases — Botrytis in buds, severe root rot (can kill plant in days)
- Environmental extremes — Temperature >90°F, RH >80%, light leak during dark period
- pH lockout — Incorrect root zone pH prevents all nutrient uptake
- Pest infestations — Spider mites, broad mites, russet mites (rapid reproduction)
- Nutrient deficiencies — True deficiencies (after pH is confirmed correct)
- Minor cosmetic issues — Natural senescence, minor nutrient imbalances, slight leaf damage
See Also: /cultivation/pest-identification | /cultivation/disease-identification | /cultivation/pests-diseases | /cultivation/nutrients | /cultivation/indoor/environment | /cultivation/indoor/grow-guide