When something goes wrong with your plant, the worst thing you can do is guess and throw solutions at it. Most cannabis problems have a specific cause, and misdiagnosing leads to making the problem worse. This guide is a systematic flowchart: start with what you see, follow the decision tree, arrive at the most likely cause, and get the specific fix.
Before you start: check these three things first. They cause the majority of cannabis problems and should be verified before investigating anything else:
- pH: is your input water between 6.0–6.8 (soil) or 5.5–6.5 (coco/hydro)? If not, fix this first — most nutrient problems are actually pH problems. See our pH guide
- Watering: is the medium wet or dry? Does the pot feel heavy (overwatered) or light (underwatered)?
- Light distance: has anything changed with your light height or intensity recently?
If all three check out, proceed to the symptom below.
Symptom 1: Yellow leaves
Where are the yellow leaves?
Bottom leaves only → Nitrogen deficiency (most likely)
- Nitrogen is mobile — the plant pulls it from old leaves to feed new growth
- Older fan leaves turn uniformly pale green, then yellow, then drop
- New growth stays green
- In late flower: this is normal. The plant naturally cannibalizes lower leaves during senescence. No action needed unless it's aggressive
- In veg or early flower: increase nitrogen in your feed. Check pH first — nitrogen lockout occurs below pH 5.5 and above pH 7.5
- See our deficiency chart
Top/new growth → Iron deficiency or light burn
- If the yellowing is between veins (interveinal chlorosis) on new leaves: iron deficiency. Common at high pH (above 6.8 in soil). Fix pH, add chelated iron
- If the yellowing is bleaching/whitening of leaves closest to the light: light burn. Raise the light 6–12 inches or reduce intensity. See our light burn guide
All leaves, uniformly → pH lockout
- When pH is outside the acceptable range, multiple nutrients become unavailable simultaneously
- The plant yellows everywhere, doesn't respond to additional nutrients
- Flush the medium with correctly pH'd water, then resume feeding at correct pH
- See our pH lockout guide
Yellowing between veins (lower leaves) → Magnesium deficiency
- Interveinal chlorosis on lower/middle leaves — the tissue between veins turns yellow while veins stay green
- Common in coco grows (coco binds magnesium) and with RO water (no mineral content)
- Fix: cal-mag supplement. See our cal-mag guide
Yellowing between veins (upper leaves) → Iron deficiency
- Same interveinal pattern but on NEW growth (iron is immobile)
- Almost always a pH issue — iron locks out above pH 6.8 in soil
- Fix: lower pH. Chelated iron as a temporary supplement
Symptom 2: Brown or burnt leaf tips
How much of the tip is burnt?
Just the very tips (1–3mm) → Mild nutrient burn
- The plant is getting slightly more nutrients than it can use
- Reduce feed strength by 10–20%
- Monitor: if it doesn't progress, you're close to the plant's tolerance. Some tip burn is normal in aggressive feeding regimens
Tips + edges browning progressively → Moderate nutrient burn
- Reduce feed strength by 30–40%
- Check EC/PPM of your feed AND runoff. If runoff EC is significantly higher than input, salts have accumulated — flush with plain pH'd water, then resume at lower concentration
- See our EC guide
Brown crispy edges (not tips) → Potassium deficiency or heat stress
- If edges brown and curl upward: heat stress. Check temperature at canopy level — should be 72–82°F. Raise the light, improve airflow, or reduce ambient temperature
- If edges brown from the outside in, lower leaves first: potassium deficiency. Check pH (K locks out below 5.5), increase K in feed
Symptom 3: Spots on leaves
What do the spots look like?
Small brown spots (rust-colored) → Calcium deficiency
- Small, irregularly shaped brown spots scattered across leaves
- Often accompanied by curling leaf edges
- Common in coco (coco binds calcium) and with RO or soft water
- Fix: cal-mag supplement, check pH (Ca locks out below 6.2 in soil)
- See our cal-mag guide
Small yellow spots (speckles) → Spider mites
- Tiny yellow dots (stippling) on the top of leaves
- Check the underside of leaves with a loupe — mites are tiny (0.5mm), round, and may have webbing
- See our pest guide
White powdery spots → Powdery mildew
- White, powdery coating on leaf surfaces
- Starts as small circular patches, spreads rapidly
- Caused by poor airflow + high humidity
- See our powdery mildew guide
Large irregular brown patches → Leaf septoria or nutrient splash
- If patches have a yellow halo: leaf septoria (fungal). Remove affected leaves, improve airflow, apply neem or copper fungicide
- If patches are random and don't spread: nutrient solution splashed on the leaf and burned it under light. Not a disease — avoid getting nutrient water on leaves
Symptom 4: Wilting / drooping
Is the medium wet or dry?
Medium is wet (heavy pot) → Overwatering
- Leaves droop uniformly, including the petioles (stems)
- The entire plant looks limp and heavy
- Let the medium dry out completely before watering again
- Check drainage — ensure the pot has holes and water isn't pooling
- See our overwatering guide
Medium is dry (light pot) → Underwatering
- Leaves droop but look thin and papery (not plump)
- Petioles may point downward while leaf blades curl slightly upward
- Water thoroughly until 10–20% runoff
- If severely wilted, water in stages — a small amount, wait 15 minutes, then water fully
Medium is wet AND leaves are wilting after days → Root rot
- The plant wilts despite wet medium because damaged roots can't absorb water
- Check roots: they should be white and firm. Brown, slimy, foul-smelling roots = root rot
- See our root rot guide
Wilting only during lights on, recovers during lights off → Heat or light stress
- The plant can't transpire fast enough under intense light/heat
- Check canopy temperature — hold your hand at canopy height for 30 seconds. If it's uncomfortably warm, it's too hot for the plant
- Raise the light, improve ventilation, or add an oscillating fan
Symptom 5: Curling leaves
Which direction?
Curling upward (taco-ing) → Heat stress or light stress
- Leaves fold upward along the midrib, like a taco
- The plant is trying to reduce light-exposed surface area
- Fix: raise light, lower temperature, improve airflow
- See our light burn guide
Curling downward (clawing) → Nitrogen toxicity or overwatering
- If leaves are dark green AND clawing: nitrogen toxicity. The plant has excess nitrogen — leaves turn very dark green and curl downward. Reduce nitrogen in feed. See our nitrogen toxicity guide
- If leaves are normal green AND drooping/clawing: overwatering. Let the medium dry out
Edges curling upward (canoeing) → Heat or low humidity
- Leaf edges curl up while the center stays flat
- Usually a VPD problem — temperature too high relative to humidity
- See our VPD guide
Twisting or irregular curling → pH swings or root problems
- Leaves that twist or curl asymmetrically usually indicate root zone issues
- Check pH of input AND runoff
- Inspect roots if possible
Symptom 6: Slow or stunted growth
When did growth slow?
Sudden stop after a change → Stress response
- After transplanting: transplant shock. Normal — wait 3–7 days for recovery. See our transplant guide
- After topping or heavy training: recovery period. Normal — wait 3–5 days
- After changing nutrients or pH: check input pH and EC. Sudden changes stress roots
Gradual slowdown → Root-bound, pH creep, or environmental issue
- Root-bound: roots circling the bottom of the pot, visible through drainage holes. Transplant to a larger container
- pH creep: pH of the medium has drifted over time. Check runoff pH — if it's significantly different from input pH, flush and recalibrate
- Low light: during winter or with aging LEDs, light output decreases. Check PPFD at canopy level
- Temperature: growth slows below 65°F and above 85°F
Growth slow from the start → Bad genetics, poor seedling conditions, or contaminated medium
- Some seeds produce weak plants. If all other conditions are correct, it may be genetics
- Check for fungus gnats in the medium — larvae feed on root hairs and slow growth in seedlings. See our pest guide
Symptom 7: Discoloration other than yellow
What color?
Dark green (almost black) → Nitrogen toxicity
- Leaves are unnaturally dark green, often with a waxy sheen
- Usually accompanied by downward clawing
- Reduce nitrogen immediately
- See our nitrogen toxicity guide
Purple stems or petioles → Genetic or phosphorus deficiency
- If the plant is otherwise healthy and growing well: genetic. Many strains have naturally purple stems
- If accompanied by slow growth, dark leaves, or poor flower development: phosphorus deficiency. Check pH (P locks out below 5.5 and above 7.0)
- See our color expression guide
Red or pink stems/leaves → Phosphorus deficiency or cold stress
- Check root zone temperature — below 60°F impairs phosphorus uptake
- Check pH — phosphorus availability drops sharply below 6.0 in soil
Brown patches on buds → Bud rot (Botrytis)
- Gray-brown spots on buds, often starting inside dense colas
- Mushy texture, gray fuzz visible under magnification
- Remove affected buds immediately — cut 1 inch below the infected area
- See our bud rot guide
The 5-step diagnostic process
When you encounter a problem:
- Check pH — test input water pH, test runoff pH. If either is out of range, fix it and wait 48 hours before doing anything else
- Check watering — lift the pot. Is it too heavy (wet) or too light (dry)? Adjust watering before adding nutrients
- Check environment — temperature, humidity, light distance. Use a thermometer at canopy level, not room level
- Identify the symptom pattern — WHERE on the plant (top, bottom, everywhere), WHAT it looks like (yellowing, spots, curling, wilting), and HOW FAST it's progressing
- Make ONE change — fix the most likely cause and wait 48–72 hours. Multiple simultaneous changes make diagnosis impossible if the problem persists
For the AI-powered diagnostic tool, visit Plant Doctor. For a visual nutrient deficiency reference, see our deficiency chart.