Diagnosis
Symptom-led decision trees: start here when something looks wrong.
Brown or burnt leaf tips on cannabis: a differential
Crispy brown tips on otherwise healthy leaves usually mean too much salt in the root zone (EC too high, or salts building up because of low runoff or drying out), not a deficiency. Check runoff or reservoir EC against input first, then watering rhythm, then heat and low humidity, and only then K or B disorders.
Brown roots in hydroponics (DWC, RDWC, NFT): a differential
Healthy hydro roots are white to cream and firm. Brown roots that are still firm and odourless are often just stained by nutrients or additives; brown, slimy, smelly roots whose outer layer slips off are root rot, usually Pythium, favoured by warm, oxygen-poor solution. Check solution temperature and dissolved oxygen first, then root texture and smell.
Cannabis bud problems: the master decision tree
Check buds for rot first: a brown, dead sugar leaf in a healthy cola, or grey fuzz inside, is Botrytis and needs the bud cut out today. Then look for white powder (powdery mildew), bananas or seeds (a hermaphrodite: a genetic defect, never normal), frass and holes (caterpillars), white or bleached tops (too much light), foxtails, and loose, airy buds (usually too little light at the bud site). Most bud problems are decided by where on the plant they appear and what the canopy climate was.
Cannabis lookalike problems: how to tell them apart
Many cannabis problems look alike in a photo. The ones growers confuse most are Ca vs Mg vs K, nitrogen toxicity vs overwatering claw, light bleaching vs Mg deficiency, spider-mite stippling vs calcium spots, heat vs light stress, Pythium vs low dissolved oxygen, Fe vs Mn, and nutrient burn vs K deficiency. Each pair separates on three things: where on the plant it started, the exact pattern, and one cheap test (a lens, a runoff EC reading, a root check, a light meter or a lifted pot).
Cannabis root problems: the master decision tree
Healthy cannabis roots are white to cream, firm and smell earthy or of nothing. Brown, slimy roots whose outer skin slides off and smell rotten are root rot (usually Pythium), driven by warm, oxygen-poor water or waterlogged media. Firm, odourless tan roots are usually stained, not sick. Check solution temperature (target 18–21 °C) and dissolved oxygen first, then texture and smell, then insects. Root problems show above ground as drooping with a wet root zone, yellowing and stunting.
Clawing leaves on cannabis: a differential
Leaves that curl down at the tips like a talon, especially if dark green and glossy, most often mean excess nitrogen and high EC, or a waterlogged, oxygen-starved root zone. Check watering and runoff EC first; distorted, crinkled clawing on new growth instead points to broad mites, calcium/boron problems or a viroid.
Drooping cannabis: overwatering vs underwatering
Lift the pot: a heavy pot with firm, swollen-looking drooping leaves is overwatered (roots short of oxygen); a light pot with limp, papery leaves is underwatered. If the medium is moist and the plant still wilts, especially one plant at a time or in hydro, suspect root rot or vascular wilt and look at the roots.
Foxtailing buds on cannabis: genetics or stress?
Foxtails (new calyx spires growing out of mature buds) are either a cultivar trait, which appears evenly through the canopy, or a stress response to heat and intense light, which shows mostly on the top colas nearest the fixture. Measure canopy leaf temperature and PPFD at the tops; evidence is mainly grower observation.
Hermaphrodite cannabis (bananas and nanners): what to do
Hermaphroditic flowers are anthers (often banana-shaped) or pollen sacs forming on a female plant. Cannabis is a dioecious species, and a well-bred female should stay female; herm-prone plants are mainly the product of bad breeding (repeated selfing, careless feminising, untested 'pollen chucker' seed). Stress such as light leaks or heat reveals that weakness rather than causing it in sound genetics. Fix the environment, remove the plant if it sheds pollen, and never keep it as a mother or breed from it.
How to describe your plant's problem so it can be diagnosed
A plant problem can be diagnosed quickly only if the description says where on the plant it started (old or new growth), the exact pattern, how fast it is spreading, and the readings: pH and EC of input and runoff, temperature, RH, light intensity or distance, medium, system, water source, stage with the day count from the flip, and anything you changed in the last week. Add a whole-plant photo and a close-up of the top and underside of an affected leaf, taken in white light.
How to diagnose any cannabis plant problem
Diagnose in a fixed order: confirm it is a real problem, note where on the plant it started (old vs new growth) and the pattern, then rule out root zone and pH, then environment and light, then pests and disease, and only then blame a nutrient. Change one thing at a time and judge the fix on new growth.
Purple or red stems and leaves on cannabis: a differential
Purple or red stems and petioles on a healthy, growing plant are usually genetics, cool nights or bright light, and need no fix. Treat it as a problem only when it comes with something else: lower-leaf yellowing (nitrogen), dark or bronzed spotting on old leaves (phosphorus), or slow growth with a cold or wet root zone. Check root-zone temperature and pH before adding phosphorus.
Purple, red, bronze or dark leaves on cannabis: the master discolouration decision tree
Most purple and red on a healthy, growing cannabis plant is anthocyanin: genetics, cool nights or strong light, and no fix is needed. Discolouration is a problem only alongside another sign: red-brown petioles with bottom-up yellowing (nitrogen), dull dark leaves with olive-bronze spots and slow growth (phosphorus uptake, often a cold or high-pH root zone), very dark glossy clawed leaves (nitrogen excess), bronze, brittle, upward-curling leaves (russet mites, or late Mg or K deficiency), or reds and oranges as leaves fade in late flower (normal senescence).
Rust-coloured spots on cannabis leaves: a differential
Rusty brown spots are most often a calcium or phosphorus uptake problem driven by pH or root-zone stress, but leaf septoria (spots with yellow halos on lower leaves, spreading in humidity) and spider-mite stippling look similar. Check pH and root zone, then turn the leaf over with a hand lens, then decide between Ca, P, Mg/Mn and disease.
Slow or stunted cannabis growth: a differential
When a plant just isn't growing, the cause is usually the root zone (overwatering, cold, small or rotting roots), too little light, or a cool room; nutrient shortages come later. Plants that stay stunted with small leaves and short internodes while neighbours thrive should be tested for hop latent viroid and checked for root aphids.
Spots, brown patches and dead edges on cannabis leaves: the master decision tree
Cannabis leaf spots and dead tissue sort into four groups: pest feeding (pale stippling or silver scars, seen with a 10–20× lens under the leaf), disease (spots with yellow halos and tiny black dots, spreading from the bottom in humidity), nutrient or root-zone disorders (Ca, P, K, Mn, B, salts, in a pattern tied to leaf age) and physical damage (spray droplets, fans, heat, light). Flip the leaf first, then check runoff pH and EC, and only then pick a nutrient.
Stretchy, leggy cannabis: a differential
Long internodes and thin stems mean the plant is reaching for light: PPFD too low, light too far away, a low red-to-far-red ratio (shade or crowding), or warm days relative to nights. Some stretch in the first 2–3 weeks after the switch to 12/12 is normal and cultivar dependent. Measure PPFD and day/night temperatures before blaming genetics.
Taco or canoeing leaves on cannabis: a differential
Leaflets folding upward along the midrib (taco, canoe) on the top of the canopy are almost always heat or light stress, often with low humidity (high VPD). Measure leaf temperature, PPFD and RH at the canopy before anything else; if the whole plant is affected, check the root zone, then calcium and magnesium.
White or grey coating on cannabis: the master decision tree
A white or grey coating on cannabis is, in order of how often it matters: powdery mildew (flour-like patches that smear and return), spray or hard-water residue (follows the drip lines), trichomes (on buds and sugar leaves only), thrips silvering, spider-mite webbing, or grey Botrytis fuzz on buds and stems. On the medium, white fuzz is usually harmless mycelium and a hard crust is salt. A rub test and a 20–60x lens settle most cases in two minutes.
White powder or white spots on cannabis leaves: a differential
Flour-like circular white patches that smear off and come back are powdery mildew until proven otherwise. Rule out spray or hard-water residue (on the surfaces you sprayed, dries in drip lines), trichomes (on sugar leaves and buds, glassy under a lens), silvery thrips scarring and mite stippling. Confirm with a 20-60x lens before treating.
Why are my cannabis leaves curling? The master curling and clawing decision tree
Which way a cannabis leaf curls is the first clue. Edges folding up (taco, canoe) at the top of the canopy is heat, light or high VPD. Tips hooking down like a talon on dark, glossy leaves is excess nitrogen with high EC; whole leaves curling down and puffy on a heavy pot is overwatering. Twisted, crinkled, stunted new growth is broad or russet mites, calcium or boron delivery, a viroid, or growth-regulator herbicide drift. Leaves angled up and flat at lights-on ('praying') is healthy.
Why is my cannabis drooping or wilting? The master decision tree
Drooping cannabis is diagnosed by one question: is the root zone wet or dry? A light, dry pot with limp leaves is underwatering; a heavy, wet pot with firm, curled-down leaves is overwatering. Wilting while the roots sit in water means the roots are failing: Pythium root rot, low dissolved oxygen, a hot reservoir, Fusarium wilt, or (in DWC and especially HPA) a failed pump, which is an emergency measured in minutes to hours. Transplant shock, unrooted clones, heat, stem damage and salt stress complete the list.
Why is my cannabis turning yellow? The master yellowing decision tree
Cannabis yellowing is diagnosed in four forks: where it started (old leaves = mobile N, Mg, K; new leaves = immobile Fe, S, Mn, Zn, Ca, B), the pattern (uniform, interveinal, margins, spots, bleached tops), the root-zone readings (pH, EC, moisture, root colour) and the stage. Most yellowing in practice is roots that cannot absorb (overwatering, cold, rot, pH out of range) rather than a missing nutrient; late-flower fade and top-only bleaching under strong light are not deficiencies.
Yellow new growth on cannabis: a differential
Yellowing at the top of the plant points to an immobile nutrient (Fe, S, Mn, Zn, Cu, B, Ca) that the roots are failing to deliver, and the usual cause is high root-zone pH or a wet, cold or damaged root zone rather than a lack of fertiliser. Rule out light bleaching, broad/russet mites and viroid stunting before adding micronutrients.
Yellowing lower leaves on cannabis: a differential
Lower-leaf yellowing is usually nitrogen shortage or, far more often, roots that cannot take nitrogen up (overwatering, cold or rotting roots, pH out of range). Uniform yellow climbing from the bottom suggests N; green veins with yellow between them suggests Mg; yellow-then-brown margins suggest K. In late flower, gradual fade is normal.
