Problem Solver

Aphids (greenfly and blackfly)

Soft, sap-sucking insects that cluster on new shoots and under leaves, leaving sticky honeydew. Squash early colonies and bring in natural enemies.

Caterpillars

Moth larvae that chew leaves and can tunnel into flowers, causing rot. Hand-pick, screen the tent and remove damaged flowers fast.

Fungus gnats (sciarid flies)

Small black flies that breed in constantly wet medium. Adults are a nuisance; the larvae can damage seedling and young roots. The fix starts with watering.

Root aphids

Aphids that feed on roots, so they're easy to miss and often mistaken for a deficiency or watering problem. You have to look at the roots.

Spider mite

Tiny sap-sucking mites that speckle leaves and spin fine webs. They thrive in hot, dry, still air and build up fast, so act at the first speckles.

Thrips

Slim, fast insects that rasp leaves, leaving silvery streaks and black dots. They pupate in the growing medium, so treat leaves and medium together.

Whitefly

Tiny white moth-like insects that fly up in a cloud when the plant is brushed. Use traps to monitor and Encarsia wasps before numbers get heavy.

Damping off

Soil-borne fungi that make seedlings collapse at the base. Infected seedlings can't be saved, so the focus is clean kit, fresh medium and careful watering.

Grey mould (botrytis) and flower rot

A fungus that rots dense flowers from the inside in humid, still air. Cut it out well past the damage and drop the humidity straight away.

Powdery mildew

A fungus that grows as white, flour-like patches on leaves. It loves stagnant, humid air and soft growth, so fix airflow early and catch it while patches are small.

Root rot (pythium)

Roots that turn brown, slimy and smelly, usually from too much water, too little oxygen or warm solution. Fix the root zone first, then help new roots grow.

Calcium deficiency

Rusty brown spots and twisted, crinkled new growth. Common in coco and with soft or RO water, and often an uptake problem caused by pH or humidity swings.

Iron deficiency

The newest leaves go pale yellow with green veins. It's almost always pH too high, not a lack of iron.

Magnesium deficiency

Yellowing between the veins of older leaves while the veins stay green. Very common in coco and with soft or RO water.

Nitrogen deficiency

Lower leaves fade evenly pale, then yellow and drop, while new growth stays green. A little lower-leaf fade late in flower is normal.

Nutrient burn (overfeeding)

Yellow, then brown, crispy leaf tips that spread inwards. The feed is too strong or salts are building up. Cut back; don't add more.

Nutrient lockout and salt build-up

Several 'deficiencies' at once that don't improve when you feed more. Usually pH out of range or salts building up in the medium.

pH drift

Feed or run-off pH creeping outside 5.5–6.5. Slight drift inside the window is fine; outside it, uptake suffers and lockout follows.

Phosphorus deficiency

Dark, dull or bluish leaves, purple-red stems and slow flower development, often worse when roots are cold or pH is high.

Potassium deficiency

Edges and tips of older leaves go yellow, then brown and crispy, with weak stems. Check pH and EC before reaching for more feed.

Cold stress

Cold roots (below about 15 °C) slow everything down and can cause purple stems and phosphorus problems. Common in UK lofts and garages in winter.

Heat stress

Leaves fold up like a taco and droop in the afternoon. Keep the canopy in 20–27 °C and act fast above 28–30 °C.

High humidity (damp air)

Humidity above your week's chart range. It slows drying, invites powdery mildew and grey mould, and matters most in late flower.

Leaf curl and claw

Which way a leaf curls points to the cause: tips hooking down like a claw usually means too much nitrogen; upward taco means heat or light; downward and heavy means over-watering.

Light burn (light stress)

Top leaves nearest the light bleach pale or white, or turn yellow while lower leaves stay green. Raise or dim the light.

Light leaks in the dark period

Light getting into the tent during the 12-hour dark period in flower can stress the plant. Find and seal leaks.

Low humidity (dry air)

Humidity below your week's chart range: leaves dry at the edges, the plant drinks fast and spider mites get a head start.

Over-watering

The whole plant droops with fat, heavy, curled-down leaves and the medium stays wet for days. Let it dry back and water by weight.

Slow growth

The plant just isn't moving. Work through the basics: pH, EC, watering, climate and light, then the roots.

Stretching (leggy growth)

Tall, thin stems with long gaps between nodes. Usually not enough light or too much heat; some stretch in early flower is normal.

Under-watering

Thin, limp, papery leaves and a light container, with dry medium pulling away from the sides. Water slowly in stages.

Wilting and drooping

A drooping plant has a handful of usual causes. Start with the lift test: heavy means too wet, light means too dry, then check heat and roots.

Yellow leaves

Where the yellow is tells you the cause. Old leaves, new leaves, between the veins, at the tips or near the light: start here and follow the link.