“Yellow leaves” is the garden’s check-engine light: it tells you something’s wrong and nothing about what. The pattern of the yellowing, though, narrows it fast. Four questions get you from panic to probable cause — and they matter, because the two most common causes (too much water, too little water) get opposite treatments.

The four questions

1. Which leaves — old or new? Plants ration nutrients. When a mobile nutrient like nitrogen runs short, the plant strips it from its oldest, lowest leaves to feed new growth — so old leaves yellowing evenly, bottom-up, while the top stays green, points to general hunger. When an immobile nutrient like iron runs short, it’s the newest leaves that come out yellow.

2. Whole leaf, or yellow between green veins? Even, all-over yellowing suggests nitrogen shortage, water trouble or light trouble. Interveinal yellowing — leaf gone yellow while the veins stay sharply green, on new growth — is the classic iron-deficiency signature. In Australia that shows up most on acid-loving plants (citrus, gardenias, camellias, many natives) growing in alkaline or limy soils, common in parts of Adelaide, Perth and coastal sand country: the iron is present but the pH locks it away.

3. What’s the watering been like — honestly? Overwatering and underwatering both yellow leaves, and overwatering is the more common killer, especially in pots. Feel the soil ten centimetres down, and look at the leaf itself. Limp, soft yellow leaves + soil that never dries + maybe a sour smell in pots = drowning roots. Dry, crisping yellow leaves + light pot + soil pulling from the pot edge = drought — and if water now runs straight through or beads off the surface, you’ve got hydrophobic soil, which mimics “I watered it, honest” drought. When you can’t tell, the finger test outranks the schedule every time — full method in the watering guide.

4. What changed recently? Just planted, repotted or moved? Expect some transient yellowing — transplant shock resolves with steady care. Moved a shade plant into sun? Bleached yellow-white patches are sunburn. Root-bound pot? Yellowing plus roots circling the drainage holes says the container’s done its dash. And scattered yellow stippling with speckles or stickiness isn’t a nutrition issue at all — that’s a pest pattern, so check the damage guide.

Matching fix to cause

• Nitrogen hunger: feed with a balanced fertiliser in the growing season, per pack directions. One caution unique to Australian gardens: many natives (grevilleas and their relatives especially) are sensitive to phosphorus, so feed natives only with a native-formulated fertiliser.
• Iron lock-out: treating the symptom (iron chelate per pack directions) greens new growth; treating the cause means addressing pH or growing acid-lovers in pots with the right mix. Don’t guess pH — an inexpensive test kit settles it.
• Overwatering: stop watering by calendar, let the profile dry down, improve drainage, and in pots, check the pot actually drains. Yellow leaves already gone soft won’t re-green; success is the next leaves.
• Underwatering / dry patch: deep soak, mulch, and fix water-repellent soil properly (here’s how).
• Light or shock: relocate gradually, or wait it out with steady water.

When yellow is fine

Every plant sheds its oldest leaves — a few yellow ones low on an otherwise vigorous plant is housekeeping, not disease. Deciduous plants yellow wholesale in autumn by design. Citrus often drops some leaves after winter without drama. Treat yellowing as a signal worth two minutes of diagnosis, not two hundred dollars of products — most cases resolve with water discipline, one correct feed, or patience.