Lesson 1. Yellow leaves, rot and the logic of diagnosis

Learn to tell whether yellowing comes from overwatering, drought, lack of light or natural aging, and make a diagnosis from all the signs together.

Differential diagnosis: yellow leaves

One symptom — different causes

Yellow leaves are the most common warning sign. But the causes can be completely different: look at the accompanying signs.

Table: cause of yellowing, accompanying signs and first action. Rows: overwatering, drought, lack of light, nutrient deficiency, natural aging. — Overwatering and lack of light cause similar symptoms — check the soil and the distance to the window.

Lesson notes

One symptom — many causes

“My leaves are turning yellow” is the most common complaint — and the most useless without details. Yellowing is like a fever in a person: a symptom of a dozen different problems. To make a diagnosis, you look at WHICH leaves, HOW they yellow, and what's going on around them. The main yellowing scenarios: — Overwatering (most often). The lower leaves turn yellow; the leaf is soft and limp; the soil is constantly wet; there may be dark spots and a bad smell from the soil. This is the most common case. — Drought. The leaf turns yellow or brown but is dry and crispy; the soil is bone-dry and has pulled away from the sides of the pot; the plant droops. — Lack of light. Leaves farther from the window turn pale and yellow; the plant is leggy. — Natural aging. One or two of the lowest, oldest leaves turn yellow and drop, while the plant as a whole is healthy and growing. This is normal; there's nothing to treat. The key to a diagnosis isn't the color itself but the combination: which leaves + soil moisture + overall look + what changed recently (repotting, a move, a change in watering).

A distinctive yellowing pattern

There's a characteristic pattern worth recognizing: the leaf turns yellow, but its veins stay green (interveinal chlorosis). This isn't “just yellowing from old age” — it's usually a nutrient deficiency, most often iron or magnesium, or root or watering problems that keep the nutrients from being absorbed. Common causes of chlorosis in a pot: — Exhausted potting mix; the plant hasn't been repotted or fed in a long time. — Water that's too hard and alkaline, or the soil becoming alkaline — iron can no longer be absorbed. — Overwatering and sick roots: even if there's iron in the soil, damaged roots can't take it up. What to do: check the watering and the roots; if the mix is exhausted, feed gently with a balanced all-purpose fertilizer (following the label, not “by eye” — overfertilizing is harmful too); if needed, repot into fresh potting mix. The next photo shows classic interveinal chlorosis: a yellow leaf blade with green veins.