Why is my begonia turning yellow?

The 4 most likely causes

The cause of begonia yellow leavesusually narrows to one of the items below, ranked by how often we see each in Growli's diagnostic chats. Work down the list — most readers find their answer in the top two.

1. Overwatering or poor drainage (Most likely)
   In most homes overwatering is more often a drainage problem than a frequency problem. Begonia needs a pot with a drainage hole, a chunky free-draining mix, and a watering rhythm of when the top 2-3cm of soil is dry, every 5-7 days. Soggy soil drowns the roots and the first symptom you see above ground is yellowing or wilting foliage.
2. Wrong light level (Likely)
   Begonia tolerates lower light than most, but tolerating is not thriving. In dim conditions it slows down, drops older leaves, and becomes more vulnerable to overwatering because the soil never dries. Give it bright indirect light.
3. Nutrient deficiency (nitrogen or iron) (Possible)
   If begonia has not been repotted or fed in a year or more, the older leaves can yellow uniformly while the newest growth stays green — a classic sign of nitrogen depletion. Yellow leaves with green veins on new growth point to iron or manganese deficiency. A balanced liquid feed during the growing season usually resolves both.
4. Underwatering or letting it dry too long (Possible)
   Underwatering looks similar to overwatering at first — both produce limp, dull leaves — but the soil tells the truth. If the soil is dust-dry several centimetres down, water deeply. Begonia prefers when the top 2-3cm of soil is dry, every 5-7 days.

Why begonia in particular

Rex begonias grow from a creeping surface rhizome and cycle their foliage hard: many cultivars enter a real winter dormancy where the oldest leaves yellow and shed while the rhizome rests, so seasonal yellowing from autumn onward is often normal. Alarming yellowing is different — it is widespread, comes with a soft, darkening rhizome, and follows soggy compost. Because these thin-leaved understory plants resent wet feet, yellowing paired with a mushy rhizome base usually points to rot rather than a nutrient issue.

Normal turnover, or real trouble?

As a Rex begonia creeps forward, its rhizome constantly retires the oldest leaf at the trailing end while pushing vivid new foliage at the tip, so one dull, yellowing leaf far from the growing point is routine turnover. The colour to watch is at the leading edge: when the newest, youngest leaves near the advancing tip pale or yellow first, the plant is telling you the roots feeding that growth are failing.

How to diagnose in 60 seconds

Run these quick checks before you change anything — the right fix depends on what you find.

• Begonia wants water when the top 2-3cm of soil is dry, every 5-7 days. Push a finger into the mix and hold it against that: still damp well before it is due, alongside the symptom, is the overwatering signature.
• Are the yellowing leaves the oldest ones at the base, or the newest at the tips? Old-leaf yellowing is usually water or nitrogen; new-leaf yellowing is usually iron or root damage.
• Look at the back of a yellow leaf with strong light — any speckling, webbing, or sticky residue? That points to pests, not water.
• Tip begonia out of its pot and look at the roots: firm and pale is healthy; brown, soft or sour-smelling is rot, which is the real cause behind most of these symptoms.

The fix — step by step

Before anything else, feel the compost. If it is wet, stop top-watering entirely and switch to bottom-watering, standing the pot in shallow water for twenty minutes so moisture wicks up while the surface-set rhizome and its crown stay dry. Let the top centimetre dry between drinks. Resist the urge to mist or feed a yellowing Rex, as wet leaves and fertiliser salts only add stress while the roots recover.

This is the recovery sequence Growli walks users through for begonia with yellow leaves. Work through the steps in order; skipping ahead is the most common reason a plant fails to bounce back.

1. Stop watering and check the roots. Don't add more water yet. Unpot begonia and look at the rootball — firm white roots mean you have time; brown mushy roots mean you need to act today.
2. Trim damaged roots and yellow leaves. Cut off any soft brown roots with clean scissors. Remove fully yellow leaves at the base — they won't green back up. Leave half-yellow leaves alone for now; the plant is still pulling nutrients out of them.
3. Repot into fresh dry mix. Repot into fresh, well-draining mix in a pot one size up. For begonia, pick a spot with bright indirect light.
4. Reset the watering rhythm. Water deeply once, then wait. For begonia, that means when the top 2-3cm of soil is dry, every 5-7 days. Use a finger or a moisture meter — never a calendar.
5. Resume feeding only after recovery. A stressed plant cannot use fertiliser and the salts will worsen the damage. Wait for at least one round of healthy new growth (4-6 weeks) before resuming a half-strength liquid feed during the growing season.

When this can't be saved

Most cases of begonia yellow leaves are recoverable, but a few red flags point to a plant that has gone past the point of return. If you spot any of these, consider propagating a clean cutting and starting over.

• Every leaf has yellowed simultaneously and the stem feels soft at the base — root rot has likely consumed the plant.
• New leaves emerge yellow and crispy and never green up — the growing tip is damaged.
• The soil smells sour or sulphurous even after a thorough drying period.

Prevention

For begonia, the single biggest preventative is matching its native rhythm: when the top 2-3cm of soil is dry, every 5-7 days, bright indirect light, and a free-draining pot with a working drainage hole. Group begonia with plants of similar needs so you can water them as a batch rather than guessing per-pot. Set a calendar reminder to feed during the growing season but never feed a sick or freshly-repotted plant — wait for healthy new growth first.

Common questions

Related guides

• PictureThis alternatives — 7 plant ID apps compared
• Is begonia toxic to cats and dogs? — toxic to pets, per the ASPCA
• How often to water begonia — the watering brief
• When to repot begonia — pot size and timing
• Yellow leaves on plants — the full diagnostic guide across species
• What's wrong with my plant? — Growli's flagship diagnosis index
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