Why is my onion turning yellow?

The 4 most likely causes

The cause of onion yellow leaves usually 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. Onion needs a pot with a drainage hole, a chunky free-draining mix, and a watering rhythm of about an inch of water per week until the bulbs swell. Soggy soil drowns the roots and the first symptom you see above ground is yellowing or wilting foliage.
2. Wrong light level (Likely)
   Onion 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 6+ hours of sun.
3. Nutrient deficiency (nitrogen or iron) (Possible)
   If onion 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. Onion prefers about an inch of water per week until the bulbs swell.

Why onion in particular

An onion pours its leaves into the bulb as it ripens, so yellowing and dieback of the tops late in the season is the crop finishing on cue, not a problem — it is the plant sealing off the bulb for storage. Yellowing earlier, before any natural fall-over, is the signal to look closer: waterlogged or nitrogen-poor soil, downy mildew greying the older leaves first, or maggots and basal rot working at the bulb below the soil.

Normal turnover, or real trouble?

Ripening yellow spreads evenly across the whole bed at once and starts on the outermost, oldest leaves as bulbs fatten, a tidy, uniform fade. The versions to investigate look patchy or back-to-front. A scatter of yellow plants dotted among green neighbours points to trouble at those individual bulbs below ground, and yellowing that begins in the youngest central leaves rather than the old outer ones means the growing heart is failing, not the crop finishing.

How to diagnose in 60 seconds

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

• Onion wants water about an inch of water per week until the bulbs swell. 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 onion 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

The fastest diagnosis is to sacrifice one yellow plant and read its base. Pink-stained roots, a browning basal plate, or small white maggots mean the trouble is below ground and the bed needs rotating off onions, not watering. If the base is sound but the soil is sodden, ease off water and improve drainage; if it is sound but the leaves are evenly pale early on, a light nitrogen feed greens them up.

This is the recovery sequence Growli walks users through for onion 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 onion 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 onion, pick a spot with 6+ hours of sun.
4. Reset the watering rhythm. Water deeply once, then wait. For onion, that means about an inch of water per week until the bulbs swell. 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 onion 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 onion, the single biggest preventative is matching its native rhythm: about an inch of water per week until the bulbs swell, 6+ hours of sun, and a free-draining pot with a working drainage hole. Group onion 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

• What's wrong with my plant? — the diagnosis apps compared
• Yellow leaves on plants — the full diagnostic guide across species
• What's wrong with my plant? — Growli's flagship diagnosis index
• Onion — curling leaves
• Onion — drooping
• Onion — brown spots
• Monstera — yellow leaves