How do I save my onion from root rot?

The 4 most likely causes

The cause of onion root rot 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. Root rot (Most likely)
   Root rot is what happens when overwatering or poor drainage goes uncorrected. The roots turn brown and slimy, the stem softens from the base up, and the plant can no longer move water to its leaves — so paradoxically it wilts even though the soil is soaking. Unpot onion now and inspect the roots; firm white roots stay, mushy brown ones get trimmed.
2. Overwatering or poor drainage (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.
3. No drainage hole or a too-dense potting mix (Likely)
   A pot without a working drainage hole — or a heavy, compacted mix that holds water like a sponge — rots roots even under a careful watering hand. Water needs to flow through the pot, not sit in it. Onion wants a free-draining mix and a pot that empties from the bottom within a minute of watering.
4. Fungus gnats (a wet-soil warning sign) (Possible)
   Small dark flies drifting up from the soil of onion are fungus gnats — their larvae live in constantly damp potting mix and nibble fine roots. The gnats themselves are a symptom: soil that stays wet enough to breed them is wet enough to rot roots. Let the top few centimetres dry out fully between waterings and the population collapses.

Why onion in particular

An onion's shallow, fibrous roots fan out from a small basal plate and need air, so a waterlogged bed or a pot that never drains drowns and rots them. Soil fungi move onto the drowned tissue — Fusarium basal rot softening the plate, pink root staining the roots, Pythium in seedbeds — and above ground the plant yellows, wilts and lifts easily because the anchoring roots have gone. Free-draining soil and restrained watering keep that root plate sound.

Normal turnover, or real trouble?

Onion roots are fine, white and short-lived, so some browning of the oldest strands as new ones push out is normal, and near harvest the whole root system dies back as the bulb cures. Mid-season is when to worry: give a growing plant a gentle tug and it should resist. One that lifts almost free, with few roots and a soft, discoloured basal plate instead of a firm creamy one, has rotted, not ripened.

How to diagnose in 60 seconds

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

• Smell the soil at the drainage hole. Sour, swampy, or sulphurous = rot is active below the surface.
• 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.
• Is the plant wilting even though the soil is wet? That paradox — thirsty-looking in soaked soil — is the root-rot signature.
• Check the pot itself: is there a working drainage hole, and does water actually run out of it when you water?

The fix — step by step

You cannot nurse a rotted onion back, so lift and remove affected plants rather than watering them. The ground is now the problem: these rots persist in the soil, so don't replant onions, garlic or leeks in that spot for several years, and where white rot is confirmed keep alliums out far longer. For the healthy rest, ease off watering and work grit into the bed, or raise it, so roots never sit wet.

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

1. Unpot and wash the roots today. Root rot doesn't pause. Slide onion out of its pot, crumble away the wet soil, and rinse the rootball gently under lukewarm water so you can see what's left.
2. Cut every rotted root off. With scissors wiped in isopropyl alcohol, cut back to firm, pale tissue — brown, slimy, or hollow roots all go, even if that means removing most of the rootball. A few clean roots beat a potful of rot.
3. Repot into fresh, dry, free-draining mix. Use a clean pot with a real drainage hole — one size smaller if you removed a lot of roots — and a chunky mix (add perlite or bark). Old soil and an unwashed pot re-infect; bin the soil, bleach-rinse the pot.
4. Water once, then hold back. Water lightly to settle the mix, then wait until the top few centimetres are fully dry before watering again. The plant has fewer roots now — it drinks far less than before. No fertiliser until new growth appears.
5. Take an insurance cutting. If the rot was extensive, root a healthy stem cutting in water now. If the parent pulls through you have a bonus plant; if it doesn't, you haven't lost onion entirely.

When this can't be saved

Most cases of onion root rot 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.

• No firm, pale roots remain at all — with zero healthy roots to rebuild from, a stem cutting is the only realistic rescue.
• The rot has climbed above the soil line into the main stem or crown, which is soft and darkening from the base up.
• The soil smells foul and sewer-like — bacterial soft rot moves faster than fungal rot and is much harder to stop.

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. Never let the pot stand in a saucer of drained water, add a third of perlite or bark to the mix at every repot, and make the finger test non-negotiable — rot is bred by watering the calendar instead of the soil. A yearly root check at repotting time catches trouble while it is still trimmable.

Common questions

Related guides

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