Root rot kills more houseplants than pests do, and it does its damage out of sight. By the time the leaves look wrong, the root system has often been failing for weeks. It is also one of the few plant problems where acting within a few days changes the outcome, so it is worth knowing how to recognise it and what to do next.
What root rot is
Root rot is not a single disease. It is the collapse of a root system, and it usually takes two things happening together.
The first is a lack of air. Roots are living tissue and they need oxygen to function. In a pot where water fills every gap between the particles, that oxygen runs out within a day or two. Roots that cannot breathe begin to die, and dead root tissue cannot take up water.
The second is opportunistic fungi. Organisms such as Pythium, Phytophthora and Fusarium are present in most potting mixes at harmless levels. They multiply in saturated, oxygen-poor conditions and feed on the weakened roots. This is why the problem accelerates: the wetter the pot stays, the faster the roots decline, and the fewer working roots remain to dry the pot out.
That feedback loop explains the single most confusing symptom of root rot, which is a plant wilting in soil that is visibly wet.
Why it happens
Almost every case traces back to one of these, and often several at once.
- Watering on a schedule rather than on need. A plant that gets water every Sunday regardless of conditions will be overwatered in winter, when growth slows and evaporation drops. Our guide on how often to water houseplants covers the checks that replace a fixed schedule.
- A pot with no drainage hole. Decorative pots without drainage hold a reservoir at the bottom that never leaves. The roots nearest that reservoir go first.
- A dense, peat-heavy mix. Fine mixes hold water tightly and release air slowly. Coarser material such as bark or perlite creates the gaps that let roots breathe, which is the subject of our guide to choosing the right potting mix.
- A pot much larger than the root ball. The plant cannot use the water held in all that surrounding soil, so it stays wet for far longer than the roots can tolerate.
- A saucer or cachepot left full. The pot may drain correctly and then sit in what it drained.
- Cold and low light. The same volume of water in a cool, dim room in January stays in the pot several times longer than it would in July.
The seven warning signs
No single sign confirms root rot on its own. Two or three together, and the picture becomes fairly clear.
- Wilting while the soil is wet. This is the signature symptom and the one worth learning. A thirsty plant wilts in dry soil. A plant wilting in wet soil has roots that can no longer move water upward. If you are unsure which you are looking at, our comparison of overwatering and underwatering sets the two side by side.
- Yellowing leaves, usually starting low. Older leaves at the base go first, often several at once rather than one at a time. Yellowing has many possible causes, and our guide to why plant leaves turn yellow works through nine of them.
- Brown patches with soft edges. Root rot damage tends to be soft and dark rather than dry and crisp. Dry, brittle brown edges point somewhere else, usually toward air that is too dry or a pot that dried out completely.
- Mushy black or brown roots. The definitive sign, and the reason the confirmation step below matters.
- A sour or swampy smell from the pot. Healthy potting mix smells of earth. Anaerobic decay smells noticeably unpleasant, and you can often detect it before anything else looks wrong.
- Stunted growth. No new leaves for weeks during the growing season, or new leaves that emerge small and never expand properly.
- A soft, darkened stem base. Press gently where the stem meets the soil. It should be firm. If it gives under light pressure, the rot has reached the crown, and that is the most serious version of this problem.
How to confirm it: look at the roots
Everything above is circumstantial. The roots settle it, and checking them takes about five minutes.
Water the plant lightly an hour beforehand if the mix is bone dry, which makes the root ball hold together. Support the base of the stem with one hand, tip the pot, and slide the plant out. Loosen the outer soil with your fingers until you can see the roots clearly. For a badly affected plant, rinse the root ball under lukewarm water so you can see what you are dealing with.
Healthy roots are firm and pale, usually white, cream or light tan depending on the species. They resist a gentle tug and they snap rather than smear when broken.
Rotten roots are brown or black, soft, and collapse between your fingers. A classic test: pull gently on a suspect root and the outer sheath slides off, leaving a thin thread of inner tissue behind. That root is gone.
Note roughly what proportion is still firm, because that number decides what happens next.
Treating it, step by step
1. Take the plant out and bare the roots. Remove as much of the old mix as you reasonably can. That mix is saturated and carries the fungal load, so keeping it works against you.
2. Rinse the root ball under lukewarm running water until you can see individual roots.
3. Cut away every soft root. Use scissors or secateurs wiped with alcohol, and cut back into firm tissue. Leaving a partly rotten root behind leaves the process running. Wipe the blades again between cuts on a badly affected plant.
4. Judge what is left. If a third or more of the root system is firm and the stem base is solid, the odds are reasonable. If almost nothing firm remains, read the recovery section below before spending more effort.
5. Reduce the foliage to match. This step gets skipped and it matters. A plant that has lost half its roots cannot support all of its leaves, and every leaf is losing water the roots can no longer replace. Removing a third to a half of the top growth on a badly affected plant gives the remaining roots a workload they can meet.
6. Repot into fresh, dry mix. Choose an open mix with bark or perlite, and a pot only slightly larger than the remaining root ball, with a drainage hole. A plant that has lost half its roots needs a smaller pot than it had before, not a bigger one. Our guide on when and how to repot a houseplant covers the mechanics.
7. Water once, lightly, then stop. Enough to settle the mix around the roots, no more. The instinct to help by watering is the one to resist here.
8. Bright indirect light, no fertiliser. Damaged roots cannot take up nutrients, and fertiliser salts in the mix draw water back out of them. Wait four to six weeks, until new growth appears, before feeding.
9. Keep it warm and leave it alone. Root regrowth is slow at low temperatures. Resist repotting again, moving it repeatedly or testing the roots every few days.
Can it recover? An honest answer
Sometimes yes, sometimes no, and the difference is usually visible at the moment you have the plant out of its pot.
Good odds. A third or more of the roots are firm, the stem base is solid, and you caught it while the plant was still holding its leaves. Plants in this state recover often. Expect three to six weeks before new growth, and expect the plant to look unimpressive throughout.
Uncertain. Only a few firm roots remain but the stem is still solid all the way down. Worth attempting, with the foliage cut back hard. Some plants come back from a surprisingly small amount of root, above all the fast-rooting species such as pothos, philodendron and tradescantia.
Poor odds. The stem is soft at soil level, or soft above it. Once rot reaches the crown, there is no root system left to rebuild from, and no treatment reverses collapsed stem tissue. A plant in this state will usually decline over the following two weeks whatever you do.
When the crown has gone, the useful move is to stop treating the plant and start propagating from it. Look for a firm section of stem well above the damage, take a cutting with two or three nodes, and root it in water or fresh mix. This is the part most advice leaves out: for many common houseplants a cutting taken today will outgrow a barely-surviving parent within a few months, and it carries none of the fungal load. Choosing that route early is not giving up, it is the faster path back to a healthy plant.
Preventing a second round
Root rot recurs when the conditions that caused it stay in place, so the fix is the watering method rather than the plant.
Check before watering rather than watering on a fixed day. Put a finger into the mix to the second knuckle, or lift the pot and learn what dry and heavy feel like. Empty saucers and cachepots twenty minutes after watering. Match pot size to root ball. Use an open mix for anything prone to rot, including most aroids and succulents. Water less in winter, and reduce it noticeably in a cool room.
Our guide to the five signs of overwatering covers the earlier warning stage, before roots start dying, which is where this problem is easiest to stop.
Not sure whether it is root rot?
This is a diagnosis people usually make in a hurry, worried about a plant that looked fine a fortnight ago. Wilting, yellowing and leaf drop each have several possible causes, and treating the wrong one costs you the days that matter.
A photo in the Gardenix mobile app can confirm the likely cause and give you a recovery plan for it. The scan works from a single photo: it names what it found, shows a confidence score so you know how much to trust the result, and matches the plan to how severe the problem looks rather than handing you a general article.
Recovery from root rot is also slow enough to be hard to judge, which is where scanning the same plant every week or two helps. Each scan produces a health score from 0 to 100, and the results build into a timeline you can step through, so you can see whether a plant is stabilising before the first new leaf opens. The plant chat inside the app keeps that plant's history too, so you can ask why it still looks poor three weeks in and the answer takes its earlier scans into account instead of starting from nothing.
Gardenix is free to download for iOS and Android, and the first 3 scans are free forever.