Root rot is a disease of plant roots in which water mould and fungal organisms break down root tissue that has been starved of oxygen by waterlogged mix. This guide explains what root rot looks like, how to grade roots by colour and texture, how to decide if the plant is recoverable and how to treat it.
What root rot looks like
Root rot looks like brown to black, soft roots that smell sour and fall apart when handled. Above the mix, the plant shows wilting in wet soil, yellowing lower leaves and sometimes a darkened stem base. Only the roots confirm it, so the plant has to come out of its pot.
The organisms behind it are mostly Pythium and Phytophthora, water moulds that swim through saturated mix, plus Fusarium and Rhizoctonia fungi. All four are present in ordinary potting mix at low levels. They take hold only once roots are weakened by a lack of air.
Warmth speeds them up. A pot of wet mix sitting on a sunlit floor in a Dubai apartment in August runs warmer at the root zone than the room air, and Pythium grows faster in warm, wet conditions than in cool ones.
To inspect, lay the pot flat, grip the plant just above the mix and ease the root ball free. Shake or rinse off enough old mix to see the roots clearly.
Grading roots by colour and texture
Grade each root on a four step scale that combines colour with a pinch test. Colour alone misleads, because healthy roots of palms, snake plants and plants grown in bark mixes are naturally tan. Texture under the fingers is the deciding factor.
The four grades:
- Grade 1 roots are white to cream, firm, with fine hairs near the tips.
- Grade 2 roots are tan to light brown yet firm and springy when squeezed.
- Grade 3 roots are brown and soft, and the outer layer slides off when pinched, leaving a thin white thread behind.
- Grade 4 roots are black, mushy and smell rotten, and they collapse to paste between the fingers.
The pinch test in grade 3 is the most reliable single sign. Pull gently along a suspect root between finger and thumb. If the brown outer sheath strips away and leaves a string of inner core, that root is dead even if it still looks whole.
Grades 1 and 2 are working roots. Grades 3 and 4 have to go.
Deciding if the plant is recoverable
A plant is recoverable if at least a third of its root mass is grade 1 or 2 and the stem is firm where it meets the roots. Below that, the remaining roots can't supply the leaves and the plant is better restarted from cuttings.
Estimate the share by eye after rinsing. A plant with half its roots firm and pale will recover. A plant with one or two firm roots in a mass of brown paste won't, however healthy the leaves still look.
The stem base is the second check. Press it with a thumbnail at the point where roots join. Firm, green or pale tissue means the plant's plumbing is intact. Soft, dark tissue that dents easily means the rot has moved into the stem, and no amount of root trimming will stop it.
When the answer is no, look above the damage for firm stem sections with healthy leaves. Those can be cut and rooted as new plants.
How to treat root rot
Treat root rot by removing every grade 3 and 4 root, cleaning what remains and repotting into fresh mix. Clean tools, a pot sized to the remaining roots and dry mix to start are the three things that decide whether the rot comes back.
- Wash all remaining old mix off the roots under lukewarm running water.
- Wipe scissors with rubbing alcohol before the first cut and between plants.
- Cut each grade 3 and 4 root back to firm, pale tissue.
- Remove leaves in proportion to lost roots, so a plant that lost half its roots loses about half its leaves.
- Let the cut roots air dry on paper for one to two hours.
- Repot into fresh, slightly damp mix in a clean pot only a little wider than the root ball.
- Hold off watering for three to five days, then water lightly.
Throw the old mix away. Scrub the old pot with hot soapy water before reusing it, since spores survive on the inside wall.
Step 4 feels harsh. It matches the leaf area to what the reduced root system can supply, so the plant isn't losing water through leaves it can't support.
Repotting to prevent a repeat
Preventing a repeat comes down to the pot and the mix draining freely. Use a pot with at least one open drainage hole and a mix that includes coarse material, such as perlite or bark, so water passes through and air follows it into the root zone.
Oversized pots are a cause of root rot in their own right, because a large volume of mix around a small root system stays wet for weeks. Size up by no more than 2 to 3 cm in diameter.
A fresh, well draining mix and a pot with proper holes are the whole foundation of the fix. The potting soil and drainage range covers both, which matters here because reusing the old mix or a sealed decorative cover brings the rot straight back.
FAQ
With a rotting plant on the bench, most owners want two answers before picking up the scissors: what the damage actually looks like, and whether it's worth the effort of saving. The root grades above settle both, and these answers summarise them.
What does root rot look like?
Rotted roots are brown to black, soft and slimy, and they smell sour. When pinched, the outer layer slides off and leaves a thin white thread. Healthy roots are white, cream or tan and stay firm and springy when squeezed between finger and thumb.
Can you save a plant with root rot?
Yes, if at least a third of the roots are still firm and the stem base is solid. Cut away all soft, dark roots, trim leaves to match, and repot into fresh mix. If the stem base is soft, take cuttings from healthy growth higher up instead.