Root rot: what it looks like and how to save your plant

What does root rot look like?

Root rot looks like a thirsty plant in wet soil. The leaves droop, the lower ones turn yellow, and the roots go brown, soft, and smelly.

Signs of root rot above the soil

• The plant wilts even though the soil is wet.
• The lower leaves droop or turn yellow first. Then the damage moves up the plant.
• New growth slows down or stops.
• In bad cases, a dark brown or black ring forms at the base of the stem, right at the soil line.

Root rot makes a plant wilt and brown, even when the soil is damp. Image: Scot Nelson, public domain

Root rot looks like thirst, so people add water and make it worse. Before you water a droopy plant, push a finger about 2 inches into the soil. If it is damp and the plant still droops, suspect root rot. If the roots turn out fine, our guide to why indoor herbs die covers the other usual causes.

White fuzz on top of the soil is usually harmless mold, not root rot. It does mean the soil stays too wet. Here is how to clear white mold on houseplant soil.

How to check for root rot

The only sure way to check for root rot is to look at the roots. Tip the pot on its side and slide the plant out. Shake or rinse off the soil so you can see them.

Now pinch a brown root and pull gently. If the outer skin slides off and leaves a thin thread behind, that root is rotten.^[1]

Rot often smells sour, but early rot can have no smell, so do not trust your nose alone.

Dark potting mix can stain healthy roots tan or brown. A healthy root is still firm, and still white inside. So judge by feel, not by color alone.

Healthy roots vs root rot

Check	Healthy roots	Root rot
Color	White, cream, or light tan	Brown or black
Feel	Firm and springy	Soft, mushy, or slimy
Gentle tug	Holds together	Outer skin slides off and leaves a thread
Fine hairs	Lots of thin side roots	Few or none
Smell	Like fresh potting mix	Sour or rotten (not always)
Stem base	Firm	Dark and soft, sometimes black

Rotten roots next to healthy ones. Healthy orchid roots are green, where herb roots are white, but rot looks the same on both: brown, soft, and shriveled. Image: Scot Nelson, public domain

Can a plant recover from root rot?

Yes, a plant can recover from root rot if it still has some firm, white roots and a firm stem. Rotten roots do not come back to life. The plant grows new roots from the healthy ones you leave.

Worth saving. Most roots are firm, and only some are soft. Cut out the soft ones and repot, using the steps below.

Take cuttings. Most roots are mush, but some stems are still green and firm. Grow new plants from those stems.

Let it go. The stem is soft and black at the soil line. No firm roots or healthy stems are left. Bag it and throw it out.

How to fix root rot, step by step

To fix root rot, cut off every soft root and repot the plant in fresh mix and a clean pot. Here is how to treat root rot, one step at a time.

1. Slide the plant out of its pot. Work over newspaper to catch the mess.
2. Rinse the roots under lukewarm running water until you can see all of them.
3. Cut off every soft, brown, or black root with clean scissors. Cut back to firm, white root. Wipe the blades with rubbing alcohol between cuts.
4. Put the old soil in the trash. Do not reuse it or spread it on other plants.
5. Pick a clean pot with a drainage hole, sized for the roots you have left. That is often smaller than before. A new pot is easiest, since plastic pots are hard to clean. To reuse a clay or ceramic pot, scrub it and soak it in 1 part bleach to 9 parts water. Then rinse it well. Our guide to choosing a pot for herbs covers sizes.
6. Fill it with fresh bagged potting mix, never garden soil. For rosemary, lavender, thyme, and sage, stir in 1 part perlite to 2 parts mix. Perlite is the small white bits of puffed rock that help water drain.
7. Water once, lightly, to settle the mix. Then wait until the soil is dry about 2 inches down before you water again.

If you cut off a lot of roots, trim back about a third of the leafy stems too. Fewer leaves means less work for the smaller roots. Hold off on plant food until you see new leaves.

When to give up and take cuttings instead

If most of the roots are mush, stop trying to save them. Grow a new plant from a healthy stem instead.

Use only firm, green stems with healthy leaves. Cut tips 4 to 6 inches long and strip the leaves off the bottom 2 inches. Push them into fresh, damp potting mix. Basil and mint also root in a glass of water. Our basil guide shows how to grow basil from a cutting.

Then bag the old plant and its soil, and put them in the trash. Root rot germs can live in old soil and dead roots for a long time.^[2]

Does hydrogen peroxide help root rot?

Hydrogen peroxide kills germs it touches, but it does not cure root rot. It breaks down into water and oxygen fast, so it does nothing about soil that stays wet. The real fix is the work above: cut off the rot, use fresh mix, and water less.

University guides from Wisconsin, Maryland, and Clemson give that same fix. None of them lists hydrogen peroxide. If you try it anyway, use only the 3% drugstore bottle, never a stronger one, and only after you trim the rot.

Fungicides are not much help either. Wisconsin does not recommend them for houseplant root rot, because few products are sold to home growers.^[2] On herbs you plan to eat, only use a product whose label lists herbs.

Is root rot contagious?

Yes, root rot can spread to other plants, mostly through old soil, reused pots, and shared water. Indoors, it moves in 4 main ways.

• Old soil and dead roots. They carry the germs, so throw them out.
• Dirty pots and scissors. Clean them before they touch another plant.
• Water. The worst root rot germs are water molds, whose spores swim in water.^[3] Do not let pots share a tray of water, and do not reuse water that drained out of a pot.^[2]
• Fungus gnats. These tiny black flies and their larvae can carry germs from pot to pot.^[3] Here is our guide to getting rid of fungus gnats.

Move the sick plant away from the others until it recovers. If you must reuse old mix, heat it first to kill the germs. Our guide explains how to sterilize old potting soil.

What causes root rot, and how to prevent it

Root rot starts when soil stays wet for days. The roots run out of air and get weak. Then germs that live in most soil move in. The worst ones are water molds with names like Pythium.

Watering on a schedule. Water by feel, not by the calendar. Water only when the soil is dry 2 inches down.

No drainage hole. Use a pot with a hole in the bottom. Gravel in the bottom does not help, and it can even slow drainage.

A pot that is too big. Extra soil that the roots cannot reach stays wet. When you repot, pick a pot only 1 to 2 inches wider than the old one.

Heavy or old soil. Garden soil packs down hard in a pot. Old potting mix packs down too, so repot into fresh mix when it sinks and goes hard. Our guide to the best potting mix for indoor herbs compares the options.

A saucer full of water. Empty the saucer after you water, so the pot never sits in water.

Cold. Plants drink less when they are cool, so the soil stays wet longer. Water less often in winter. Keep basil away from a cold window at night.

Going away. Some self-watering setups keep the soil too wet. Here are safe ways of watering plants while you’re away.

Which herbs get root rot most?

Rosemary, lavender, thyme, and sage get root rot most, because their roots need soil that dries out fast. Basil gets it when it sits in cold, wet soil.

Herb	Why it rots	What helps
Rosemary	Too much water is the top killer indoors	Clay pot, gritty mix, water only when dry
Lavender	Prone to root rot when water stands around it	Gritty mix, sunny window, empty saucer
Thyme	Roots rot in soggy soil	Small pot, gritty mix
Sage	Does not cope with wet or slow-draining soil	Gritty mix, let the top dry out
Basil	Roots and stem base rot, and the plant collapses	Keep it above 50°F, and do not let it sit wet

For full care, see our guides to growing rosemary indoors and growing lavender indoors, and our thyme plant profile.

Root rot in hydroponics and water gardens

Herbs grown in water get root rot too, when the water sits still, runs warm, or runs low on air. The roots turn brown and soft, with few fine hairs.

Keep the pump or air stone running so the water stays full of air. Keep the water at 68 to 72°F.^[4] Between crops, clean the whole system with a cleaner made for hydroponics.

Questions & answers

Can root rot be reversed?

Not the rotten part. Soft, brown roots are dead. But once you cut them off and repot, the plant grows new roots from the firm ones left.

Can you fix root rot without repotting?

Only when you catch it very early and most roots are still firm. Let the pot dry out, then water less often. Once the roots are mushy, you have to cut off the rot and repot.

What does root rot smell like?

Rotten roots smell sour or foul, a bit like a swamp or bad eggs. Healthy roots smell like fresh potting mix. Early rot may not smell at all, so check how the roots feel too.

What is the best treatment for root rot?

Cut off the rotten roots, then repot in fresh mix in a clean pot with a drainage hole. After that, water less often. No spray or drench fixes root rot on its own.

Should you water a plant after treating root rot?

Yes, once, lightly, to settle the new mix. Then wait until the soil is dry about 2 inches down before you water again.

References

1. University of Maryland Extension, Root rots of indoor plants. https://extension.umd.edu/resource/root-rots-indoor-plants ↩︎

2. Wisconsin Horticulture, Division of Extension, Root rots on houseplants. https://hort.extension.wisc.edu/articles/root-rots-houseplants/ ↩︎ ↩︎ ↩︎

3. UC IPM, Pest Notes: Houseplant problems (Publication 74172, 2020). https://ipm.ucanr.edu/legacy_assets/PDF/PESTNOTES/pnhouseplantproblems.pdf ↩︎ ↩︎

4. Colorado State University Extension, PlantTalk Colorado, Root disease problems and management in hydroponic systems. https://planttalk.colostate.edu/topics/miscellaneous/2043-root-disease-problems-management-hydroponic-systems/ ↩︎