How to save a potted plant with root rot

A plant with root rot has soft brown or black roots and stays wilted after you water it. If only some of the roots are affected, lift it out, cut the rotted roots away, and repot it in clean, sterile mix in a clean pot with a drainage hole; if most of the roots are gone, replace the plant. Not sure whether the plant is drowning or thirsty? Overwatered vs underwatered sorts that out before you unpot anything.

Confirm it is root rot

Two signs matter: the roots, and how the plant answers to water.

Clemson Cooperative Extension says that with root or stem rot, leaves and stems show noticeable wilt, stems may be girdled at or near soil level by a ring of brown or black tissue, and infected roots are brown to black and may be soft. Slide the plant out and look before you decide.

The second sign is how the plant answers to water. UC IPM says that unlike plants suffering from water stress, plants with Phytophthora root rot do not recover when irrigated. A plant that was simply dry perks up; a plant that sits limp in damp mix has roots that cannot move the water.

Clemson lists several causes of wilting: a lack of water, excess water, root rot, too much fertilizer, or salt build-up. So a wilted plant on its own proves nothing, and the roots are the test.

The rescue, in order

Clemson’s repotting factsheet says not to repot ailing plants, but its houseplant disease factsheet makes root rot the exception: if only a few roots are infected, cut them out and repot in sterile soil. The rotted roots and the wet mix around them are the problem, so leaving the plant in place does not help.

1. Stop watering and unpot

Tip the pot and slide the whole root ball out. Shake or tease off as much old mix as you can, so you can see every root.

2. Cut out the rot

Cut away the roots that are brown to black or soft, back to firm root. Use clean scissors or secateurs, and wipe the blades afterwards so you do not carry the fungi to the next plant.

3. Prepare a clean pot and fresh mix

Clemson says to use sterilized soil and pots. In practice that means fresh bagged potting mix, not the mix you just took off the roots, and a pot that has been washed. If the pot is the old one, scrub it clean first. Whatever pot you use needs a drainage hole; Clemson suggests covering the hole with a piece of broken clay pot so the mix does not wash out, and says not to add a layer of clay pieces or rocks, because that slows water movement through the pot. The drainage hole guide covers pots you cannot drill.

4. Repot at the right size

Clemson’s rule for repotting is a pot 1 or 2 inches larger in diameter than the current one. After trimming, the plant has fewer roots drinking from the mix, so stay at the low end of that, or keep the same pot. A big pot of damp mix around a few roots is how rot starts again. Fill around the roots and press the mix gently with your fingers; Clemson says not to pack wet media. The full routine is in repotting herbs without shock.

5. Water once, then let it dry

Water the new mix so the excess drains out, then empty the saucer. Clemson says overwatering kills roots through lack of oxygen and advises providing drainage and reducing watering. UC IPM’s garden advice is to let the top few inches dry thoroughly between waterings; in a small pot, check at fingertip depth instead, as overwatered vs underwatered explains. Check with a finger rather than by the calendar.

What about fungicide and peroxide?

Clemson notes that fungicides are available, but says most indoor gardeners will find these chemicals probably cost more than a new plant. Neither Clemson nor UC IPM gives a home dose for a fungicide or a hydrogen peroxide soak. For a plant you plan to eat, choosing a pest treatment for edible plants explains why the label decides what you can use.

When to stop and replace it

If the roots are mostly brown and soft, there is little to rescue. UC IPM says a plant with a substantial Phytophthora infection rarely recovers. That statement is about Phytophthora in the garden rather than a pot of basil, but the reasoning carries over: once most of the roots are gone, the plant cannot take up the water it needs. Cheap, short-lived plants such as annual herbs are usually better replaced with fresh seedlings in fresh mix. Keep the rescue for a plant that has value to you and that still has a decent share of firm, pale roots.

Stop it coming back

In garden soil, UC IPM says Phytophthora can develop in as little as 4 to 8 hours of saturation. A pot standing in a full saucer or over a blocked hole stays saturated far longer than that. Keep to a short list:

• Use a pot with a working drainage hole.
• Empty the saucer after watering.
• Let the top of the mix dry before the next watering. How often to water potted herbs turns that into a routine by plant.
• Use fresh sterile mix and a clean pot for every new plant.

If the roots are in water rather than mix, the causes are different. Hydroponic root rot covers that case.

Sources

• Clemson Cooperative Extension: Houseplant Diseases & Disorders
• Clemson Cooperative Extension: Indoor Plants – Transplanting & Repotting
• UC IPM: Phytophthora Root and Crown Rot in the Garden