If a potted plant wilts or yellows while the mix is still wet, suspect root rot. The cause is usually a waterlogged mix, which lets soil-borne organisms attack roots that are short of air. The only sure way to look is to slide the plant out and inspect the roots.

Plants with some firm, pale roots left can often be rescued. Plants with mostly mushy, dark roots seldom recover, so it is worth knowing which you have before you act.

What is root rot in a pot?

“Root rot” describes roots dying and decaying, not one single disease. Iowa State University Extension lists several organisms that can be involved, including Pythium, Phytophthora, Rhizoctonia and Fusarium, and says they thrive in wet, poorly drained soils and potting mix.

Phytophthora and Pythium are often described as water moulds or fungus-like organisms. The Dieback Working Group notes that Phytophthora spores swim through soil water towards roots. That is why standing water matters so much in a container, where excess has nowhere to go.

Waterlogging alone can also kill roots by starving them of oxygen. In a pot, the two problems often overlap.

What are the signs above the soil?

The signs are easy to misread, because a rotting root system cannot supply water even when the mix is soaked.

• Wilting that does not improve after watering, or wilting when the mix is wet. Iowa State describes it starting with the lower leaves and moving upward.
• Yellowing, dull or sparse leaves, and leaf drop.
• Weak or stalled growth, or shoots dying back, sometimes on only part of the plant.
• A soft, dark stem base on some plants.

The Royal Horticultural Society warns that above-ground symptoms often appear only after root decay is well advanced. Oregon State notes that flooded plants may show no symptoms for several months.

How do you check the roots?

Check before you water again. Wilting from dry mix and wilting from rotten roots look alike, and extra water makes rot worse.

1. Push a finger into the mix. If it is wet well below the surface, hold off watering.
2. Tip the pot and ease the plant out, supporting the base of the stem.
3. Look at the roots and the bottom of the root ball, where browning often starts.
4. Smell the mix. A sour or swampy smell supports the idea of a waterlogged root zone.

The first two rows match the firm-versus-mushy test in the Iowa State and Royal New Zealand Institute of Horticulture guidance.

Can you tell which pathogen it is?

No. A visual check cannot confirm which organism is involved, or even whether a pathogen is involved at all rather than waterlogging damage. The RHS says above-ground signs are not specific and that laboratory testing is often needed to confirm the cause.

Oregon State’s diagnostic handbook goes further: even lab results are one line of evidence, because the organism may fail to grow from diseased tissue. If a valuable or unusual plant is declining and you want a firm answer, ask your state agriculture department or a plant diagnostic service about lab testing.

For most home gardeners the practical response is the same either way: fix the water problem, remove dead roots and start with clean mix. The name matters less than the drainage.

How do you rescue a plant with root rot?

Work in shade and keep roots from drying out while you work.

1. Unpot and shake off the old mix. Do not reuse it in other pots.
2. Cut away all mushy, brown or blackened roots with clean secateurs, back to firm tissue. Wipe the blades between cuts.
3. Trim the top to match. With fewer roots, the plant cannot support all its leaves, so remove dead shoots and thin the canopy modestly.
4. Choose a clean pot with large drainage holes, the same size or one size smaller than the root ball now justifies. A big pot of mix holds more water than a reduced root system can use.
5. Repot in fresh, free-draining potting mix. The RNZIH guidance recommends good-quality potting mix and careful watering afterwards.
6. Water lightly to settle, then let the mix dry slightly before the next drink. Keep the plant in bright shade, out of hot afternoon sun, until new growth appears.

Be realistic. New leaves are the sign of recovery, and it can take weeks. If a plant still declines, check the roots again before assuming another cause. If you consider a chemical treatment, read the product label and check with your state agriculture department. This guide does not recommend products.

What causes it, and how do you prevent it?

Plastic-lined surfaces, blocked drains and saucers can leave pots sitting in puddles, which Oregon State identifies as a common route to infection.

Does climate change the risk?

Yes, but the principle holds across Australia. Warm, humid conditions in tropical and subtropical areas, including coastal Queensland, mean pots can stay wet during long wet-season spells, and heavy downpours can fill saucers and sit-in trays. Move pots under cover or tilt them to drain during extended wet weather.

In temperate and cool zones, risk often rises in winter. Plants use less water in cold, low-light weather, so the same watering routine leaves the mix wet for longer. In arid areas, the risk is lower outdoors but still real for pots on drip lines or with plates left full.

Local conditions vary, so judge by the mix, not the month.

Common mistakes

• Watering a wilting plant without checking the roots or the mix.
• Moving straight to a much bigger pot.
• Leaving rotted roots in place “to see what happens”.
• Reusing old mix or dirty pots.
• Assuming all yellow leaves mean a nutrient problem. Some leaf problems, such as the black coating described in the guide to sooty mould on leaves, have entirely different causes.

Root rot is one of many issues covered in the overview of common Australian garden pests and problems. If your in-ground beds also stay wet, see how to improve soggy beds that never drain.