Root rot is what happens when roots sit in water long enough to suffocate, and the fungi and water moulds that live in wet soil move in. It kills more Australian garden plants than any pest, and it is nearly always a drainage or watering problem rather than bad luck. This guide covers how to recognise it, why the cause differs across the country, how to save a plant that still has roots, and the drainage habits that stop it coming back.
What root rot is
Healthy roots need air as well as water. In waterlogged soil or a pot standing in a saucer, the air spaces fill, the fine roots die within days, and organisms such as Pythium and Phytophthora colonise the dying tissue and spread to the rest. The plant above ground then shows the symptoms of drought, because the roots it has left cannot take up water, which is exactly why so many people respond by watering more.
Causes
- Overwatering, especially in winter, when growth slows and soil stays wet for far longer. The same pot can need half the water in July that it needed in January, and our guide to adjusting your watering schedule in winter covers the change.
- Heavy or compacted soil that holds water around the roots. Sandy soils, with their larger particles, drain quickly and rarely cause it, and our guide to soil types and how to identify them includes a simple test for working out which one you have.
- Pots without drainage, or standing in water. Never leave a pot standing in a full saucer, and browse our pots and planters if the container has no holes at all. A pot far too large for the plant has the same effect, because the unused mix stays wet; our guide to choosing the right size pot explains the sizing.
- Planting in a low spot where water collects after rain, or in a hole dug into clay that fills like a bathtub.
- Phytophthora already in the soil, which attacks even well-drained plantings of susceptible species (see below).
Recognising it
- Yellowing and wilting despite wet soil: the giveaway. Roots cannot take up water even though there is plenty.
- Wilting in the afternoon that does not recover overnight.
- Leaf drop starting from the top or the outside and progressing inward.
- Dark, soft, mushy roots that smell sour, instead of firm white ones.
- The plant lifts easily because the root system has gone.
- Sudden collapse of one plant in a row while the rest are fine, usually a local wet spot.
The trap is that wilting looks like drought, so people water more, which accelerates the problem. Always check the soil by hand before watering a wilting plant. Our guide to why plant leaves turn brown helps you rule out the several other problems that look much the same.
Root rot in Australian conditions
The pattern differs sharply by region, and knowing which version you are dealing with matters, because the responses are different.
| Region | Typical cause |
|---|---|
| Brisbane and subtropical Queensland | Wet summers on heavy clay; humidity and warmth |
| Sydney and coastal NSW | East coast lows delivering sustained heavy rain |
| Melbourne and Adelaide | Winter waterlogging on clay soils |
| Perth | Phytophthora in sandy soils, especially on natives |
| Anywhere, in pots | Saucers left full; mix that holds too much water |
Phytophthora dieback
Worth knowing about specifically because it is a major issue in Australia rather than a garden nuisance. Phytophthora cinnamomi is a soil-borne water mould that attacks roots and is listed as a key threatening process under national environmental law. It has devastated native vegetation across southwest Western Australia and parts of Victoria and Tasmania. In gardens it is the reason Western Australian grevilleas and banksias die on the humid east coast, why avocados fail in wet ground, and why some plants decline steadily with no visible pest. Once it is in your soil it is effectively permanent; management is about prevention and drainage rather than cure. Practical steps: buy plants from reputable nurseries, do not move soil or plants between properties, clean tools and boots when visiting bushland, and never plant susceptible species in ground that stays wet.
Prevention, which is most of the answer
- Plant on raised mounds on clay soils in Brisbane, Sydney and Melbourne. This single practice prevents most cases.
- Never plant in a low point where water collects after rain.
- Improve the whole bed, not individual holes; a well-dug hole in clay is a bathtub. Our guide on how to prepare the soil for planting covers the drainage work that is far easier to do before planting than after.
- Match plants to drainage. Natives from dry-climate regions, agaves, yuccas and Mediterranean plants all need sharp drainage; melaleucas, callistemons and Lomandra hystrix tolerate wet.
- In pots, empty saucers and use a free-draining mix. Standing water is the commonest cause in containers. Our potting mixes, mulches and fertilisers range has free-draining blends by plant type.
- Do not overwater in winter, when growth slows and soil stays wet for far longer.
Indoor plants
Most indoor plants that die, die of root rot, because the pot is watered on a schedule rather than when the mix is dry, and because the saucer catches the run-off and holds it. Feel the mix three to five centimetres down before watering, water until it runs through and then empty the saucer, and move any plant away from a cold draught in winter, when wet mix stays cold and wet for weeks.
If a plant is already affected
Recovery depends on how much root is left. Stop watering, improve drainage immediately, and remove obviously rotted material. A potted plant can sometimes be saved by tipping it out, cutting away every dark, soft root with clean scissors until only firm tissue remains, and repotting into fresh, free-draining mix in a clean pot no larger than the root ball needs, following the method in our guide on how to repot your plants like a pro. Keep it out of direct sun for a week and water lightly only when the surface dries. A plant in the ground in saturated soil usually cannot be saved without fixing the drainage first; if the site stays wet, replacing it with a species that tolerates wet feet is the more sensible decision than trying again with the same plant.
Ready to plant?
If a plant is too far gone, the kindest thing is usually to start again with the drainage sorted first. Browse our potting mixes, mulches and fertilisers for free-draining mixes and soil conditioners, and our pots and planters for containers with proper drainage.
