Root rot isn't one disease with one cause. It's what happens when soil stays wet for too long: roots are starved of the air they need, and once they're weakened, one of several different fungi already living in most soils takes the opportunity. That's why "root rot" can mean a soggy pea bed, a rosemary bush that's browned off over winter, or a specific named disease on radish or asparagus, and why none of them are quite the same thing under a microscope. What they share is the fix: drainage, not a spray.
What it looks like
Above ground, the signs are easy to mistake for drought stress: dull or yellowing leaves, wilting even when the soil is obviously wet, browning that often starts at the base or along leaf midribs, dieback, and a plant that's stunted or failing to thrive with no obvious pest damage anywhere on it. Below ground is where it becomes unambiguous. Healthy roots are firm and pale; roots affected by this turn dark, soft and sour-smelling, and the bark peels away easily. In severe cases there are barely any healthy roots left to take up water at all, which is the reason a plant sitting in waterlogged ground can still look like it's dying of thirst.
Why it happens
Saturated soil pushes air out of the spaces between soil particles. Roots respire and need that air; once it's gone, they stop functioning within as little as a day or two of sustained saturation. Weakened, oxygen-starved roots are then vulnerable to whichever opportunistic fungus or fungus-like organism is already present in the soil, and several different ones can do it: Phytophthora, Pythium, Fusarium and Rhizoctonia between them account for most cases. Telling them apart usually needs a lab test, not a garden inspection, and for a home gardener it rarely changes what to do next.
This is also why the exact same underlying problem gets described so differently from crop to crop. Peas and beans get "foot and root rot", a recognised disease complex of the soil-dwelling organisms above. Radish is affected specifically by Rhizoctonia. Asparagus gets violet root rot, a distinct disease with its own name, separate from the generic Phytophthora-led picture above. The herbs, rosemary, lavender, thyme, oregano, sage, sorrel and lemon balm, are mostly just affected by plain wet-soil root death without one particular organism standing out, because for a gardener the practical cause and the practical fix are identical either way.
🌱 Plants at risk
Root rot also affects these crops:
Why this matters in Scotland
The trigger here is rainfall and drainage, not cold, and that's a genuine disadvantage in a Scottish garden for most of the year, not just over winter.
Scotland's defining climate feature is rainfall rather than extreme cold, and heavy clay is common across a lot of the country's growing ground. Both raise the odds of the kind of prolonged saturation that starves roots of oxygen, in a way a milder-but-drier southern English garden on free-drainingSoil that lets water pass through quickly rather than pooling, so roots aren't sitting in wet ground after rain. Full definition → soil doesn't face nearly as often. None of these crops are actually killed by Scottish cold. Rosemary and lavender especially are only really at risk here because of what wet ground does to their roots over winter, not the temperature above them.
A raised bed or a gritty, free-draining spot does more for these plants than any amount of winter fleeceA lightweight, breathable fabric laid or draped over plants to trap a little warmth and keep off frost, wind and some flying pests. Full definition →, because the thing actually putting them at risk is water sitting around the roots.
What actually prevents it
All four of these are well-evidenced, not repeated folklore.
Improve drainage before planting, not after symptoms appear
Raised beds, avoiding soil compaction, and formal drainage wherever there's somewhere for the water to go are the practical fixes. Plant in well-drained soil or use raised beds from the start: drainage is far easier to get right before a bed goes in than after.
Blueberries are the exception worth naming directly: they need consistently moist, ericaceous soil, and "free-draining" here doesn't mean "dries out". A raised bed still helps by keeping the roots out of standing water, but pair it with ericaceous compost and a moisture-holding mulchA layer of material spread over the soil around plants to suppress weeds, retain moisture, and improve soil structure as it breaks down. Full definition → rather than gritty, fast-draining mix, or you'll trade root rot for drought stress on a plant that tolerates neither well.
Water deeply but infrequently, and avoid letting containers sit in a saucer of water
Overwatering is named directly as a cause alongside poor drainage, with avoiding excess moisture around the roots recommended as a primary control. For the herbs here especially, a container standing in its own runoff after rain recreates the same waterlogged conditions a raised bed is meant to solve.
There is no reliable chemical treatment
No chemical treatments are available to gardeners for Phytophthora root rot, and no chemical option exists for waterlogging-driven root death generally either. Once roots have rotted, the damage doesn't reverse.
What actually works: Improve drainage for whatever is left of the plant, remove the worst-affected roots or the whole plant if damage is severe, and replant into a well-drained spot rather than the same wet ground.
After a flood, don't eat raw crops from affected ground for two years
RHS: 'it is prudent to avoid growing salads and other uncooked crops for two years' after flooding, since disease spores and contamination can remain in the soil even once the plant itself has recovered. Cooked crops are considered safe again from the following year. This is a food-safety point, not a plant-health one, and it's easy to miss since the plant can look fine well before two years are up.
Frequently asked questions
What causes root rot?
Waterlogged soil, not a single pathogen. Saturated soil pushes air out of the gaps between soil particles, and roots need that air to function. Within a day or two of saturation, oxygen-starved roots stop working and become vulnerable to whichever opportunistic fungus is already in the soil, commonly Phytophthora, Pythium, Fusarium or Rhizoctonia. The organism varies; the setup is always the same wet, airless soil.
Is there a spray or chemical treatment that works?
No. There's no chemical treatment gardeners can reach for. Once roots have rotted, that damage doesn't reverse. The only real lever is improving drainage, for what's left of the plant and for whatever gets planted there next.
How is this different from a plant just being underwatered?
Above ground, the two can look almost identical: wilting, yellowing, a plant that seems thirsty no matter what you do. The difference is below ground. An underwatered plant has dry soil and firm, pale roots; a plant with root rot is often sitting in wet soil with roots that have turned dark, soft and sour-smelling. Checking the roots themselves is the only reliable way to tell them apart.
Is my garden safe to grow salad crops in after flooding?
Not straight away. Salads and other crops eaten raw are best avoided on flooded ground for two years, because of contamination and disease spores that can remain in the soil, even though the plants themselves may recover well before then. Crops that are cooked are considered safe again the following year. This is a food-safety point separate from whether the plant itself survives.
Sources
2 sources, recorded with what each was used for
- RHS: mechanism (oxygen deprivation from displaced soil air), above- and below-ground symptoms, at-risk soils and situations, drainage improvements and raised beds, absence of chemical treatment, post-flood food-safety guidance
- UC Integrated Pest Management Program: corroborating symptoms and drainage-first management independently; naming Fusarium, Pythium, Phytophthora and Rhizoctonia as the common causative fungi; overwatering as a direct cause