Root rot

The trap this problem sets

A plant with rotted roots wilts. Its leaves go limp, the whole thing droops, and every instinct says it is thirsty. So it gets watered, and it wilts harder.

The wilting is real and it is caused by drought, but the drought is inside the plant rather than in the pot. Roots that have died can no longer take up water, so the leaves lose more than the plant can replace and go slack while the compost around them is sodden. Reading that as thirst is the commonest way a houseplant dies in Britain, and it is why this page leads with the test rather than the treatment.

Put a finger in the compost before you reach for the watering can. Wet compost under a wilting plant means stop watering it.

What is going wrong underground

Roots breathe. They take oxygen from the air spaces between particles of compost, and they use it to run the cells that pull water and nutrients in against a gradient. That process costs energy, and energy needs oxygen.

Water fills the air spaces. In saturated compost the oxygen dissolved in the water is used up within a day or two, and after that the fine feeding roots start to die. They are the youngest, thinnest and most active tissue in the plant, and they go first.

Water moulds arrive next. Pythium and Phytophthora are always present in soil and compost, they swim through a film of water, and they colonise weakened root tissue readily. That is where the word rot comes from. They are the consequence rather than the cause, which is why no fungicide sold to an amateur addresses this and why changing how the plant is watered does.

How to read a root

Take the plant out of the pot. There is no way to judge this from the surface, and the five minutes it takes is the difference between a diagnosis and a guess.

A sound root is white, cream or light tan, firm between finger and thumb, and smells of nothing much. A rotted root is brown or black and soft. Pinch one and pull, and the outer layer slides off like a sleeve, leaving a fine thread of central tissue in your fingers. Nothing else does that.

Smell the compost while you are there. Waterlogged compost turns anaerobic and smells sour, stagnant or faintly of drains. A healthy root ball smells of earth.

Whether it can be saved

Count what is left. A plant keeping a third or more of its roots firm and pale will usually come back. Trim off everything soft with clean scissors, wash the old compost away rather than repotting the rot along with the plant, and put it into fresh open mix in a pot that just fits what remains.

Then take off a third of the leaves. It feels wrong, and it is what makes the difference. A halved root system cannot supply a full head of foliage, so the plant will shed leaves whichever of you decides it, and choosing yourself lets it put what it has into new roots.

Water once, lightly, and leave it for a fortnight somewhere bright and out of direct sun. Do not feed it. Fertiliser draws water out of damaged roots and adds nothing a plant with no working root tips can use.

Where the stem has gone soft and dark at compost level, the plant is generally finished, but the top may be sound. Cut above the damage and root the healthy section as a cutting.

Why it keeps happening

Almost every case traces back to one of five things, and all five are fixable in an afternoon.

A pot with no drainage hole holds every drop. A saucer left full keeps the lower centimetre of compost saturated between waterings. A plant moved into a pot three sizes too big sits surrounded by compost its roots cannot reach or dry. A watering routine run to a day of the week ignores that a plant needs perhaps a quarter as much water in January as in July. And a dense peat-free multipurpose compost used straight from the bag compacts within a season into something closer to mud than to soil.

Outdoors the same thing happens on clay and it happens in winter. Lavender, rosemary, sage, cistus and most silver-leaved Mediterranean plants are killed by sitting wet at 4 degrees C far more often than by cold itself. Planting on a low mound with grit worked in, or into a raised bed, is what gets them through.

Why there are no predators listed on this page

The biological tier is empty because there is nothing to introduce. This is not an organism doing something to the plant; it is the plant suffocating, with opportunists moving into the wreckage afterwards. Mycorrhizal products are sold for it and there is no good evidence they rescue a waterlogged root ball.

The fix is drainage, pot size, compost and habit. All four are free.

What it gets mistaken for

Underwatering produces the same wilt, and the compost tells the two apart in seconds. Dry, light and shrunken away from the side of the pot is drought; heavy and wet is this.

A nitrogen shortage yellows the oldest leaves first, exactly as root rot does, but the plant stays turgid and the compost is not saturated. Feeding a plant with rotted roots to fix yellow leaves reliably makes it worse.

Honey fungus kills woody plants outdoors from the roots up and is often blamed on wet ground. Look under the bark at the base for a sheet of white fungal growth smelling of mushroom, and in the surrounding soil for black bootlace strands. Neither appears with plain waterlogging.

Vine weevil grubs eat the roots of container plants and produce the same sudden collapse. Tip the pot out and look. Rot leaves black mush and no animals; vine weevil leaves white C-shaped grubs with brown heads.