Why does my Swiss cheese plant have a black, mushy stem?

On a Swiss cheese plant this is a section of stem, usually at or just above the compost, that has turned dark brown to black and gone soft and wet, so that it gives under a gentle squeeze. The leaves above may droop or yellow, and the stem may smell sour. A stem that is dark but dry and hard is older damage, which is a different matter.

The most likely cause is rot travelling up from the roots. The Royal Horticultural Society names overwatering and waterlogging in poorly drained compost as what makes this plant's roots rot and the plant collapse. Its roots want air, and in a mix that stays wet the base is the first thing to go, because that is where the water sits. Winter makes it worse. The interval stretches from about 7 days to about 12 as the plant slows, and a watering habit that has not stretched with it drowns the base.

To confirm it in a minute, squeeze the stem gently between finger and thumb, working up from the compost, to find where firm becomes soft. Push a finger into the mix and lift the pot: wet and heavy points to the roots. If you can, slide the plant partway out of its pot and look. Pale and springy roots are healthy. Brown, slimy ones that smell sour are rotten.

Two other causes are worth ruling out. A wound is one: blackening that starts at a pruning cut, a snapped stem or an old leaf scar and works downwards, often after cutting with dirty scissors. Cold is the other. A Monstera left against a winter window, or brought in late from outdoors, can go dark and limp overnight and rot in the days after.

Recovery depends on how far the soft tissue has gone. Cut back to firm, pale tissue and the plant may shoot again from what is left, slowly, over weeks in the growing season rather than days. The surest rescue is to root a healthy top as a cutting, with a node and an aerial root, which the entry's propagation section says takes three to six weeks. If the soft tissue has gone all the way up, there is nothing left to root.

What this plant wants

The fix

Unpot, cut away soft brown roots, repot in fresh chunky mix and water sparingly until new growth appears.

Read on

Black, mushy stem: where to cut, or start again

The same sign on any plant, with the causes in the order they are likely.

Swiss cheese plant care

Light, water, feeding, repotting and the rest of the entry.

Check it in the diagnoser

Opens with the plant, the part and the sign already chosen.