How far back should I cut a rotten stem?

Higher than looks necessary. The visible blackening marks where the damage has finished, not where it has reached. Cut into firm pale tissue and look at the cut face: any staining or softness and you cut higher again.

Will a plant with a mushy stem recover?

The stem itself will not. If the rot came up from the roots, take the healthy top as a cutting and treat the rest as lost. If it came down from a pruning wound, cut into clean tissue and the base will usually shoot again.

Ailment · the stems

A black, mushy stem

Soft rot working up a stem. Where to cut, how to know you have gone far enough, and when the honest answer is to start again from a cutting.

This one is worth acting on today rather than at the weekend. A week of waiting changes what is left to save.

What it usually is

1 Rot travelling up from the roots

How to tell	The blackening begins at the compost and moves upwards, the tissue is soft and wet rather than dry, and the compost is damp. Squeezing the stem gently between finger and thumb tells you where firm becomes soft.
The fix	Cut with clean scissors into firm, pale tissue well above the soft part, and look at the cut face: it should be uniformly pale. If it is stained, cut higher. Root the healthy top as a cutting and treat the parent as lost.

2 Rot down from a wound or a cut

How to tell	The blackening starts at a pruning cut, a snapped stem or a leaf scar and works downwards, often after the plant was cut with dirty scissors or in damp still air.
The fix	Cut back into clean tissue and let the wound dry in air rather than sealing it. Clean your scissors with surgical spirit between plants, which takes seconds and prevents most of this.

3 Cold damage that then rotted

How to tell	The stem went dark and limp after a cold night and softened over the following days. Common on tender plants left near a window or brought in late from a balcony.
The fix	Cut back to firm tissue, move the plant somewhere steady and warm, and keep it barely moist until it recovers. A plant that has lost its stem to cold usually shoots again from the base if the roots were not frozen.

Through the year. Autumn and winter, when compost stays wet, rooms are cool and a cold night can damage tissue that then rots in the following week.

A stem that has gone black and mushy is dead tissue with a live organism in it, and it does not stop where you can see the edge. That is the single most useful thing to know when you pick up the scissors: the visible blackening marks where the damage has finished, not where it has reached. Cut at the visible edge and it will be black again in a week.

So cut higher, and use the cut face to tell you. Firm, pale, uniform tissue means you are past it. Any staining, any softness, any darker ring, and you cut higher again. This wastes stem and saves plants.

What you do with the two pieces depends on where the rot began. If it came up from the roots, the part below is not worth saving; take the healthy top as a cutting, let the cut dry for a few hours so it seals, and set it in fresh, barely damp compost or in a clean jar of water. Most of the plants this happens to, philodendrons, pothos, monsteras, tradescantias, root readily and you lose a season rather than a plant.

If the rot came down from a wound, the base is probably fine. Cut into clean tissue, leave the wound open to the air rather than sealing it with anything, and let the plant shoot again from lower down.

Prevention is mostly hygiene and dryness. Clean the scissors with surgical spirit between plants. Do not cut a plant and then mist it. Water the compost rather than the crown. Keep pots far enough apart for air to move. And in winter, keep tender plants away from cold glass, because cold damaged tissue is what soft rot moves into.

A last honest word. Some plants will not come back and the sensible thing is to stop trying. Cacti and succulents that have gone soft at the stem are usually finished, and every day the plant sits in the collection is a day for the organism to reach the next pot. Take a cutting from the firm top if there is one, and put the rest out.

Diagnose your plant

Entries that name it

What the journal says about this symptom on each plant.

Heartleaf philodendron	Root and stem rot from compost that never dries. Cut back to firm tissue, take healthy cuttings from what is left, and start again in fresh compost with better drainage.
Inch plant	Rot from compost that stays wet, most often in winter. Take cuttings from the healthy tips at once and start again. A rotted crown does not recover.
Swiss cheese plant	Root rot from waterlogged mix. Unpot, cut away soft brown roots, repot in fresh chunky mix and water sparingly until new growth appears.

Questions from the margin

How far back should I cut a rotten stem?: Higher than looks necessary. The visible blackening marks where the damage has finished, not where it has reached. Cut into firm pale tissue and look at the cut face: any staining or softness and you cut higher again.
Will a plant with a mushy stem recover?: The stem itself will not. If the rot came up from the roots, take the healthy top as a cutting and treat the rest as lost. If it came down from a pruning wound, cut into clean tissue and the base will usually shoot again.

Checked against RHS, Soft rot; Missouri Botanical Garden, Stem and crown rot.