Why does my ZZ plant have a soft or mushy base to the stem?

On a ZZ plant this shows as stems that are soft, wet and brown or black at the base where they meet the compost, so that a stem leans, flops or comes away in your hand. The leaflets above go yellow, and there may be a sour smell from the pot.

The most likely cause is rhizome rot from compost that has stayed wet for a long time. The rhizomes are fat water stores that hold weeks of reserve, and the only thing that kills this plant is compost that stays damp around them. It is watered about every 18 days in the growing season and about every 35 in winter, so the compost is meant to be dry for most of the time. A pot watered often, or a cold room in winter, keeps it wet far longer.

To confirm it in a minute, press the stem gently just above the compost. Healthy tissue is firm, and rot gives under a finger and may weep. Lift the pot, which stays heavy when the compost is wet. If you can tip the plant out, sound rhizomes are plump and pale, and rotten ones are brown or black, soft and sour smelling.

Two other causes are worth ruling out. A decorative pot with no drainage hole, or a saucer that has held water since the last watering, leaves the bottom third of the compost sodden. And a free draining mix matters, because the entry asks for a third perlite or fine bark, and a heavy compost on its own stays soggy around the rhizomes.

The rot does not heal, so the recovery is from the firm rhizomes that remain. Each piece with a plump rhizome and a stem can be potted separately, as the entry says for division, and new stems come slowly. A ZZ makes few stems even when healthy, so allow a whole growing season, and expect little in winter.

What this plant wants

The fix

Unpot, cut every soft brown rhizome back to firm white tissue, let the cuts dry for a day and repot into fresh gritty mix. Water sparingly until new stems appear.

Read on

A soft or mushy base to the stem

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

ZZ 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.