The ZZ plant — Zamioculcas zamiifolia, if you want to be formal — is sold to people as the unkillable houseplant. The plant you can ignore for weeks. The plant that survives in dim corners. All of that is true, more or less. But it leads to a strange paradox: the people most likely to kill a ZZ plant are not the negligent ones. It’s the attentive owners. The ones who water on a Sunday schedule. The ones who think the leaves looking a bit dull means it needs a drink.

ZZ plants don’t want a drink. They want to be left alone, in dry chunky soil, until they’re properly thirsty. And by the time you realise you’ve been overwatering one, the rhizomes underground are usually mush.

How You Know It’s Overwatering, Not Something Else

Yellowing leaves are the first sign — but the pattern matters. Overwatered ZZ leaflets yellow from the bottom of the stem upward, and the whole stem softens at the base. If you grab a stem near the soil line and it bends easily where it should be firm, that’s rhizome rot below.

Underwatering, by contrast, causes the leaflets to crisp at the tips and curl in slightly. The stems stay firm. If your ZZ is sad with crispy edges, give it water and it’ll bounce. If it’s sad with yellow lower leaves and a soft base, you have a problem.

Stem drop — where individual canes fall over from the base — is late-stage rot. The rhizome that was supporting that cane has decomposed. You can sometimes save the rest of the plant. Sometimes you can’t.

Step One: Get It Out of the Pot

Don’t try to fix this in situ. Tip the plant out, knock off as much soil as you can, and look at the rhizomes. Healthy ZZ rhizomes are firm, light brown to creamy, and the size of small potatoes. Rotted rhizomes are dark, soft, sometimes hollow, sometimes oozing. They smell. You’ll know.

Wash the root mass under a slow tap to clear away soil so you can see what’s living. Take photos if you want — being able to compare progress over weeks helps.

Step Two: Cut Out the Rot, Aggressively

Use a clean, sharp knife. Sterilise the blade between cuts with a wipe of methylated spirits. Cut every soft rhizome back to firm, healthy tissue. If a stem is attached to a fully-rotted rhizome, sacrifice the stem. You can sometimes propagate it from leaflets later, but a stem with no rhizome is not going to recover.

Be ruthless. People always try to save more than they should and end up with rot reappearing two weeks later. If half the rhizome mass is gone, half the plant is gone. Accept it.

After cutting, let the cut surfaces air-dry for a couple of hours on a clean piece of newspaper. Some growers dust the cut surfaces with cinnamon or a sulphur-based fungicide; I’ve had fine results just leaving them dry. The ABC Gardening Australia factsheet on root rot in indoor plants covers similar techniques and is worth a read.

Step Three: Repot Into Bone-Dry Mix

This is non-negotiable. Do not water-in. Use a chunky, free-draining mix — I use two parts premium indoor potting mix, one part coarse perlite, one part orchid bark. The mix should fall through your fingers, not clump.

Pot size: smaller than you think. ZZ rhizomes like to be snug. A pot that’s only 2-3cm wider than the remaining rhizome mass is correct. Big pots full of damp soil around tiny rhizomes is exactly how the rot started.

Terracotta is better than plastic for a recovering ZZ because it breathes and dries the mix faster. Drainage holes are mandatory. Decorative pots with no drainage are how plants die.

Step Four: Don’t Water for Two Weeks

Read that again. Two weeks. The rhizome and remaining roots need to seal their cut surfaces and start producing new feeder roots. Watering immediately re-introduces the moisture that caused the problem.

After two weeks, water lightly — maybe 100ml for a medium pot — and then leave it another fortnight before watering again. You’re retraining the plant and yourself. ZZ plants in Melbourne winter conditions might only need water every 5-6 weeks. Yes, really.

Light and Position During Recovery

Bright indirect light, no direct sun while the plant is rebuilding. Stable temperature, no cold drafts. Don’t fertilise for at least two months — the plant doesn’t need a push, it needs to repair. Once you see new growth (a fresh chartreuse stem emerging from the rhizome), you’ll know the recovery has taken.

A friend who works on AI tools at Team400 — a Sydney AI consultancy — asked me whether plant identification apps could diagnose root rot from a photo of leaves. The honest answer is no, not reliably. The visible symptoms overlap with several other problems and the diagnosis really depends on what’s happening below the soil line, which a leaf photo can’t see. There’s something to be said for actually taking the plant out of the pot and looking.

Preventing the Repeat

Going forward, water ZZ when the top 5cm of mix is bone dry, and only then. In winter, that might be once every six weeks. In summer with heating off, once every three. Forget the schedule. Stick a finger in the soil. If it’s dry, water. If it’s not, don’t.

A ZZ that’s slightly underwatered looks fine. A ZZ that’s slightly overwatered is dying. The asymmetry is the whole secret to keeping these plants alive. They’re not unkillable. They’re tough in one direction only.