One thing to know before you diagnose anything: a yellowed ZZ leaflet is not coming back to green regardless of the cause. Fixing the underlying issue protects the rest of the plant — it won't rescue the leaflet that's already gone.
Rhizome rot from overwatering
Check trapped water, drainage and the condition of the base. Wet mix alone does not identify a pathogen, but soft, collapsing tissue or a bad smell needs prompt attention. Do not keep watering a saturated pot.
Inspect gently rather than squeezing healthy tissue. Retain firm roots and rhizome tissue; remove clearly decaying areas with clean tools if needed. Colour alone is not a reason to cut. Repot viable sections in fresh, freely draining mix and a pot suited to the remaining roots. Extensive decay needs specialist advice; drying the pot cannot restore tissue already rotted.
Natural aging (not actually a problem)
ZZ plants are slow but steady growers, and as new stems mature, the plant will sometimes let an old, lowest leaflet or two yellow and drop on its own. The tell is that it's isolated and slow — one leaflet going at a time while the rest of the plant, including any new growth, looks completely healthy.
Fix: nothing needed. Trim the spent leaflet off if you'd like a tidier look, or leave it — either way it isn't a sign to change your care routine.
Underwatering
If the root ball is dry and the plant has been left without water for a prolonged period, water thoroughly and allow it to drain. Check that the centre is wetted, not just the gap around the edge of the pot.
Wait for the mix to dry appropriately before the next watering. If leaves remain limp in wet mix, investigate roots rather than soaking again. Old damage or lost leaves may not recover.
Cold damage or drafts
ZZ plant is comfortable at normal room temperature but doesn't tolerate cold well. A spot too close to a drafty window, an exterior door, or an air-conditioning vent can produce dark, almost water-soaked patches on the leaves that yellow and eventually turn black — a different look from the gradual, even yellowing of rot or aging, and one that tends to appear suddenly after a cold snap or a change in the plant's position.
Fix: move the plant away from the cold source. Affected leaflets won't recover, but the rest of the plant typically resumes normal growth once it's back in a stable, comfortably warm spot.
Nutrient deficiency
Review feeding history, but do not diagnose a deficiency from yellowing alone. Check moisture, roots, light and pests first. Healthy active growth may benefit from feed at the label rate; a declining plant with decayed roots will not be repaired by extra fertiliser.
Keep a short care record
Photograph the whole plant and any damage, and note moisture checks and recent changes. Compare later growth rather than expecting an old scar to disappear. Continuing collapse, expanding lesions or extensive decay deserves specialist help.
Reference material
Reference material consulted for this independently written guide. The publishers listed have not reviewed or endorsed it.
- NC State Extension — ZZ plant
- University of Maryland — watering
- University of Maryland — symptom checks
- University of Maryland — root rots