Zamioculcas zamiifolia stores water in thick, potato-like rhizomes just under the soil line. That single fact explains almost everything about how to keep one alive. Because the plant is carrying its own reserve, watering on a fixed weekly schedule is the most common way to kill it. A ZZ in a normal-sized pot, in an average indoor room, often only needs water once every three to five weeks — less in winter, when growth all but stops. The right test isn't the calendar, it's the soil: let it go fully dry, then water thoroughly and don't touch it again until it's dry all the way through. Dry all the way through is easy to check and hard to guess at. A wooden skewer pushed to the bottom of the pot comes out clean and dry when the plant is ready, and cool and darkened when it isn't. The weight of the pot works too, once you have lifted it wet and lifted it dry a few times. A fingertip tells you about the top inch and nothing about the rhizomes, which is where the decision actually sits. Yellowing stems are almost always a symptom of overwatering, not underwatering — the instinct to water a droopy or yellowing ZZ is exactly backwards. If a stem at the base goes soft or mushy, that's rhizome rot, and it usually means the pot has been sitting wet. Rot is worth catching before a stem goes. Soil still dark and sour-smelling a week after watering, or a base that gives slightly under a thumb, comes earlier than yellowing. If it has gone further than that, tip the plant out and look. Firm, pale rhizomes are fine; brown, soft ones come off with a clean blade. Let the cut surfaces dry for a day, repot into fresh mix on the dry side, and leave it unwatered for two or three weeks. A genuinely thirsty ZZ is a different picture. The stems thin and wrinkle and the leaflets fold up a little. It looks tired rather than sick. That one will take a thorough watering and be fine. Light is flexible. ZZ plants tolerate low light better than almost any common houseplant, which is why they show up in windowless offices, but they'll grow noticeably faster in bright indirect light without needing it. New stems come up in flushes rather than steadily, several at once from the rhizome and then nothing for months. A ZZ that hasn't moved since spring is usually not stuck. A plant kept dim simply has fewer of those flushes. One more thing worth knowing: every part of the plant is mildly toxic if chewed or ingested, which matters if pets or small children are around it.
The ZZ Does Not Want Your Attention
Most ZZ plant deaths are drownings, not neglect.