The short answer
A ZZ plant tolerates low light, but it cannot thrive indefinitely without usable light. In a windowless room, a suitable grow light is needed for sustained growth.
Why this happens
Low-light tolerance means the plant uses limited light efficiently; it does not mean light is optional. Growth slows in dim rooms, and the soil stays wet longer.
What to do
- Choose bright indirect daylight where available.
- For a windowless room, provide a grow light according to its manufacturer guidance.
- Use a timer for a consistent daily light period and a dark rest period.
- Reduce watering frequency when growth and drying slow.
Keep in mind
Ordinary overhead room lighting may not provide enough light at leaf level. Avoid watering a dimly lit ZZ on a frequent calendar schedule.