The short version: mold loves damp, still air and fabric that stays wet too long. In a humid city, your goal is not just to keep rain off the cushions—it is to help every layer dry fast and stay ventilated.
Start with the cushion itself. If you are buying new ones, look for quick-drying foam, mesh or drainage panels on the bottom, removable covers, and breathable outdoor fabrics such as solution-dyed acrylic or olefin. Thick, sponge-like foam and cotton-rich covers feel nice but hold moisture and are much more likely to go moldy on a city balcony.
Give the cushions airflow. Don’t leave them flat against the furniture or stacked together. Prop them on edge when they are not in use, leave gaps between cushions, and avoid wrapping them in plastic. A waterproof cover can be useful for rain, but it must be breathable or left partly open; otherwise it traps humidity underneath and makes the problem worse.
Make drying part of your routine. Bring cushions inside before a heavy rain if you can, and after any damp weather, let them dry fully in sun and moving air before sitting on them or storing them. In small urban spaces, a fan or a dehumidifier nearby can help. Wipe down frames and tables too—wet leaves, dirt, and AC drip can keep cushions damp from below.
Clean them before mold gets a grip. Brush off dust and pollen, wash removable covers with mild soap and water, rinse well, and dry completely. For small mold spots, test a diluted white vinegar or oxygen bleach solution on a hidden area first, then clean outdoors, rinse, and dry in the sun. Avoid harsh bleach on colored fabrics unless the manufacturer says it is safe.
Store smartly. Never put cushions away damp. Use a breathable storage bag or a dry indoor spot, and keep them off a cold concrete floor. Rotate cushions occasionally so one side does not stay pressed against a damp surface. If your balcony is very shaded, consider bringing cushions in daily during the humid season. It is a little extra effort, but it saves a lot of scrubbing later.