Not always. Urban planter boxes need saucers underneath only when runoff would otherwise cause a problem. On balconies, terraces, rooftops, and paved courtyards, a saucer or drip tray is often useful because it catches excess water before it stains concrete, drips onto neighbors below, creates slippery surfaces, or damages facades. In these settings, choose a saucer with a raised grid or pot feet so the planter is not sitting in water, and make sure the tray has an overflow or can be emptied easily. A full saucer can become heavy, so consider structural load on roofs and balconies. Empty trays after heavy rain and check for mosquito larvae in warm weather.
In ground-level landscape beds, free-draining planters, or systems with integrated drainage, a separate saucer is usually unnecessary. If water can drain safely into soil or a designed drainage layer, a saucer may do more harm than good by keeping the root zone soggy, encouraging root rot, and concentrating salts. Outdoors, rain can fill a saucer quickly; without overflow and maintenance, the planter may sit in stagnant water. Saucers also collect sediment, fertilizer residue, and debris, so they need periodic cleaning and flushing.
The decision depends on four practical questions: Where will the runoff go? Is the surface sensitive to stains or water? Who maintains the planter? And does the planter already have a reservoir, internal tray, or drainage design? For urban balconies and rooftop gardens, a saucer with a spacer, overflow, and easy emptying is a smart runoff-control accessory. For public sidewalk planters, saucers are often omitted because they are hard to maintain, can collect trash, and may breed mosquitoes; engineered drainage is usually better.
If you use a saucer, do not treat it as a permanent water reservoir unless the system is designed for that. Keep drainage holes open, use pot feet or a grate, empty standing water after rain, and flush the tray occasionally. If mosquitoes are a concern, empty the saucer within a few days or use a safe mosquito control product labeled for containers. In short, urban planter boxes do not always need saucers, but saucers are recommended when catching runoff protects surfaces, neighbors, and the building—provided you manage overflow, weight, and standing water.