The short answer is yes—if water collects and sits in an urban planter box, it can attract mosquitoes and become a breeding site. Mosquitoes do not need a large pond to reproduce; even a small amount of standing water in a saucer, drainage tray, clogged drainage hole, uneven soil surface, or plastic liner can be enough. If that water remains for about a week, especially in warm weather, mosquito larvae may begin to develop.
That does not mean planter boxes are inherently pest problems. The key issue is standing water, not the planter itself. Urban planters can also attract other pests when conditions stay too wet or messy. Soggy soil may encourage fungus gnats, and accumulated organic debris or food waste can attract ants, cockroaches, or rodents. However, mosquitoes are most directly linked to water that collects and stays still.
To reduce the risk, check that every planter box has working drainage holes and that they are not blocked by soil, roots, or liner material. Empty saucers and catch trays after rain, or use designs that do not allow water to pool. Fix leaky irrigation systems, avoid overwatering, and keep the soil surface reasonably level so water does not gather in low spots. If a planter has a decorative water feature or a difficult-to-empty reservoir, use mosquito dunks or granules containing Bti, following the product instructions. Routine checks after storms and weekly emptying of small water containers are simple but highly effective.
So urban planter boxes can attract mosquitoes when water collects, but with good drainage and regular maintenance, they can remain productive, attractive, and low-risk.