Yes, an urban planter box can be used to grow mosquito-repelling plants like citronella, but it must be the right kind of planter and the right expectations. The plant usually sold as citronella is citronella grass, a tropical clumping grass from the Cymbopogon genus. It grows best in a large container—at least 12 to 18 inches deep and wide—with drainage holes. A shallow, narrow balcony box will stunt it, and it may dry out too quickly. Citronella grass also needs full sun, warm weather, rich well-draining potting mix, and steady moisture. It is not frost-hardy, so in cold climates you will need to bring it indoors before the first frost or grow it as an annual.
If you mean citronella scented geranium, often sold as a "mosquito plant," that is easier to keep in a smaller urban planter box. However, its repellent effect is generally weaker than true citronella grass. Other mosquito-repelling plants that work well in containers include lemon balm, mint, lavender, catnip, basil, rosemary, marigold, and lemongrass. Give sun-loving herbs at least six hours of direct light, and plant mint in its own pot because it spreads aggressively.
The key limitation is that living plants do not usually create a strong mosquito-free zone. The aromatic oils are released mainly when leaves are crushed, rubbed, or heated. A pot of citronella on a balcony may look and smell lovely, but mosquitoes can still bite nearby. For better results, place pots near seating areas, gently brush or crush a few leaves before you sit outside, and use the plants as part of a broader strategy. Never rely on plants alone.
To reduce mosquitoes around an urban planter box, empty saucers and trays so water does not collect. Mosquitoes breed in standing water, even small amounts. Refresh water in plant reservoirs, fix leaks, and keep gutters or drainage areas clear. Use screens, fans, or an EPA-approved repellent when needed, especially at dusk.
For container care, water citronella grass when the top inch of soil feels dry, but do not let the pot sit in water. Feed it monthly during the growing season with a balanced liquid fertilizer. Trim brown leaves and divide crowded clumps in spring. In winter, move tender plants indoors to a bright window and reduce watering. With enough space, sun, and warmth, an urban planter box can indeed grow citronella and other mosquito-repelling plants—just treat them as a helpful garden addition, not a complete mosquito control solution.