Yes, an urban planter box can be used hydroponically, but it usually needs a few adaptations rather than being filled with water and plants. A planter box is just a container; hydroponics depends on how you manage water, nutrients, oxygen, and root support. If the box is waterproof, has a lid or light-blocking cover, and can hold a reservoir, it can work well for passive wick systems, Kratky-style setups, or a small ebb-and-flow or drip system. If it has drainage holes, line it with a waterproof tray or pond liner and add an overflow hole so the nutrient solution does not flood the root zone.
The basic setup is simple. Use inert growing medium such as coco coir, perlite, clay pebbles, or rockwool instead of soil. Place plants in net pots or fabric pots, set them so roots can reach the nutrient solution, and keep the water level slightly below the root crown to avoid stem rot. For passive systems, a wick can pull moisture up into the medium. For active systems, add a small pump, a timer, and an air stone; aeration matters because roots need oxygen as much as water. Check pH and EC or nutrient strength regularly, top up with fresh nutrient solution, and change the reservoir every one to three weeks depending on plant size and temperature.
Material and location also matter. Choose food-safe, UV-stable plastic or line the box if you are growing edibles. Keep the reservoir covered to block light and reduce algae. Outdoors, place the box where it gets appropriate sun but not excessive heat, because small reservoirs warm quickly and can stress plants. Indoors, use a grow light and keep water away from electrical outlets. Watch for mosquitoes, especially in warm climates, and never let stagnant water sit uncovered. Support tall plants with stakes or a trellis, since hydroponic roots have less anchorage than soil roots.
In short, an urban planter box can absolutely be used hydroponically if it is converted into a proper watertight, aerated, light-blocked growing system. It is a practical way to grow herbs, lettuce, peppers, and small fruiting plants on balconies, patios, and rooftops, as long as you manage nutrients, oxygen, and drainage carefully.