Generally, yes—but only with conditions. Living on the 20th floor does not make a planter box inherently unsafe, but it changes the risks. The main issues are weight, wind, drainage, and fixing. A large box filled with wet soil can be much heavier than people expect, and balconies are designed for specific loads. Use lightweight potting mix, choose an appropriately sized container, and keep the total load—including plants, water, and drip trays—well within your building's limits. If you are planning several boxes or a built-in bench planter, ask a structural engineer or your building manager to confirm the balcony's load capacity.
Wind is the biggest high-rise concern. At 20 floors, gusts can be stronger and more turbulent than at ground level. Never hang a planter box on a railing unless the railing and fixings are explicitly rated for it. Secure the box to the floor or wall using wind-rated brackets, keep it low, and avoid tall, top-heavy plants that catch the wind. In typhoon or storm seasons, move lightweight pots inside or add extra tie-downs before storms arrive.
Drainage and falling objects also matter. Use saucers or a self-watering reservoir so runoff does not drip onto neighbors or damage the balcony surface, and check that the box cannot shift or tip. Follow strata, lease, and insurance rules, and check whether your balcony is open, enclosed, or partly glazed—each affects wind and fire safety. Finally, inspect fixings and brackets regularly. If the planter is small, lightweight, securely fixed, and the balcony is in good condition, an urban planter box can be a safe and enjoyable option. If it is large, heavy, or mounted high on a railing, treat it as a safety risk until a professional confirms otherwise.