It depends on the planter box, but on an 8th-floor balcony, wind is a real concern. At that height, gusts can be stronger and more turbulent than at ground level, especially around railings, corners, and open balcony edges. A lightweight plastic or hollow planter box with dry soil and tall plants is much more likely to tip or slide than a heavy, wide, low-profile box filled with moist soil.
To reduce the risk, choose a planter with a broad base and a low center of gravity. Heavier materials such as wood, concrete, or thick resin are generally safer than thin plastic. Fill the box properly with soil, keep plants reasonably low or prune them to reduce wind catch, and make sure drainage is good without letting the box become top-heavy. If the planter has wheels, lock them or remove them.
Most importantly, secure the planter. Depending on your balcony design, you can anchor it to the railing, wall, or floor with proper brackets, straps, or wind clips approved for outdoor use. Avoid simply placing it loosely against the railing. A windbreak or balcony screen can also help reduce direct gusts, but it must itself be safely installed.
Check the weather forecast before storms, and move lightweight planters indoors or to a sheltered spot during strong wind warnings. If you live in a high-rise building, also check your building’s balcony rules and insurance requirements. In short, your urban planter box will not automatically blow over, but on a windy 8th-floor balcony, it does need weight, a stable design, and secure fastening to be safe.