Securing a planter box on a high-rise balcony is really a wind-load and anchor problem, not just a weight problem. On upper floors, wind can be stronger and more turbulent than at ground level, and a heavy planter can still tip or slide if it is not tied into something structural.
The best method depends on your building rules and whether you can drill. If you can drill into the concrete slab or a structural wall, use corrosion-resistant anchors rated for outdoor use. For concrete, wedge anchors or sleeve anchors are common; for a wall, expansion anchors or through-bolts into studs or concrete. Bolt or bracket the planter to the slab or wall, then use large washers and lock nuts so vibration does not loosen them. If the planter has legs, secure each leg with a bracket. If it is a large trough, use at least two anchor points on opposite sides, preferably four.
If you cannot drill, use a non-penetrating system. A heavy, low-profile base filled with pavers or sand can add ballast, but ballast alone is not enough in strong gusts. Combine it with rated straps, steel cables, or turnbuckles attached to a structural railing or wall anchor. Check with your building manager first: many high-rise railings are not designed to hold lateral loads, and some balconies prohibit drilling. Use marine-grade stainless hardware and UV-resistant nylon or polyester straps. Avoid bungee cords, rope, or adhesive mounts; they fail under wind and sun.
Another key step is choosing the right planter. Use a wind-rated, low-center-of-gravity box made of heavy resin, concrete, or metal. Keep tall plants and lightweight pots away from the edge. Place heavier planters against the wall or in a corner where wind pressure is lower. Do not hang planters over the railing unless the bracket is engineered and approved. For very large or tall installations, consult a structural engineer or contractor to calculate wind load for your height and exposure. A simple rule: if you can push it and it moves, wind can move it too.
Finally, prepare for storms. During severe weather warnings, move small planters inside or to a protected area. Inspect anchors, straps, and brackets twice a year, and after any major storm. Tighten loose bolts, replace frayed straps, and check for rust. If you rent, use removable, non-damaging anchors where allowed and get written approval. The best answer is a combination: a heavy wind-rated planter, structural anchoring, rated hardware, and storm-day removal for anything light or loose. That gives you beauty on the balcony without creating a falling-object hazard.