Short answer: yes, an urban planter box can tip over in strong wind, but it won’t automatically do so. Whether it tips depends on how much wind reaches your balcony, how much sail area your plants create, and how stable the planter is.
Wind force rises quickly as wind speed increases, and a balcony on a high floor, at a corner, or in a narrow gap can funnel wind much harder than ground level. A tall, narrow, lightweight plastic box with dry soil and a bushy plant is the riskiest combination. A wide, low, heavy box with wet soil and a low center of gravity is far more stable.
To reduce tipping risk: choose a wide, low planter; fill it with compacted soil and add a heavy base layer if the structure allows; avoid wheels or lock them; keep tall plants trimmed and use a permeable windbreak rather than a solid screen; group planters together if they cannot be moved; and anchor the box with straps, brackets, or a secure railing system when safe and permitted. During storms or very gusty days, move lightweight planters indoors or to a sheltered spot.
No planter is completely windproof. Check it after every strong wind, and if your balcony regularly gets severe gusts, consider smaller, heavier containers or secure built-in planters instead. If you are unsure, test gently: with the planter empty, try pushing it sideways. If it moves easily, it needs more weight or anchoring. With the right setup, an urban planter box can usually stay put—but in extreme wind, anchoring is not optional.