The short answer is: only a small, narrow, lightweight, noncombustible planter box that is securely fixed and does not reduce the required clear space—and only if your local fire code and building management allow planters on fire escapes at all. In many cities, fire escapes are strictly emergency egress routes, so anything that can obstruct movement, add weight, or become a falling hazard may be prohibited. Check the rules before buying anything.
For a fire escape, the best urban planter box is a purpose-built rail-mounted or low-profile floor-standing box made from powder-coated aluminum or steel. It should be narrow enough to leave handrails, steps, and landings completely clear, shallow enough to hold small herbs or flowers rather than large shrubs, and fitted with strong locking brackets so it cannot shift in wind. Avoid heavy terracotta, large timber troughs, and loose plastic pots that can blow over. Wood and some plastics can also add fire risk, so noncombustible or fire-resistant materials are safer.
Weight matters as much as size. Use a lightweight growing mix with coco coir, perlite, and compost rather than dense garden soil, and choose shallow-rooted plants such as herbs, succulents, or small annuals. Do not use the planter as storage, and never let vines or foliage block the path or handrail. A built-in drip tray or sealed drainage system is important because water on fire escape steps can create a slip hazard, stains, and ice in cold weather.
If planters are permitted, secure the box to the railing or floor using manufacturer-approved clamps or brackets—never rely on hooks or wire. Keep the box low, stable, and away from the outer edge. Inspect it regularly for rust, loose fittings, and dead foliage. The safest choice may be to skip the fire escape entirely and use an approved balcony, windowsill, or indoor growing setup instead. In short, the only workable fire escape planter box is a compact, noncombustible, firmly secured one used with strict attention to egress, load limits, drainage, and local regulations.