Yes—you can hang an urban planter box from a pergola or overhead beam, but only if the structure and fixing method are safe. A lightweight planter plus soil, plants, water, and brackets can become surprisingly heavy, and that load is permanent. Before hanging anything, check what the beam is designed to carry and whether it is structural. If you are unsure, ask a structural engineer or a qualified contractor; do not rely on guesswork, especially on rented or shared property.
Choose a planter that is designed for hanging or can be reinforced for it. Use galvanized brackets, chains, cables, or straps rated for well above the expected wet weight, and attach them to solid timber, steel, or masonry with the correct fixings. Avoid hanging from thin slats, gutters, loose fascia, or decorative trim. Spread the load across more than one fixing point where possible and use a backing plate or spreader bar to reduce point stress.
Think about wind, drainage, and access. Hanging planters swing and catch wind, so leave clearance and use a stable mounting system. Make sure drainage water will not stain the pergola, drip onto people, or cause rot. Position the box where you can water, prune, and remove it safely—ideally without a ladder. Use lightweight potting mix and drought-tolerant or trailing plants to keep weight down. Check the fixings regularly and after storms, and reduce the load if you notice movement, cracks, rust, or loose hardware. If the beam flexes, creaks, or shows any sign of stress, stop and get professional advice.