There is no single schedule that fits every urban planter box. In most cases, a full soil replacement is needed every 2 to 5 years, with a lighter annual refresh in between. For planter boxes growing annual flowers or vegetables, replace most or all of the soil at the end of each season, or at least renew the top third with fresh potting mix and compost. For perennials, shrubs, or small trees, top-dress yearly and repot with fresh mix every 2 to 3 years; a complete replacement every 3 to 5 years is usually enough. Small containers dry out, compact, and lose nutrients faster, so check them every year. Large built-in urban planters can last longer if drainage is good and fertility is maintained.
Replace the soil sooner if you notice poor drainage, water pooling or running off the surface, hard compacted soil, stunted growth despite feeding, yellowing leaves, salt crust on the surface, foul smells, pests, disease, or roots circling the pot. In busy urban areas, especially when growing edible plants, avoid using soil of uncertain quality and refresh it more often if dust or pollution is a concern.
Good maintenance can extend soil life. Remove old mulch, loosen the top layer, add compost, confirm that drainage holes are clear, and avoid overfilling the box. When you do replace soil, you do not always need to remove every bit. If roots are healthy, shake off loose old soil, trim damaged roots, and refill with fresh container mix. Water deeply after repotting. A practical rule is to check the soil every year, refresh it annually, and plan a full replacement every few years based on plant type, box size, drainage, and plant health.