Start with a simple rule: repair the planter only if the crack is cosmetic or localized and the box still holds its shape. Replace it if the crack runs through a load-bearing corner, the sides are bowing, water pours out, or the material is rotten, crumbling, or delaminating. Urban planter boxes take a lot of stress from soil weight, freeze-thaw cycles, irrigation, and foot traffic, so a repair that ignores structural damage usually fails.
First, empty the planter and move it to a flat work area if possible. Remove loose soil, clean the crack with a stiff brush, and let the surface dry completely. Check the inside as well as the outside. A crack that is only in the finish or a small surface chip is usually easy to fix. A crack that goes all the way through the wall, especially near a corner or base, needs reinforcement or replacement.
Match the repair to the material. For plastic or resin planters, use a two-part epoxy made for plastics or a plastic welding kit; rough up the area first so the repair bonds well. For wood planters, use waterproof wood glue plus stainless screws or a metal brace, then seal the repaired area with outdoor-rated wood sealer. For concrete or stone planters, clean out the crack and fill it with masonry epoxy or a flexible concrete patching compound. For fiberglass planters, use a fiberglass resin repair kit and add a patch on the inside if you can reach it. Metal planters should be treated for rust, patched if needed, and repainted with a rust-resistant coating.
After filling or patching, smooth the repair, let it cure fully, and apply a waterproof sealant suitable for the material. If the planter has drainage holes, make sure the repair does not block them. Refill with fresh soil only after the repair has cured. In many cases, adding a brace or liner inside can extend the life of the planter, especially if the crack is near the base.
Choose replacement instead of repair when the planter is large and heavy, the crack keeps reopening, the walls are bowed, or the repair cost is close to the price of a new box. Replacement is also the safer choice for public or high-traffic urban spaces where failure could cause injury or property damage. If you repair it, monitor the crack after the first rainy season or freeze-thaw cycle. If it spreads, replace the planter rather than patching it again.