A cracked urban planter box can often be repaired for much less than the cost of a new one, but the right method depends on what it is made of and whether the crack is cosmetic or structural. Start by emptying the planter, removing loose soil, and cleaning the crack with a stiff brush. Rinse and let it dry completely; sealants and epoxy will not bond to damp, dirty surfaces. Put on gloves and eye protection. If the crack is in a rigid material such as concrete, stone, or hard plastic, you can drill a small relief hole at each end of the crack to stop it spreading. Do not over-drill, and avoid this step on thin flexible plastic unless you are confident.
For wooden planters, remove any splinters and check for rot. If the wood is soft, chisel out the rotten section and patch it with epoxy wood filler; if it is only split, apply exterior wood glue to the crack, add a dowel or spline for strength if needed, and clamp it until cured. Add an internal brace with galvanized screws or a wooden batten behind the crack. Once repaired, seal the inside with a waterproof planter liner or a non-toxic waterproof sealant, and make sure drainage holes remain open. For plastic, resin, or fiberglass planters, roughen the area with sandpaper, clean it again, and use a two-part epoxy or a plastic repair compound. For a long crack, embed fiberglass mesh in the epoxy for reinforcement. Flexible plastic may need a flexible plastic adhesive rather than a rigid epoxy. Seal the finished repair with a waterproof polyurethane or silicone suitable for the material.
Concrete or stone planters usually need a hydraulic cement, concrete patching compound, or epoxy injection for structural cracks. Clean the crack, dampen the surface if the product requires it, press in the patch, and smooth it before it sets. For larger cracks, reinforce with wire mesh or fiberglass mesh. Metal planters should be sanded to remove rust, treated with a rust converter, patched with metal epoxy or a riveted metal plate, then painted with a rust-resistant exterior paint. In all cases, a waterproof inner liner can protect the repair and extend the planter's life, but never seal over drainage holes. After repairing, let the planter cure for the time recommended on the product, then fill it and watch for leaks. If the crack is very large, the planter is bowing, or it holds a heavy load such as a small tree, add an internal brace or consult a professional; safety matters more than a cosmetic fix. With the right preparation, reinforcement, and waterproofing, most urban planter boxes can be repaired instead of replaced.