Start by emptying the planter and removing any soil, grit, or loose glaze around the crack. If the planter is outside, move it to a dry, shaded workspace. A small hairline crack that is not leaking can often be repaired successfully. A crack that runs through the base, has shifted pieces, or opens when you lift the planter may need replacement because it may fail under wet soil and freeze-thaw cycles.
Clean the crack thoroughly. Wash with mild soap and water, rinse, and let it dry completely. Wipe the area with isopropyl alcohol to remove dust and oils. If the edges are loose, gently pick out any crumbling ceramic with a plastic scraper. Do not enlarge the crack unnecessarily.
For most ceramic urban planter boxes, a two-part epoxy putty or ceramic epoxy adhesive is the best choice. It bonds to ceramic, dries hard, and resists moisture. Mix it according to the package directions, then press it firmly into the crack with a plastic scraper or gloved finger. Slightly overfill the crack so you can sand it flush later. For a wider crack, first fill the deeper part with epoxy, then add a second layer after the first begins to set.
If the crack is structural or likely to spread, reinforce it from the inside. Roughen the inside surface, apply epoxy, and embed a piece of fiberglass mesh or stainless steel mesh over the crack. Coat the mesh with more epoxy. Let everything cure fully, usually 24 to 48 hours, depending on the product.
Once cured, sand the repaired area with fine wet/dry sandpaper until it is smooth and level with the surrounding surface. Clean away dust. If you want a neat finish, paint the repair with an exterior-grade acrylic or enamel paint matched to the planter. After the paint dries, apply a waterproof ceramic or masonry sealer over the repaired area and around the inside if the planter will hold wet soil.
Wait at least 48 to 72 hours before adding soil and plants. Check that drainage holes are clear, and avoid overwatering. For outdoor planters, protect them from standing water and freeze-thaw stress. If the repair cracks again or the planter leaks badly, replace it rather than risking further damage.