First, decide whether the crack is cosmetic or structural. A hairline crack that does not leak can often be repaired, but a long crack that runs through the base, loose pieces, or a planter that already leaks may need replacement or professional reinforcement. Empty the planter, remove the plant and soil, and brush away dirt from both sides of the crack. Clean the area with isopropyl alcohol and let it dry completely; epoxy and sealant will not bond to damp ceramic.
For a small crack, use a two-part ceramic or stone epoxy. Mix it according to the package directions, work it into the crack with a craft stick, and press the edges together if they are stable. For a wider or chipped area, use epoxy putty because it fills gaps and can be sanded after curing. Avoid ordinary superglue, which is brittle and may fail outdoors or when wet. If the planter holds water, reinforce the inside with a compatible waterproof patch or fiberglass mesh embedded in epoxy, then smooth it before it sets.
After the epoxy cures, apply a waterproof, plant-safe sealant to the inside if the planter is designed to hold soil and water. Make sure the drainage holes remain open. Let the repair cure fully, usually 24 to 72 hours, then fill the planter with water and check for leaks. If it still leaks, dry it and apply another thin layer of sealant. Replant only after the repair is completely cured, and avoid moving the planter by its rim or dropping it, since impact and freeze-thaw cycles are common causes of new cracks.