I would use a nickel rod in the stick welder. Prep the joint like you would with steel, clamp across the crack at the top so it stays even as you weld. Preheat the whole thing with a torch, any kind even propane. Just need to get the cast temp up. I would weld in 1-2" beads and move around. When your done hit the whole thing with the torch again until it seems to be even temp on the whole thing. Then let it cool slowly. For a flower pot, I only see the need to weld one side. Your choice if you want the crack visible on the outside or weld on the outside.
Bill
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