I bought a 5' oleander grown as a standard at the end of last summer. Wintered it over in a sunny room and then moved it outside when the weather warmed. It has bloomed ever since, but each flower is sort of rimmed with a black edge. I don't know if that means too much sun, not enough water, not enough fertilizer or what. It's in a pot 14" in diameter and 12" deep, potted in topsoil. Any advice will be greatly appreciated.