• Cluster formation of charged grains in magnetized dusty plasma with external force field. • Long-time trajectories of the charged grains in the cluster form regular flower patterns. • The patterns tend to become simpler in time until a steady state is reached. Long-time evolution of charged grains in a two-dimensional magnetized plasma under a spatially harmonic external force is investigated using molecular dynamics simulation. It is found that the grains can converge into a single quasi-steady crystal disc, and their trajectories trace out a complex but regular flowery pattern that gradually becomes simpler in time until a quasi-steady rotating pattern is reached. [ABSTRACT FROM AUTHOR]