Lilacs (Syringa) are often pruned too late, and this will remove the flower buds for next year’s crop of flowers. The proper time to prune Lilacs, Forsythia, double flowering Plum, Cherry and any other shrubs that flower on old wood is right after flowering. Other flowering facts to remember: To set flower buds, Lilacs need plenty of direct sun in summer. Cold snaps in late spring may damage flower buds on Lilacs and other shrubs.