First, before you attempt to pipe anything, be sure to let your cake cool completely! Even the sturdiest buttercream will melt on a warm cake. Of all types of buttercream (Swiss meringue, Italian meringue, and American), American is the sturdiest for piping flowers and such because the butter can be firmed up with cold temperature. In fact, I recommend piping any flowers ahead of time and freezing them before decorating.