My geraniums the stems below the flower head are slitting open and flowers dying
The geraniums have been bought from various sources and the problem effects all plants
If this issue of splitting is also marked with browning stems it could be Bacterial Stem Rot.
If you don't have any browning spots on the stems and plant; verify the growing conditions.
On a personal note; I do like to feed my Geraniums with Epsom salt through the summer; light feedings help them retain a good colors and stay strong looking.