Does Islam support putting flowers on a grave to reduce punishment?

On a recent visit to a Muslim graveyard, I see people put rose petals on the graves of their loved ones. On inquiry, I was informed that the Prophet (SAW) once put a leaf on the grave to reduce the punishment of that grave's occupant. So my queries are: 1. Does putting leaves/flowers reduce one's punishment? 2. Can leaves as used by the Prophet be substituted by flowers?