What is the origin of placing flowers on a grave?

Flowers are offered to honor spirits.

ANSWER:

In modern times, we place flowers on graves as a sign of respect or remembrance, but the origin of this practice is not so pleasant. It was done as matter of necessity. Evidence of flowers having been placed on graves can be traced back to the Stone Age. The flowers were placed in an attempt to alleviate the stench of the rotting corpse beneath and hopefully to prevent animals picking up the smell of the corpse and digging it up.