Praise be to Allah.
There is no basis for this. If trees or other plants grow on graves it is not a sign that their occupants were righteous. That is a false notion. Trees may grow on the graves of righteous and evil people alike, not only on the graves of the righteous. We should not be deceived by the claims of the myth-makers and followers of deviant beliefs who claim otherwise. And Allaah is the One Whose help we seek.