What is the ruling on buying flowers and placing them on graves and pouring water over them, and is it recommended to place green plants on them?

Placing flowers on graves is not permissible because it is a waste of money without benefit and involves imitating Christians. Giving the money spent on them in charity for the deceased is better. As for pouring water on the grave, if it is to solidify the soil and protect the body from predators, then there is no harm in it. But if it is for any other reason, then there is no basis for it.