Is it permissible to plant a tree on a non-Muslim relative's grave

Answer

According to the majority of the fuqaha of the four schools, it is permissible, and even encouraged, to plant small plants on graves, asking Allah to ward off the punishment of the grave for the deceased for as long as the plant remains; if shrubs or grass grow naturally, removing them is disliked, since they glorify Allah in a way imperceptible to us, and that glorification brings barakah to the grave. This is based on the hadith of Ibn 'Abbas, in which the Prophet passed two graves and said the occupants were being punished for minor matters -- one for not protecting himself from urine, the other for spreading tale-bearing -- then split a fresh palm stalk and planted one piece on each grave, saying it might lighten their punishment so long as the pieces did not dry out.