Is it permissible to plant a tree at a deceased person's grave?

Planting trees at graves with the intention of worship is an innovation (bid'ah) because it was not part of the guidance of the Salaf (pious predecessors). As for the action of the Prophet, peace and blessings be upon him, of placing palm branches on two graves to alleviate their torment, that was specific to him. However, if the intention behind planting trees is for those who come to the cemetery to bury the dead to shade themselves under them, then it is a good deed and an ongoing charity (sadaqah jariyah).