What is the ruling on placing plants, such as cacti, on a grave, by analogy with the action of the Messenger of Allah (peace and blessings be upon him) when he placed palm fronds on two graves?

It is not legislated to plant trees or crops on graves, for the Messenger of Allah, peace and blessings be upon him, and his rightly-guided caliphs did not do so. His action, peace and blessings be upon him, with the two tormented graves by planting a palm frond was specific to him and to them.