The ruling on planting trees on graves following the Prophet (peace and blessings be upon him)

Question:
Honorable Shaykh, may Allah grant you success. A questioner from outside this country says: In our cemetery, they plant olive trees on the graves because they remain evergreen, following the action of the Prophet (peace and blessings be upon him) when he placed a branch on a grave. He asks: What is the guidance on this?

Answer:
The Prophet (peace and blessings be upon him) was given knowledge by Allah, so he knew that it would lighten the punishment for them as long as it did not dry out. As long as it did not dry out, this green thing that the Messenger (peace and blessings be upon him) placed was a cause for lightening the punishment of the one buried. The Messenger knew this because Allah taught him. But we do not know, so we do not place anything on the graves.