It is said in one of the Bukhari hadith that the prophet once was passing and pointed to graves and said that they were being punished. One because of backbiting and other because of the impurities while urinating. Then the prophet took palm plants and planted on the graves.
The ulama (scholars)say that everything in this world praises Allah including the plants, which will reduce the pain of the people in the grave. I learned that scholars in some Arab countries forbid the act of putting roses or leaves on the graves, when the scholars in the Indian subcontinent take reference to the above Hadith and support the putting of rose leaves on the graves. Please answer these differences in detail.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

This matter, i.e. putting roses and fresh branches on graves, is a matter of a difference opinion among the scholars. Some of them said that it is permissible to put fresh roses and branches on graves given the evidence stated in the Hadith mentioned in the question. The Prophet (Sallallahu Alaihi wa Sallam) put palm branches in order to reduce the punishment of those in the graves since these palm leaves bring blessings because they make Tasbeeh (remembrance of Allah). So based on this, it is permissible to put fresh tree branches and the like on graves.

However, some other scholars said that it is not permissible to put any of the above on graves. We think this is the preponderant opinion for many reasons:

1.     Imam Muslim reported the Hadith narrated by Jabir, the meaning of which is that the torture is reduced because of the intercession of the Prophet (Sallallahu Alaihi wa Sallam) and not because of the Tasbeeh (remembrance) of the palm branches. The Prophet (Sallallahu Alaihi wa Sallam) said: "I passed by two graves, and those two persons (in the graves) were being tortured. So I hoped by my intercession, that their punishment be lessened till the branches become dry." (The Prophet (Sallallahu Alaihi wa Sallam) had put one branche on each grave). So this means that the reduction of the punishment was due to the intercession of the Prophet (Sallallahu Alaihi wa Sallam), and the period of the reduction of the torture is until the leaves become dry. This is how Al-Albani interpreted this Hadith.

2.     Putting roses on graves for the purpose of reducing the punishment is a bad assumption about the dead person, because we do not know whether he is having pleasure or is being punished. This is a matter of the unseen. So if we say that it is permissible to put roses on his grave, this means that we consider him among the people who are being punished. He (the dead) could be in reality in the pleasures of the grave, and we would have thought badly of him.

Putting roses especially on graves is not permissible if the person doing so intended to imitate the Kuffar (disbelievers). The Prophet (Sallallahu Alaihi wa Sallam) warned us against imitating the Kuffar saying: "He is not amongst us who imitates the people different from us. Do not imitate the Jews or the Christians." Reported by Thirmidhi.

Allah knows best.