Answer:
As Salaamu Alaikum wa Rahmatullahi wa Barakatuhu
Your query comprises of two questions, is it permissible to plant flower/plant on the grave and the means of conveying reward to the deceased.
As for the first question: It is narrated in the Hadith of Bukhari that Rasulullah Sallallahu Alaihi wa Sallam requested for a date palm leaf, tore it into two and placed half on each of the two graves and said, “it is likely that the punishment of the grave will be decreased until these leaves stay moist”. (Bukhari Shareef vol. 1 Pg: 35)
The commentators of Hadith say, this action is exclusive to Rasulullah Sallallahu Alaihi wa Sallam as there is no specialty in the leaf. It was the blessing of the hand of Rasulullah Sallallahu Alaihi wa Sallam. That’s why Ulama such as Allamah Khataabi have disapproved of people putting leafs on the grave. Mufti Taqi Usmani Sahib writes, although it is proven from Hadith that Rasulullah Sallallahu Alaihi wa Sallam placed a leaf on the grave, it was a one off occasion. Therefore, it should not be made habit or a continuous practice. (Dars e Tirmizi vol. 1 Pg: 286) (References: Fatawa Mahmoodiyah vol. 9 Pg: 175, Malfoozat of Mufti Mahmood Sahib (RA) vol. 2 Pg: 324)
As for conveying reward to the deceased, there many ways of doing that. It is mentioned in the Hadith of Muslim, Rasulullah Sallallahu Alaihi wa Sallam says: when a person passes away, his book of deeds is closed (and no reward reaches him) except in three cases; in the form of Sadaqah Jariyah, or he has left behind the legacy of knowledge that is benefitted from, or a pious child that makes dua for him (Mishkaat vol 1, pg 32).
Allamah Shaami Rahmatullahi Alaihi mentions that a person can convey the reward of Salaah, fasting, charity, recitation of the Quran, zikr, Hajj and Umrah and any righteous deed to the deceased. (Raddul Muhtaar vol 4, pg 10)
Hence it can be taken from here that there are numerous ways of conveying reward to the deceased, and the reward reaches the deceased.
And Allah Ta’aala Knows Best
Muhammad Z Panchbhaya
11th February 2013
30th Rabi’ al Awwal 1434