Is it permissible to plant flower trees around a grave?

Note: Placing flowers on graves is considered a bid‘ah (innovation in religion). As mentioned in Fatāwā al-Lajnah al-Dā’imah (the collection of fatwas issued by the Permanent Committee for Islamic Research and Ifta, one of the highest fatwa authorities in the Arab world):

وضع الزهور على قبور الشهداء أو قبور غيرهم- من البدع التي أحدثها بعض المسلمين في الدول التي اشتدت صلتها بالدول الكافرة، استحسانا لما لدى الكفار من صنيعهم مع موتاهم، وهذا ممنوع شرعا لما فيه من التشبه بالكفار،
 

Placing flowers on the graves of martyrs or others is a bid‘ah (innovation) introduced by some Muslim countries that maintain close ties with non-Muslim nations. They imitate the disbelievers, who place flowers on their graves. In Islamic law, this practice is prohibited because it involves resemblance to non-believers. (Fatāwā al-Lajnah al-Dā’imah, Fatwa No. 4023)

In many regions of our country, there are also some superstitions and misconceptions about the trees, flowers, and fruits found in graveyards. Some people believe that using or eating the flowers or fruits from graveyard trees can cause misfortune or physical or mental harm.

From an Islamic perspective, these beliefs have no basis. The flowers and fruits of graveyard trees are just like those from any other trees  they may be used or eaten without any harm or restriction.