Question: I have a fruit tree that has started blossoming. Do I need to wait until the month of Nissan to recite the beracha on blossom? Does it matter that I have already seen the blossom, or can I still recite the beracha?
Answer: The Gemara (Berachos 43b) teaches that one should recite a beracha on a tree when it blossoms in the month of Nissan. The Shulchan Aruch (OC 226:1) writes that this beracha can only be recited once a year.
According to some poskim (Halachos Ketanos 2:28; Kaf Hachaim OC 226:1) the beracha should only be recited in the month of Nissan, though most poskim (Ritva, Rosh Hashana 11a; Mishna Berura 226:1; Aruch Hashulchan OC 226:1; Tzitz Eliezer 12:20:3) write that one should say it whenever when one first sees the blossom. R’ Ephraim Greenblatt (Rivevos Ephraim 5:173) writes that he witnessed R’ Moshe Feinstein say the beracha in Iyar.
Thus, R’ Ovadia Yosef (Yabia Omer OC 8:8:4; Yechave Daas 1:1) writes that one should ideally wait until Nissan to recite the beracha, though one can still do so afterwards. R’ Yitzchak Yaakov Weiss (Minchas Yitzchak 10:16) writes that where trees blossom at different times, such as in Australia, one recites the beracha then (See Har Zvi OC 1:118; Teshuvos Vehanhagos 1:190).
While the Kaf Hachaim (OC 226:9) writes that one can only recite the beracha upon first seeing the blossom, most poskim (Machatzis Hashekel 226; Shaar Hatziyun 226:3; Halichos Shlomo, Nissan 2:5) write that one can recite it later if necessary.
In conclusion, one can say the beracha upon seeing blossom on a fruit tree even before the month of Nissan, though some are particular to wait for Kabbalistic reasons.