The gemorah in Brochos 43b says that when one is walking in the month of Nisan and sees the trees beginning to flower, he should make a brocha. (see Shulchan Aruch 226:1 for the exact wording of the brocha). According to the Mishna Berura this brocha can be made any time upon seeing a tree flowering, even if not during Nisan. Kaf Hachaim argues. The tree must be a fruit tree. If the brocha was not made upon seeing the tree the first time, the brocha can still be made as long as the fruit is not ripe.

Rabbi Tzvi Aryeh Hyman, Mir Yeshiva
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