During the month of Nissan, trees start to blossom (in the Northern hemisphere).
The first time a year that one sees edible fruit trees blossoming one says:
Once the flowers have fallen off and the fruit is visible then one can no longer say this Bracha.
One makes this Bracha only once a year.
Source: Shulchan Aruch, Orach Chaim, 226:1,
Wednesday, 2 Nissan 5770