Siman 226

One who goes out during the days of Nisan and sees trees that are producing blossoms says: "Blessed are You, Hashem our God, King of the universe, Who has withheld nothing from His world, and has created within it goodly creatures and goodly trees for human beings to enjoy." This means that he gives praise and thanks to the Blessed Name for creating for the sake of man even things that are not essential for human life, such as the fruit of trees. Therefore, we do not recite this blessing over seeds and vegetables, as these are like essentials, whereas fruit is not so. And he blesses at the time of blossoming, for then it is evident that they will produce fruit. In our countries, it is not in Nisan but in Iyar or the beginning of Sivan, and then we recite the blessing. This blessing is only once a year even if he sees other trees, for this blessing is a general blessing of thanksgiving for His kindness and goodness, may He be blessed. And regarding one who is careful with this blessing, it is said of him: "See, the fragrance of my son is like the fragrance of a field which Hashem has blessed" (Eliyah Rabbah).

The Tur and the Shulchan Aruch wrote that if one delayed reciting the blessing until after the fruits grew, he should no longer recite the blessing, but the Mordechai wrote that he should recite it. And some say that they do not disagree, for the Mordechai is dealing with a case where he did not see them at all until after their growth, and the Tur and Shulchan Aruch are dealing with a case where he saw them previously and did not recite the blessing, and then he should not recite the blessing (Bach). And there is one who rejected this matter, for certainly it does not depend on the first sighting (Prisha). And it is also possible to say that the Tur and Shulchan Aruch are dealing with a case where he did not see them until after the completion of their growth, for then there is the blessing of 'shehecheyanu', and the Mordechai is dealing with a time before the completion of their growth (and so it appears in Eliyah Rabbah see there and see Prisha), and so one should act. However, regardless, this blessing is weak among the masses (and see Bedek HaBayit who wrote in the name of the Ri that they were not accustomed to recite the blessing, but every Torah scholar and those who fear Hashem are careful with this blessing).