Shulchan Arukh Orach Chayim · Siman 226

English

One who goes out in the month of Nisan and sees trees that have put forth flowers says, "Blessed Are you....who has not left anything lacking in the world and who has created in in good creations and good trees from which people can benefit." And this blessing can only be made once each and every year and if one delays until after the fruit has grown on the tree it is too late.