Siman 226
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א הַיּוֹצֵא בִּימֵי נִיסָן וְרָאָה אִילָנוֹת שֶׁמּוֹצִיאִין פֶּרַח, אוֹמֵר: בָּרוּךְ אַתָּה ה' אֱלֹהֵינוּ מֶלֶךְ הָעוֹלָם שֶׁלֹּא חִסֵר בְּעוֹלָמוֹ כְּלוּם, וּבָרָא בוֹ בְּרִיּוֹת טוֹבוֹת וְאִילָנוֹת טוֹבוֹת לֵיהָנוֹת בָּהֶם בְּנֵי אָדָם; וְאֵינוֹ מְבָרֵךְ אֶלָּא פַּעַם אַחַת בְּכָל שָׁנָה וְשָׁנָה, וְאִם אִחֵר לְבָרֵךְ עַד אַחַר שֶׁגָּדְלוּ הַפֵּרוֹת, לֹא יְבָרֵךְ עוֹד.
1 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.