Rosh Chodesh Nissan (Sunday April 6th) marks the beginning of the season for Birkas Ha'Ilanos - the blessing we recite once a year upon seeing fruit trees in bloom. Men, women and children should make this beracha. The beracha may only be said upon seeing the flower bloom that precedes the growth of the fruit.
In a leap year, when trees may begin to bloom in Adar (particularly in Eretz Yisroel), or in locations where Spring takes place in Elul or Tishrei (ie. Australia, Argentina, Brazil) the beracha may be said in those months.
The text of the beracha is as follows: "Boruch ata Hashem Elokeinu melech haolam shelo chisar ba'olamo klum, uvara vo beriyos tovos v'ilanos tovim lehanos bahem b'nei adam."
If one did not make the beracha timely in Nissan because he did not see a tree, one may bi'dieved make the beracha until such time that the fruit of the tree has not yet ripened. However, if one did see a tree in Nissan and neglected to make the beracha, he may only make the beracha until the fruit begins to grow.
The beracha should not be said on Shabbos unless it is the last day of the month of Nissan and one has not yet made the beracha.
Before the blessing is recited some have the minhag to say V'yehi Noam (last posuk of Tehillim 90) followed by Hallelukah hallelu Keil min ha-shamayim (Tehillim 148). The beracha should preferably be said with a minyan of men (with a kaddish following), but it may be said individually too.
The Chida writes that one should especially concentrate when saying this beracha because it is misakain (repairs) the neshamos that have been sent back to this world in the trees and plants, and one has the opportunity to ask for mercy on these souls at this time.
Shulchan Aruch w/Mishnah Brurah 226:1, Kaf Hachaim 226, Moed L'chol Chai 1:6, S'V Har Tzvi 226, Shulchan Hatahor 226, Sdei Chemed Vol. 2, Maareches Berachos pg. 265.