The Halacha says that if someone sees Ilanot Korchim, trees that have began to bud, so he must make a Beracha; “Baruch Ata Hashem Elokenu Melech HaOlam Shelo Chisar BeOloamo Kelum, Uvara Vo Briyot Tovot V'Ilanot Tovot L'Hanot Bahem B'nei Adam.” This is the Beracha we call Birkat HaIlanot.
Preferably, the Beracha should be made in the month of Nisan. Although the Beracha can really be made so long as you can see the trees budding, but according to the Kabala, there is a lot of mysticism behind the Beracha Birkat Hailanot.
Read more: http://www.dailyhalacha.com/m/halacha.aspx?id=247
You can click here to see HaRav Ovadia Yosef fulfilling the mitzvah at the earliest opportunity, this past Tuesday on Rosh Chodesh Nissan.