Birkath Ha-Ilanoth (the blessing on the trees) should be done in the month of Nissan, and "Zerizin Maqdimin Lemiswoth" (the earlier it can be done the better).
While it is best to go outside the city to recite this blessing, if this is not practical, or cannot be done, the blessing can be recited inside the city. In order to recite the blessing, you need to have two fruit trees -- they can both be the same type of fruit tree -- but their buds must have flowered. Till the buds have flowered you cannot recite the blessing and once the flowers have fallen, you can no longer recite the blessing.
Ideally, the blessing should be recited with a Minyan (quorum). Sometimes, when the weather is cold, the buds do not flower till after Nissan. In such a case, the blessing can still be recited, but without Shem Umalkhuth (without mentioning G-d's Name in the blessing).
(See Sefer Orah Hayyim, Haggadah, Hilkhoth Birkath HaIlanoth)
