From Chabad.org:
A special mitzvah, which can be fulfilled only once a year, is to recite the bracha (“blessing” or prayer) made upon seeing a fruit tree in bloom:
“Baruch Ata Ado-nai, E-loheinu, Melech Ha’olam,
shelo hasair b’olamo kloom,
ubara bo briyot tovot v’ilanot tovot,
leihanot bahem b’nai adam.”
“Blessed are You, G‑d, our Lord,
King of the Universe,
that His world is not lacking anything,
and He created in it
good creations and good trees
for the pleasure of mankind.”
It can be done anytime during the month of Nissan (referred to by the Torah as “the month of spring” ).
Many visit botanical gardens during this time, so as to avail themselves of an opportunity to observe this beautiful mitzvah.
Corrections:
shelo cheeseir (not chaseir)
uvara vo (not ubara bo)
very beautiful mitzvah, not to be missed if at all possible.