"A special mitzvah which can be fulfilled only once a year—anytime during the month of Nisan—is to recite the berachah...made upon seeing a fruit tree in bloom." (source)

"Nisan marks the birth of spring, signaled by the arrival of new buds and the birth of the Israelite nation. Physical and spiritual seeds are planted in Nisan—seeds for the summer crop and seeds of freedom and liberation." (source)