In the opening lines of Milton's "Lycidas," the speaker declares, "Yet once more, O ye laurels, and once more / Ye myrtles brown."
The laurel leaves are symbolic of poetic accomplishment. This is an association that dates back to ancient Greece, when poets (as well as successful athletes and military commanders) were crowned with wreaths of laurel to signal their skill. This is why we have poet laureates today. The word "laureate" derives from the word "laurel."
Myrtle, in Greek myth, is associated with the goddess of love, Aphrodite. Myrtle is thus associated with love but also, because of its white flowers, with purity and innocence.
When at the beginning of "Lycidas" the speaker plucks the berries from the laurels and the myrtles and "shatter[s] [their] leaves before the mellowing year," he is metaphorically representing the early death of his friend, for whom this elegy is composed. That friend was named Edward King. Milton and King met at Cambridge University, but the latter tragically died in 1637 at the age of just 25.
The laurels and the myrtles are plucked from the ground too early, just as Edward King was, as it were, plucked too early from the world. The myrtles represent the innocence and purity that Milton associates with King and which were destroyed too soon, and the laurels represent perhaps the poetic inspiration that Milton feels he needs to compose an elegy befitting his friend. He symbolically plucks this inspiration from the ground because he feels that he needs all the help he can get to do his friend justice.