Poets use apostrophe so the speaker can address an abstract idea, quality, imaginary person or non-human object. This is a kind of extended personification because the speaker supposes that the idea or object he’s speaking to actually understands him. In this poem, the speaker is promoting a “seize the day” approach to life. He is frustrated that the object of his affection is wasting time, either by rejecting his advances or because of shyness.
Using apostrophe, addressing a rose, implies that the speaker is literally not with this woman. He and she are detached. He could have spoken directly to her or been thinking these things in her presence. Since he is asking a rose to speak to her, we know that they are apart. This use of apostrophe in this poem is supported by lines which indicate they are currently separated and that he wishes they were together.
Tell her that’s young,
And shuns to have her graces spied,
That hadst thou sprung,
In deserts where no men abide,
Thou must have uncommended died.
He asks the rose to tell that a rose in a water-less desert with no men to care for it will die “uncommended” (unloved or unnoticed). The speaker wants the woman to forget her shyness and accept his love. Thereby, she will be noticed, loved and “commended.” The use of apostrophe illustrates the current separation between the speaker and the woman he loves.