There's a lot of nature in this poem. The first half of "To the Virgins" talks about flowers and the sun. Even though the speaker talks about rosebuds and about how the sun rises, he also reminds us that flowers die – that their bloom is only temporary – and that the sun sets. People are a lot like the natural world in this poem; they too have their "prime" (15), in which everything is pretty, but they also have their "setting," that time when the bloom starts to fade.
The speaker suggests that human beings are like natural objects (the sun, flowers), but also that they are different. In a way, it both endorses and questions its own terms (i.e., that man is like a flower, or the sun).
The flower is an appropriately double-edged metaphor in this poem. It represents both the virgins, who might suddenly die and should therefore take advantage of their bloom, and the opportunities – the "rosebuds" – their bloom affords.