In the following 10 lines the speaking voice starts immediately with a simile:  he compares his lover to a vegetable that should grow up vaster than empires but last more than empires do. A vegetable can be a simple metaphor for his love. Or, these lines could mean that time acts as nourishment for his love.

Going on reading you have a hyperbole, an temporal exaggeration that has a strong ironical tone. In fact he mocks her. The poet does not agree with her coyness because the time is very little and he can't wait. Time pasts and they become older, the useful time to live a relationship becomes day after day less convenient for both. With his ironical tone he tries to convince her by his love and to persuade her to have a closer relationship.