The central idea in Andrew Marvell's poem "To His Coy Mistress" is that the two lovers have only a little time in which to enjoy their love. The first part of the poem is taken up with the speaker's protestations of how he would behave if this were not the case. If they had hundreds or thousands of years together, he would be quite happy to proceed slowly with his courtship.
This idea of addressing one's mistress in these terms is not an original one and is particularly common in the cavalier poetry of the seventeenth century. Marvell's poem would probably not be read today if it were not for the compelling and unusual imagery with which he makes his case. Much of this is grotesque, like the notorious image of worms violating his mistress's "long-preserved virginity" in the grave.
The idea of vegetable love is not as shocking as this, but it is unusual, striking, and slightly grotesque. Love is often compared to a beautiful flower, seldom to a slow-growing vegetable. The meaning of the image is clear: vegetable love is love that increases so slowly that its growth is imperceptible. Its poetic value, however, lies in the fact that the image is original enough to stay in readers' minds, perhaps even changing the way they think about love.