What kind of figure of speech is My vegetable love should grow Vaster than empires and more slow?

The line "My vegetable love should grow Vaster than empires and more slow" employs a metaphor as its primary figure of speech. The speaker compares their love to vegetables, suggesting it is natural and nurturing, while also contrasting its growth with the vastness of empires and the slow, steady pace of nature. This juxtaposition highlights the depth and enduring quality of their love, emphasizing its richness and gradual development.