The speaker of a poem (or any work of literature) is always tricky. Obviously, the speaker of the poem is always technically the poet. We mean, they wrote the thing. But this poem gives us some other options, too.
The first stanza is told from an outside observer. However, distant as they may be, they still totally have an opinion on what's going down. The speaker uses exclamations to describe the flower just the way that Persephone saw it and strongly emphasizes her isolation (7-8). In line 8, the speaker pronounces judgment on Persephone, so this is someone who knows her well, either literally or metaphorically. They can get inside her brain a bit, and then they feel confident enough to say tell us why this all went down.
It's the parenthetical statement (9-12) that gets really murky. The speaker is absolutely a mother, or at least a child remembering her mother's words. She demands and commands even to the point of overprotection. We can imagine a lot of eye-rolls going down on the part of the listener.
This could be Demeter, Persephone's mother, but might also be the poet Rita Dove connecting with Persephone's experience by remembering her own childhood. The final two lines sound like that same voice at the end of the first stanza that knows, but is above both of these figures and is able to comment on the whole scene. Lucky her.