Which flower did the poet see in the bower?

I'm assuming the poem you're referring to is "Lines Written in Early Spring." It's a well-known poem by Wordsworth, and it fits the description in your question. If that's the case, the poet actually describes seeing two kinds of flowers in the bower: primrose and periwinkle. Here's the full quote: "Through primrose tufts, in that green bower, / The periwinkle trailed its wreaths." (9–10)
Broadly speaking, "Lines Written in Early Spring" is a poem that laments the gap between man and the natural world. Like many Romantic writers, Wordsworth describes a world in which humanity has deserted its innate and spiritual relationship with nature as a result of industrialization, urbanization, etc. The primrose and periwinkle are therefore symbolic of an innocent and happier state of existence; Wordsworth talks, for instance, about the flowers "enjoy[ing] the air [they] breathe." (11) On that note, it's perhaps significant that the poem specifically takes place in "early" spring, and that the primrose is in fact one of the first flowers to come into bloom each year; the implication is that humanity has fallen away from its own purer beginnings.