It's nice outside today. The daffodil is opening its petals.
Sounds like a line from the Handmaids Tale.
Those 2 are unrelated
The daffodils are blooming
Yes, that's fine. It has a lyric quality to it. Over 99% of people would undoubtedly say, "The daffodils are opening their petals." And that would be grammatically fine, too. But your sentence focuses the reader's attention on one particular flower, and thus paints a more vivid picture in the reader's mind.
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But, doesn't that imply that the daffodil is actively doing something? I would say, "The daffodil's petals are opening." I think "petals" should be the subject, not "the daffodil."
I'm not a botanist, but it would seem to me that the opening of the petals is done by the entire flower. The stem must carry nutrients and moisture up from the soil to nourish the flower; The leaves exchange carbon dioxide and chorophyll and whatever in a process that I have long forgotten; and eventually the flower is able to open its petals. So while the sentence is more figurative than literal, it would seem to also be correct on a literal level. Let's ask @firefly5 .
I'm okay with the phrase "the daffodil is smiling."
And yet again.