You've probably heard the phrase "in the blossom of youth" before. It makes sense to us because we've all seen the unblemished beauty of a newly-blossomed flower. Take a rose, for example. If you've ever touched the blossoming petals, you understand how soft and smooth they are—kind of like the face of a young person, before age moves in.
When we see the flowers in the first and fourth stanzas of this poem, we're definitely meant to make those associations with Laura, who is constantly surrounded and bejeweled with them. For Petrarch, she is forever young and beautiful, always blossoming like the flowers around her. It's a great way to be, if you can manage it.