In what way are the blossoms pledges of the fruitful tree?

Blossoms come before the fruit. Their presence means fruit will appear.

What is personified in the poem blossoms

Blossoms are flowers. A pledge means a promise. When the poet says ‘Fair pledges of a fruitful tree’, he is connecting the blossoms to being the promise of a fruitful tree. A promise is made keeping the future in mind. So, the blossoms are making some kind of a promise to the trees. When blossoms appear on a tree, only then will the tree go on to bear fruit. Therefore, when the flowers appear, it is nature’s assurance that at a later stage, fruit will definitely grow on the tree. Hence blossoms are ‘pledges’ of a fruit-bearing tree.