By the expression the ‘flag of breezes free’ the poet compares the leafy green branches of a tree to a flag flying freely in the breeze. 

As a flag flutters during a gentle breeze,the green leafy branches of the tree also flutter and sway along the breeze.

The leaves are compared with the fluttering flag during a gentle breeze.As the breeze blows the leaves also flutters like the flag providing the fresh air.