Petrie's work tends to be diverse, in form, technique, and subject matter. His poems, like the world they illuminate, are multifaceted and so are hard to sum up. If there is one theme that appears to be pervasive, it is mutability, the transience of everything, and the various ways we humans attempt to deal with it in our lives[1].
In some of his pieces he employs traditional forms and meters (though with considerable latitude), and in others he uses more open forms and freer rhythms, while still maintaining a voice uniquely his own[2]. In general it is not inaccurate to say that he puts more stress on the musical elements of poetry than is currently common.
Above all, he dislikes dogmas, whether in theory, subject matter or technique, and uses whatever seems best suited to the matter at hand.