The influence of Roethke on Plath's poetry is no secret, however in an essay by Roger Elkin, 'Hidden Influences in the Poetry of Ted Hughes and Sylvia Plath', Elkin explores the idea that Roethke's poetry and creative theories greatly influenced Hughes also.
It's a fascinating essay detailing not only the similar themes and techniques such as animism and minimism and use of Jungian symbolism in the poetry of all three but also the role/purpose of poetry to tap into and explore the primitive, pre-conscious thus universal self. He obviously analyses Plath's 'Poem for a Birthday' but also thoroughly analyses Hughes' 'Wodwo' in the light of Roethke. You can find the essay here.