The Special Collections Division of the Allen Library at the University of Washington, Seattle, has a small cache of Sylvia Plath and Ted Hughes materials. These are held in the Theodore Roethke papers . Theodore Roethke (1908-1963) met Sylvia Plath and Ted Hughes in London at a party in London on 1 February 1961. She wrote about meeting him to her mother the following day, saying "I find he is my influence" ( Letters Home 407). Plath described him a bit, as well: "He's a big, blond, Swedish-looking man, much younger-seeming than his 52 years . . . Ted and I got on well with him and hope to see him again" (407). Plath's poems written at Yaddo were largely influenced by Roethke's "greenhouse poems" from his collection The Lost Son (Doubleday, 1948). Often Roethke's influence on Plath starts and stops with the likeness between her "Poem for a Birthday" sequence in The Colossus (Heinemann, 1960). However, perhaps Plath continue...
Sylvia Plath Info Blog by Peter K. Steinberg. The blog of A celebration, this is.