Blossoms from the bell jar

The Journals of Sylvia Plath, 1950-1962 edited by Karen V Kukil (Faber & Faber, £30, 732pp)

One of Ted Hughes's most important books is his edition of the Collected Poems of Sylvia Plath, published in 1981. There are few precedents - only Mary Shelley springs immediately to mind - for a creative artist taking on the academic role of textual editor of their dead spouse's works.

A combination of personal knowledge and patient scholarship enabled Hughes to arrange Plath's poems in chronological order. To the chagrin of some feminists, he took it upon himself to relegate her early work to the status of "juvenilia", beginning the run of her mature poems in 1956 - the year of their now legendary first encounter at a Cambridge gathering of student poets.

By Hughes's reckoning, Plath wrote 224 poems between 1956 and her suicide at the age of 30 in the freezing February of 1963. He argued that her breakthrough into that uniqueness of voice which constitutes poetic greatness came with the seven-part sequence "Poem for a Birthday", composed in late 1959 while the two were in residence at Yaddo, the artists' retreat in upstate New York. The title of Birthday Letters is, among other things, a tribute to this moment in Plath's career.

Hitherto, her poems had been highly accomplished but somehow brittle. A self-description in a journal entry of late 1955 is harsh but apt: "Roget's trollop, parading words and tossing off bravado for an audience." Very few poems written before Yaddo haunt the reader; almost all the 100 or so thereafter sear themselves into our consciousness. Years earlier, Plath had dreamed of "gathering forces into a tight tense ball for the artistic leap". At Yaddo, she made that leap.

On 10 October 1959, she wrote in her journal: "Feel oddly barren. My sickness is when words draw in their horns and the physical world refuses to be ordered, recreated, arranged and selected. When will I break into a new line of poetry? Feel trite." And on the 13: "Very depressed today. Unable to write a thing. Menacing gods. I feel outcast on a cold star". But then on the 22, walking in the woods in the frosty morning light, she found the "Ambitious seeds of a long poem made up of separate sections: Poem on her Birthday. To be a dwelling on madhouse, nature. The superb identity, selfhood of things. To be honest with what I know and have known. To be true to my own weirdnesses."

Within a fortnight the sequence was "miraculously" written. What was it that released the flow? Hughes pointed to the influence of the poetry of Theodore Roethke, but the entry of 19 October offers a wealth of other clues. Plath records there that she has written two poems that "please" her, "one a poem to Nicholas, and one the old father-worship subject": one to the father who had died when she was eight, the other to the unborn child in her womb - for whom a boy's name has been chosen (although it turns out to be a girl). She was about 18 weeks pregnant. Did the baby quicken and give its first kick at this time? That earlier phrase "oddly barren" becomes explicable as the journals turn to a sense of new life.

There is also confidence in her husband: "Ted is the ideal, the one possible person." Yet in the same entry we find the following: "Involvement with [the novelist] Mavis Gallant. Her novel on a daughter-mother relation, the daughter committing suicide." Plath goes on to make her own plans for a novel. Everything comes together: father, mother, husband, unborn child, new poetic voice, prospective novel about a girl's suicide attempt (which would become The Bell Jar). During the previous months, Plath has been in psychoanalysis, describing her mother as "a walking vampire", exploring her own "Electra complex" and wondering "how to express anger creatively". Back in 1956, a week after meeting Hughes, she had confided to her journal: "I would live a life of conflict, of balancing children, sonnets, love and dirty dishes." With the venting of the anger against her parents and the kick of the child in the womb, the balancing act could begin. Out of it came great art.

Plath's journals were first published, under the aegis of Hughes, in 1982. That edition was incomplete. There were extensive cuts, and two volumes, from August 1957 to November 1959, were excluded. It was Hughes's intention to keep them sealed until the 50th anniversary of Plath's death. Reading the matricidal notes from the time of her psychoanalysis, one can see why.

Shortly before his own death, Hughes changed his mind, as part of that same process of release and reparation which led to the publication of Birthday Letters and the writing of Alcestis (a small masterpiece of simultaneous translation and autobiography). So it is that the surviving journals are now published in full.

Two further bound volumes, covering Plath's final three years, remain lost. One of them may yet conceivably turn up, but the other - which ran up to within three days of Plath's suicide - was destroyed by Hughes. We will never know what she wrote about the process of composing her extraordinary last poems ("The woman is perfected./ Her dead/ Body wears the smile of accomplishment").

This new edition reveals Plath as a much better prose writer than Hughes gave her credit for (he frowned on her stories, smeared as they were with the garish lipstick of Mademoiselle magazine). The woman writing here can be feisty and funny as well as maudlin and self-indulgent.

Inevitably, though, her death casts its shadow back on entries long before - not only the portentous ("I desire the things which will destroy me in the end"), but also, more poignantly, the superficially inconsequential ("Marilyn Monroe appeared to me last night in a dream as a kind of fairy godmother").

The greater the artist, the more we will want to explore that complex alchemy whereby experience is transmuted into imaginative creation. An entry such as that of 19 October 1959 offers a revelation about Plath's discovery that art could be made from truth to her own "weirdnesses". That in itself seems sufficient justification for the publication of material not intended to be seen by strangers. Shamefully, though, the memory of both Plath and Hughes has been traduced by their publisher's connivance in a newspaper serialisation that utterly trivialised this very remarkable book.

Jonathan Bate's book 'The Song of the Earth' is published this month by Picador