These days, Joni Mitchell's appearances in the news are more often prompted by her opinions (rebarbative) or her health (erratic) than her music. But after her hospitalisation in Los Angeles last week, her fans had reason to wonder if they were about to lose her, which inevitably concentrated minds on her career. "Folk legend" was the phrase frequently invoked in coverage of the story, alongside an emphasis on her lasting impact upon women performers. It's not exactly damning her with faint praise, but it does misrepresent and undervalue her.

To call Joni Mitchell undervalued is, on the face of it, an obvious nonsense. No one questions her greatness. Her seat atop Parnassus is assured. It's more a matter of what she's valued for. Mitchell's most enduringly popular work is found on the series of albums she recorded between 1969 and 1974: "Clouds", "Ladies of the Canyon", "Blue", "For the Roses" and "Court and Spark". Play chronologically through the best-remembered songs from those records and you'll hear a bright, slightly mannered but evidently special folk artist, sometimes keeping only just the right side of the line between the charming and the twee, as she develops an astonishing depth and range, both musical and emotional. Soon the songs unhook themselves from their skeletons. They flip and they fly. Her voice takes on an acrobatic naturalism, suddenly leaping with unfettered joy or folding itself back into marvellous and unlikely shapes. Of all the 1960s folk artists who remained within that idiom, only Bert Jansch, with his wild, eerie, intense music and flying finger-picking style, did anything like as much to stretch its limits.

That, then, is what Mitchell is celebrated for, and rightly so. She made wonderful records and she reconfigured the boundaries of her chosen music as she did it. And she was the inadvertent godmother to generations of female singer-songwriters - many of whom, as is the way with these things, homed in on and mimicked her confessional introspection, while lacking the imagination and talent that licensed her to express it. That's no fault of hers, of course. That's simply how artistic influence works.

Yet it's here that we fetch up against the deficit in her reputation. When Mitchell's influence is mentioned, it is almost exclusively in terms of how she inspired those female singer-songwriters, as if a female artist may be the creative begetter only of other women. Elvis Presley or The Beatles or Bob Dylan, we are customarily told, revolutionised popular music. Not one part of it. Not for men alone. Popular music, in its entirety. But Joni Mitchell simply set her own people free. Which she did, and it's no small thing. But it's not the whole story.

The rest of the story takes place, mainly, in the second half of the 1970s - although it begins with "Court and Spark", just as the first part ended with it. Jazz sashayed into Mitchell's music as early as "Blue" (1971), where you can hear its liberties with tune and tempo, its improvisational loucheness, flicker in and out like a candle flame in the corner of the room. By "Court and Spark" (a wonderfully apt title) it had sauntered much closer to the centre of things. That album began a run of releases - including "The Hissing of Summer Lawns" and "Hejira" - which, in their ease and delicacy, their light-footed exhilaration, their exquisite melancholy and sheer mind-bending fearlessness, rank alongside the greatest bursts of invention in pop: Dylan's first electric phase, say, or Stevie Wonder's astonishing series of early 1970s recordings.

Even if Mitchell's final two albums of the decade, "Don Juan's Reckless Daughter" and "Mingus", went so far over into experimental territory that they lost their hold on her mainstream audience, why aren't the previous three feted as they should be, rather than much admired? While the impact of Mitchell's folk-ish records is direct and clear, these albums offered to posterity not so much a sound as a spirit: an opening up of possibilities, an abandoning of border posts. This trio of albums is one of the most artistically successful adventures in musical fusion ever undertaken by a star with so much to risk. They rival Miles Davis and John Coltrane at their sublime best, and constitute the high watermark of a tide that carried jazz back into modern pop.

It might be Mitchell's gender that has prevented this music receiving its due, or it might just be one of those things: a cultural preference for the pretty over the audacious. I suspect a combination of both: the high-cheekboned high priestess of dear-diary dreamgirls sits more easily in our canon than the strange, unnerving and uncompromising explorer who succeeded her. Odder incarnations have come along since, including her present one. But none can undo those earlier guises, each in its own way a marvel.

David Bennun is a critic and the author of the African memoir, "Tick Bite Fever"