From Zappa Wiki Jawaka
Joni Mitchell (born: Roberta Joan Anderson November 7, 1943) is a Canadian folk singer, guitarist, songwriter and painter. She is best known for her landmark album "Blue" (1971) and songs like "Big Yellow Taxi", "Free Man in Paris" and "Help Me". She is highly respected both by music critics and fellow musicians and a huge influence on several rock, pop and folk musicians.
Joni sat in with us during the second show and we improvised a thing that was really good - ending it with her singing 'Duke of Earl'. She came on stage, we did a few chords for her and she started reciting this poem: 'Penelope wants to fuck the sea...
Zappa named "Blue" an impressive record and played Mitchell's song "All I Want" on WSTM, Chicago.
Her "Refuge Of The Roads" concert DVD includes Vinnie Colaiuta.