I'm trying to identify a book written prior to the 1980's, about a child who finds a witch and then has an adventure with her. The witch teaches her how to use plants to heal and then, I believe, they fight some grumpy old man. It was illustrated beautifully and I remember the witch having very long black hair.
There was a poem in the book it reads.
Under moon and under star, the wind blows where the birch trees are.
Springtime earth waits for the rain and spring will.come to earth again.
I think the witch was discovered in the child's cellar.
The cover of the book was pale blue but my copy had lost its dust jacket.
The witch was beautiful, the illustrations quite stylized but serious, not cartoony. In fact the tone of the book was rather serious.