Natural Dye from Wild Chicory

What part of the wild Chicory plant should I use to get a blue dye?

Rita Buchanan in her book, "A Dyer's Garden" (available from Richters), says that the wide range of colours of garden flowers and wild plants rarely translates into the same colours when dyeing. This is true of the blue chicory flowers that are a common sight along roadsides and in meadows in summer.
The Canadian natural dyer, Karen Leigh Casselman, says that chicory produces an "excellent range of colours," but blue is not one of them. Yellow-greens, tans and browns are obtained from the whole plant or the roots, depending on the mordant used. She also says that the fastness is excellent.
The wild chicory is also a valuable medicinal plant and wildflower for land restoration and natural gardens. It is easy to grow from seeds sown in spring or fall.