Historical clue formulations

• Indigo plants — 2015-09-20
• Dye-yielding shrubs — 2006-04-16
• Dye-yielding plants — 2005-12-28
• Dye-yielding shrubs — 2004-10-14
• Blue dyes — 2003-01-16
• "___ Ghost" (Michael Ondaatje novel) — 2002-05-02
• Indigo-yielding shrubs — 1998-12-06
• Dye-yielding shrubs — 1998-03-14
• Indigo-yielding shrubs — 1997-09-06