Historical clue formulations

• Poplar tree — 2006-12-03
• White poplar — 2005-10-30
• White poplar — 2004-02-14
• White poplar — 1998-09-26
• Poplar variety — 1997-03-08
• White poplar — 1996-07-11
• White poplar — 1995-12-06
• White poplar — 1995-10-30
• White poplar — 1995-10-01