The title references the Judas Tree or Cercis siliquastrum, a flowering deciduous tree.
In most episodes you are shown the victim and told that they will die. In the beginning of this one, "The Adventure of the Wary Witness" and "The Adventure of Veronica's Veils," the victim is already dead.
When Ellery comments on how fast Russo got the scene of the death, Russo compares himself to Glenn Cunningham. Cunningham was a track athlete in the 1930s who set world records in the mile (outdoor and indoor) and the 800 meters.