The mariachi tree was the result of Phineas and Ferb planning their day's activities after drinking too much chocolate milk. Phineas introduced it to Perry the Platypus as, "The latest party craze to sweep the nation."
It consisted of one Guitarron Mexicano (literally "Mexican large guitar") player, two guitarists, two violin players, two trumpet players and Ferb playing trumpet while sitting in the tree in the backyard. On the crescendo, all the musicians, including Ferb, fall to the ground in a pile. Phineas thanked the musicians, including one named Arturo, as they walk away presumably in pain. Afterwards, the boys build a haunted house to scare Isabella's hiccups away. ("One Good Scare Ought to Do It!")
In "Picture This", they can be seen at the Mexican-Jewish Cultural Festival.
The Mariachi tree song has become Pinky's theme song, as every time he is seen this song is played. Also when Mexican events are mentioned this song is heard is well.
McGregor's Gorge/Holy Mackerel