Police said the victims of the freak accident, all students at a local high school, had been swimming during a storm when the tree fell and crashed into the revellers.
A further 11 people were injured, among them one of the school's administrators.
Emergency workers have been trying to free the victims by cutting through the tree with chainsaws, according to the Ghana Starr News.
"Most of them are students of the Wenchi Senior High School, others are tourists," said one eyewitness.
Ghana National Fire Service spokesman Prince Billy Anaglate said that 18 people died at the scene and two others died in hospital - but the overall death toll remains unclear.