From Encyclopedia of Occultism (OOP, 1920) - (Formatting / spelling retained)

St. John's Wort : St. John’s Wort. In classical mythology the summer solstice was a day dedicated to the sun, and was believed to be a day on which witches held their festivities. St. John's Wort was their symbolical plant, and people were wont to judge from it whether their future would be lucky or unlucky ; as it grew they read in its progressive character their future lot. The Christians dedicated this festive period to St. John's Wort or root, and it became a talisman against evil. In one of the old romantic ballads a young lady falls in love with a demon, who tells her --

"Gin you wish to leman mine
Lay aside the St. John's Wort and the vervain."

When hung up on St. John's Day together with a cross over the doors of houses it kept out the devil and other evil spirits. To gather the root on St John's day morning at sunrise, and retain it in the house, gave luck to the family in their undertakings, especially in those begun on that day.