This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Leaves boiled in wine were supposed to cure and heal up wounds. Perhaps also the perforations were thought to resemble wounds, when by Doctrine of Signatures the plant would in the older days therefore cure wounds. In the Netherlands the people gathered it before dawn, and it was reputed to take away the ill effects of lightning. It was believed that it revealed a witch, and on St. John's Eve, when they were active, it was worn as a charm. For similar reasons they call it Devil Chaser in Italy, and doorways and windows were decorated on that day. The name Devil's Flight sums up the idea that it drives away evil spirits. If one trod on it at night in the Isle of Man a fairy horse would appear and carry one about all night. On Midsummer Eve it was employed as a love charm.
It is placed under the beams in the roof in Denmark for divination by lovers, one for each, and if they grow together it is considered a good omen. On St. John the Baptist's Day it was hung up over the doors of houses, according to Stowe (Survey of London), to drive away witches. The red pods are connected with John's beheadal as drops of blood. It was dedicated to St. John. The plant was also called Peterwort. In the Middle Ages they called it Fuga Daemonum. It formed one of the ingredients of " Save " of Chaucer's day, and was used by knights for their wounds. It was used for wounds in the same way as balsam. Red and yellow dyes are given by the plant. Steeped in turpentine a red varnish is produced. An essential oil is secreted in the perforations of the plant. Spirits and oils are tinged purple by the flowers. It is bitter and astringent, and acts as a diuretic, having been used for ulcerated kidneys.
Essential Specific Characters: 61. Hypericum perforatum, L.--Stem erect, 2-angled, leaves oblong, with pellucid dots, flowers yellow, sepals acute, erect, not fringed, styles equalling the capsule, petals oblong.