Periwinkle
The periwinkle is a trailing evergreen plant, also called myrtle or creeping myrtle, often used as a ground cover in gardens. In folklore, periwinkle has powers pertaining to Ghosts, witches and bewitchment. Periwinkle, which blooms in five-petaled blue-purple or white flowers, is called violette des sorciers (violet of the sorcerers) in France, where it wards off evil spirits. Hung over doorways, periwinkle is said to keep out both witches and spirits. It is effective in the Exorcism of Demons.
In Welsh superstition, if one plucks periwinkle off a grave, one will be haunted by the dead in DREAMS for a year. In order for the plant to be magically effective, one must harvest it only on the first, ninth, 11th and 13th nights of the moon. The harvester must not be unclean.
Taken from :The Encyclopedia of Ghosts and Spirits– Written by Rosemary Ellen Guiley – September 1, 2007