Rowan trees and their wood, particularly the sub-species called mountain ash (Sorbus scopulina), is a species known to have mystic properties when used in the correct rituals. In their naturally occurring form, these plants are fairly harmless; however, when mystically activated, they can become a potent ward against mystic creatures and magical energies.
There is a large amount of mystically activated mountain ash growing in the wildnerness around Beacon Hills, CA.
Warding lines and circles of Mountain Ash will create barriers that bar most werewolves, demons, and divines from crossing--unless they have sufficient power to expend pushing past said barriers. More powerful alpha werewolves, especially "True Alphas," are known to be able to break such barriers with sufficient effort and concentration. Immortals and Slayers are not affected by Mountain Ash barriers, perhaps because they are sufficiently close to human. Witches, wizards, and other practitioners are not affected personally by Mountain Ash barriers. Their magic is generally impeded by it, but powerful enough magical sources may overwhelm such a barrier, especially if multiple practitioners are acting together. However, anyone actively channeling magic--such as an Immortal harnessing the Quickening or a practitioner actively casting magic--are barred by mountain ash barriers.
Mountain ash, due its powerful warding properties, can also be used to create poisons that are highly toxic to some supernatural creatures, such as most forms of werewolves.
Logs featuring Mountain Ash: