Mountain Ash likely refers to the Sorbus scopulina, a small to medium-sized deciduous tree common in Northern California.
The tree, also known as Rowan, is woven throughout the folklore of many regions as a ward against evil beings.
- Peter Hale could not pass over a balustrade made of the wood in Formality.
- Isaac and Erica could not cross a line of Mountain Ash powder in Raving.
- Capsules filled with Mountain Ash powder and ingested by Gerard cause him to violently reject a werewolf bite in Master Plan.
Teen Wolf Creator Jeff Davis has called Mountain Ash “magic” but the mechanism through which it exerts power is unclear.