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.
Teen Wolf Creator Jeff Davis has called Mountain Ash “magic” but the mechanism through which it exerts power is unclear.
Mountain Ash Effect
We've seen, several times, that thrown Mountain Ash falls either in a circle or a straight line.
This suggests, under proper conditions, the material will form a barrier with very little effort.
- Druids Ms. Morrell and Jennifer Blake throw down circles of Mountain Ash dust to create a personal shield.
- Peter Hale could not pass while a balustrade made of the wood was down, completing the circle of Mountain Ash built into the Vet Clinic in Formality.
- Capsules filled with Mountain Ash powder and ingested by Gerard cause him to violently reject a werewolf bite in Master Plan.
- Dr. Deaton says his animal clinic is lined with Mountain Ash "making it difficult for someone like Scott to cause me any trouble."
- Sometime after Lunar Ellipse, Deaton installed Mountain Ash baseboards throughout the McCall home. Spreading Mountain Ash along front door jam completes the circle and (mostly) protects those inside from supernatural creatures.
- Lorraine Martin's boat house is apparently made of Mountain Ash. Spreading ash along the opening to the dock completed the circuit in Time of Death.
- With great effort, Scott McCall was able to cross a Mountain Ash barrier.
- Peter Hale, and later Deucalion, showed that supernatural being can still throw items through a Mountain Ash barrier.
- Several Oni, with great effort, were able to break through the barrier around the McCall home.
- Tracy and the other Chimera are able to cross Mountain Ash because the rules don't apply to the Dread Doctors creations because they are not truly supernatural.