Hypericum perforatum
St. Johnswort is a perennial that spreads by seeds or short runners; the creeping roots have small pink buds. It has bright-yellow, five-petaled flowers that are ½”- ¾” in diameter and often with over a dozen stamens. The leaves are very distinct because they are covered with small glands and, when held up to a light source, appear to be covered in pin holes. The glands produce photoactive oil that covers the leaf surface. When browsed it can cause rashes or blisters in fair-pigmented animals and may negatively impact behavior.
St. Johnswort has been called Klamath or Tipton weed, so named because of the dense infestations this plant has previously formed in the Pacific Northwest. It was introduced to the continental United States in the early 1700’s for its medicinal value and has since shown the ability to negatively impact native plant communities.
In Teton County, St. Johnswort occurs in limited infestations and is being actively controlled in nearly all situations. It typically invades gravelly, well-drained soils and slopes with moderate amounts of sunlight.
If you find this weed on your property or around Teton County,
please report it at 733-8419.
Back to Noxious Weeds