COMMON ST. JOHNSWORT - Hypericum perforaturm L.

Clusiaceae - (St. Johnswort family)

Common St. Johnswort is a plant common to the Pacific coast but also thrives in Wyoming growing up to 3 ft. tall. 
Up to one inch long, the leaves are generally bright green and grow directly from the stem having no leaf stalk.   The leaves are oval with smooth edges, grow opposite one another on the stem and are covered with transparent dots.

Perennial roots support many upright branching stems and have short runners.
The numerous yellow flowers, whose stamens stand up from the petals, are almost as big as the leaves.  The five petal flowers often have tiny black dots on the petal edges and produce ¼ inch long, rust colored, three chambered, seed pods filled with seeds. 

White haired animals that eat St. Johnswort may suffer from weight loss or a skin irritation that occurs as a reaction with sunlight.

Control is much easier than eradication as wide infestations do not respond well to remediation.

The following is courtesy of Weeds of the West:

A perennial reproducing by seeds or short runners. Stems are 1 to 3 feet high, erect, with numerous branches, somewhat 2-ridged, rust-colored, woody at their base. Leaves are opposite, sessile, entire, elliptic to oblong, not over 1 inch long, covered with transparent dots. Flowers are 3/4 inch in diameter, bright yellow, numerous in flat-topped cymes, with 5 separate petals with occasional minute black dots around the edges. Petals are twice as long as the sepals. Stamens are numerous, arranged in 3 groups. Seed pods are 1/4 inch long, rust-brown, 3-celled capsules, each with numerous seeds.

St. Johnswort, originally from Europe, is frequently found in the Pacific Northwest, often on sandy or gravelly soils. The weed contains a toxic substance which affects white-haired animals. Affected animals rarely die, but will often lose weight and develop a skin irritation when exposed to strong sunlight. St. Johnswort is an abundant weed in Australia where research has identified three insect species native to Europe which selectively feed on the plant. Insects introduced to the western United States and Canada have provided partial control of the weed.

Clusiaceae synonym: Hypericaceae.
(Courtesy of Weeds of the West)

No identified infestations in Fremont County.