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St. Johnswort (Hypericum perforatum)
ODA rating: B
Oregon St. Johnswort distribution
Other common names
Klamath weed, goat weed, common St. Johnswort
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Images courtesy of Eric Coombs, Oregon Dept. of Ag.

If images are downloaded and used from the ODA web site please be sure to credit the photographer.
Perennial; blooms June to July. Grows 1 to 3 ft tall. Stems erect, numerous branched, somewhat 2 ridged, rust-colored and woody at their base. Leaves opposite, oblong, not over 1 in long and covered in transparent dots. Flowers 3/4 inch in diameter, bright yellow, numerous in flat-topped cymes with five petals with occasional small black dots around the edges. Stamens numerous, arranged in three groups.
This difficult to control pest is found throughout most of the western U.S. It invades pasture and rangeland and is poisonous to livestock. Livestock seldom eat the plant unless pasture is scarce. It can invade rangelands to open timber. Infestations spread rapidly on disturbed, well drained sites such as roadways, trails, overgrazed range, and logged areas.
Known hazards
The plant contains a toxic compound that causes photo-sensitivity; poisoning appears to be cumulative. Light pigmented animals develop blisters and scabs on the mouth eyes, ears, nose and feet, especially when exposed to sunlight. Dark colored animals are generally less susceptible.
A European native, this weed is abundant in the Pacific Northwest on sandy and gravelly soils.
Distribution in Oregon
The first documented site of this plant in Oregon was 1877 in Clackamas County.

Biological controls
There are five biological control agents approved for use on this plant. Four of these, three beetles and a moth, have been successfully established in Oregon. Currently the biological agents are not effective above the 2500 foot level.
Agrilus hyperici
Aplocera plagiata
Chrysolina hyperici
Chrysolina quadrigemina
Zeuxidiplosis giardi