Alturas Field Office

Noxious Weeds

Klamathweed or St. Johnswort
(Hypericum perforatum)     St. Johnswort Family  (Hypericaceae)
 

Klamathweed or St. Johnswort
Photo courtesy of the California Department of Food and Agriculture Botany Lab

 
 

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Description:  Perennial 1 to 3 feet with erect, two-ridged stems and numerous rust-colored branches. Leaves opposite, oblong, entire, covered with tiny transparent dots. Flowers numerous, bright yellow, with five separate petals that twist after flowering. Peltals about 1/2 inch long with occasional minute black dots around the edges. Stamens numerous, arranged in three groups.

Habitat:  Native to Europe. Usually found in sandy or gravelly soils and along roadsides. Contains a toxic substance which causes animals that eat it to lose weight and develop a skin irritation when exposed to strong sunlight.

Distribution:  Klamathweed ranges from the Pacific Ocean east to Nevada with the largest infestations being found in Humboldt and Trinity counties. In northeastern California the infestations are small, usually less than an acre. Scattered along Highway 44 and 89. The size of the infestations vary from year to year due to the Klamath beetle, Chrysolina quadriggemina, and other biocontrol agents.

Flowering Period:  June to September