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Arcata Field Office

Klamath Weed (Hypericum perforatum)

St. Johnswort Family  (Hypericaceae)

  Photo of klamath weedCalifornia Department of Food and Agriculture Botany Lab

Description: Klamath weed is a perennial that survives year round due to its taproot. It stands 1 to 4 feet (3 to 12 dm) tall, with erect, two-ridged stems and numerous rust-colored branches. Leaves are opposite, oblong, entire, and covered with tiny transparent dots. Flowers numerous, bright yellow, with five separate petals that twist after flowering. Petals are about .5 inches (1.2 cm) long with occasional minute black dots around the edges. Stamens are numerous and arranged in three groups.

Habitat: Native to Eurasia, Klamath weed grows in pastures, abandoned fields, and disturbed places. This species may produce seed without fertilization. Also known as St John's Wort (which means St. John's plant), it is seriously toxic to livestock. However, in humans, the toxin hypericin inhibits human immunodeficiency virus and also combats depression. But to the landscape as a whole, it is a noxious weed that can be controlled by the introduced flea beetle Chrysolina quadriggemina, and other biocontrol agents.

Distribution: Pacific Northwest at less than 4721 feet (1500 meters). On public lands administered by the Arcata Field Office, Klamath weed can be found mostly along roadsides in small numbers. The plant locations are noted but do not seem to be compromising the resources at this time.

Flowering Period: June through August