Invasive Focus: Hypericum perforatum, Common St. Johnswort


Invasive Focus: Hypericum perforatum, common St. Johnswort. An erect, semiwoody, perennial, rhizomatous herb. The stems are often winged. The leaves are simple, sessile, opposite, and net-veined. The blade has pinnate major veins and numerous punctate dots that can be viewed when the blade is held up to the light. The base of the blade is obtuse and clasps the stem. The flowers are perfect and produced in clusters at the tips of the branches. The petals are bright yellow and often black dotted. The fruit is a capsule with many, small, brown to black cylindrical seeds. Plants grow in cultivated fields, pastures, waste areas, and often in the edges of forests.

The rhizomatous perennial habit of this species makes it a difficult weed to eradicate and allows it to compete with and replace natives. The plant is usually avoided by grazing animals, which gives it a competitive edge in pasture competition. Plants contain compounds that when ingested by light-colored animals cause the skin to be photosensitive and produce severe sunburn conditions. The leaf feeding beetle (Chrysolina quadrigemina (Suffr.) has been used to control this weed along the Pacific Coast.

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