St. John’s Wort

Hypericum perforatum

St. John's-wort (Hypericum perforatum) is a perennial invader of grazed and disturbed lands that creates dense stands, displacing native plant species and reducing available forage for livestock and wildlfie. Plants contain a toxin that causes skin irritation and blistering in light-coloured livestock when exposed to the sun. 

Mature plants grow up to 1 m high and turn a rusty red, prefering dry sandy soils and full sun. It can be found at low- to mid-elevation areas of the coast, grasslands, and forest regions of BC, namely the Kootenays, Okanagan, Boundary, North Thompson, Cariboo, Skeena, Fraser Valley and Vancouver Island. 

St. John's-wort spreads aggressively by a lateral-reaching root system that can form new buds that are separate from the parent plant. Seeds also have a gelatinous coating that aids long-distance dispersal and can survive in the soil for 10 years. One plant can produce over 100,000 seeds per year, spreading by wind and water.

The flowers of this herb have bright yellow petals with black dots that are produced in clusters at the tips of the branches. There can be 25-100 flowers per cluster. The fruit is a capsule with many small, brown to black cylindrical seeds. Stems are often winged with simple veined, opposite leaves.

Gallery: St. John’s Wort

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