St. John’s Wort (Hypericum perforatum L.)

St. John’s Wort, Hypericum perforatum L., invasive plant

General: A branching plant that has become popular as a herbal remedy for a variety of health issues.

Height: Grows up to 1m tall. 

Flowers: Small yellow flowers (2cm) in flat-topped clusters. They have 5 petals.

Leaves/Stems: Leaves grow across from each other on the stem (opposite), 1-3cm long. They are oval in shape with veins and transparent dots all over them. 

Root: Short creeping roots.

Klamath weed, Goatweed.


Where did it come from? Eurasia.

Where does it grow here? Grows in a variety of environments and prefers dry, gravelly or sandy soils. It is found near housing as well as lake shores in our region.

Reproduction: By seed and root. The root can form buds that break off from the parent plant. One plant can produce 15-30,000 seeds.

When does it grow, flower & seed? Sprouts April-May. Flowers June-August. Seeds September-October.

Spreads By: People have been known to plant it. Seeds have a gelatinous coat that allows it to travel long distances. Seed can stay viable in the soil for 6-10 years so soil movement or disturbance can expose seeds and plant growth can occur.

Plant Type: Perennial.

  • It contains a toxin that can cause skin irritation in livestock if they eat this plant. The skin irritation is triggered in light-coloured livestock when they are exposed to sunlight.
  • Reduces forage for livestock & wildlife if stands become large in pastures or forested areas.
  • It can compete with native species and reduce their numbers.
  • Review your property regularly for this species. Do not plant it.
  • Treatment Remove small patches before it flowers & sets seed. Tillage in fields can be a good control method. There are biocontrol agents (beetles, moth & aphid) that have been released on infestations. 
  • Cover bare patches or disturbed soil by planting or seeding with non-invasives.
  • Check areas where you have removed invasives for any new plants that year and in future growing seasons.
  • Dispose of invasive plants responsibly. Bag them for disposal at the local landfill.  Composting and burning are not recommended.
  • Contact LRISS for specific treatment recommendations.

Southern Interior Weed Management Committee. 2016. Invasive Plants of the Southern Interior BC. 86pgs.

Okanagan Invasive Species Online website.