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St. John's Wort

St. John's Wort

St. John's Wort

(Hypericum perforatum)

Priority: -  Control

Tags: Terrestrial

Identification and Reproduction


  • St John’s Wort is an herbaceous perennial that grows upright up to 1.5 metres high.
  • It has a woody root crown that produces up to 30 stems, with 25-100 flowers per stem.
  • Stems are woody at the base and smooth. 
  • The plant has narrow, lance-like leaves that are 1-2 inches long, and are opposite in arrangement. When held up to the sun tiny dots will be seen on leaves. 

  • Flowers are yellow, star-like with black dots on margins.
  • Seedpods are bright green, sticky and contain many seeds. 


  • Spreads easily by seeds carried by wind, water, humans and even animals. 
  • Seeds can remain viable for decades. 
  • Is also capable of spreading laterally through roots.

Habitat & Ecology

This weed spreads over forests, woodlands, rangelands, disturbed and well-drained sites like trails, meadows, and overgrazed ranges. It prefers dry, sandy soils, and full sun sites. 



  • St. John's wort is poisonous to most livestock: can cause photosentivity that can lead to blindness, weight loss swelling and soreness of the mouth. 
  • As the plant matures it becomes impalatable forage for livestock. 
  • Crowds out native forage species in pasturelands. 


Mechanical/Manual Control: 

  • Hand pulling or digging will only be effective for small, young infestation. 

Biological Control: 

  • There are two Chrysolina species that are used as a biocontrol, but they are unable to eradicate infestations entirely. 
  • Effects from a moth, aphid, gall midge and another beetle are currently being researched. 

Chemical Control: 

  • Currently isoxaben is registered for use on St John's wort. 
  • May need to do repeat herbicide applications.


Download the Alberta Invasive Species Council's Factsheet on St John's-wort here

Check out this video made by the Montana Department of Agriculture to help identify St. John's wort.  

Header photo (Isidre blanc).