Identification and Reproduction

Identification:

• 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. 

Reproduction: 

• 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. 

Impacts

Social: 

• 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. 

Management

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.

Resources

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).