common St. Johnswort

Hypericum perforatum

   

Family: Clusiaceae

Other common names: St. John's wort, klamath weed, common goatweed and tipton weed

Weed Class: C

Year Listed: 1988

Native to: Europe, Asia and Northern Africa

Toxic: humans, livestock

additional photos

 

   

Why is it a noxious weed?

St. Johnswort spreads easily to new sites. Over-exposure to St. Johnswort can cause various animal health problems including severe skin lesions and necrosis when their skin becomes hypersensitive to sunlight.

How would I identify it?

General Description: Common St. Johnswort is an upright perennial herbaceous plant which typically grows 1 to 2.5 feet in height. It has tap roots and short rhizomes and its stems are freely branched.

Flower Description: Flowers are yellow, star-like and have 5 petals with tiny black dots on the margins. Flowers occur in clusters at the ends of stems with 25 to 100 flowers per cluster.

Leaf Description: Leaves are oppositely arranged on stems, narrow, lance shaped and 1 to 2 inches long. They are oppositely stalkless and have pointed tips. Each leaf is spotted with tiny translucent or purplish-black dots.

Stem Description: Stems are reddish, single or multiple, smooth, somewhat two-edged, woody at the base, and branching out toward the top of the plant.

Fruit/Seed Description: Flowers form capsules that contain small (1 mm) dark brown seeds.

May be confused with:

How does it reproduce?

St. Johnswort spreads both by underground rhizomes, above-ground creeping stems, and by seeds that are dispersed by wind and animals. One plant can produce up to 100,000 seeds per year that are viable for 10 to 30 years.
 

Where does it grow?

Infestations spread rapidly on disturbed, well drained sites such as roadways, trails, meadows, grasslands, overgrazed range, logged areas, and similar type sites. Please click here to see a county level distribution map of common St. Johnswort in Washington.
 

How do I control it?

Mechanical Control

Pulling should only be considered an option on new or small infestation sites. Repeated pulls will be necessary to ensure removal of the whole plant and any lateral roots. Do not leave plants at the site since vegetative growth will occur, and the seed source will remain. Tillage is effective when repeated in croplands. Mowing is a limited option depending both on site accessibility and whether seed formation has occurred. Repeated cuts are necessary.

Cultural Control

St. Johnswort seedlings will readily establish in disturbed situations. The combination of site-specific range management (which includes encouragement of beneficial plants species as well as a grazing management plan) will prevent new infestations and re-infestations.

Biological Control

The Klamath weed beetles, Chrysolina quadrigemina and Chrysolina hyperici, adults and larvae feed on the leaves of common St. Johnswort. The St. Johnswort moth, Aplocera plagiata, larvae feed on the leaves. The St. Johnswort root borer, Agrilus hyperici, larvae feed within plant roots. For more information about the biological control of common St. Johnswort, please visit WSU Extension Integrated Weed Control Project.

Herbicide Control

Please refer to the PNW Weed Management Handbook, or contact your county noxious weed coordinator.


 

For more information

 
See our Written Findings for more information about common St. Johnswort (Hypericum perforatum).

Additional Photos


 
 

common St. Johnswort forming fruit

common St. Johnswort flower buds

common St. Johnswort leaves

common St. Johnswort forming fruit

common St. Johnswort leaves

in flower

developing seed capsules

developing seed capsules

field with old flowering stems

new growth

    

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