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Class C Noxious Weed

Common St. Johnswort
Hypericum perforatum
 

Image of St. Johnswort flower

Image courtesy of Ben Legler

Other common names: St. John's wort, Klamath weed, common goatweed, and tipton weed
 Plant family: Clusiaceae
Year listed: 1988
 Native to: Eurasia

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: An erect perennial herb typically growing 1 – 2.5 feet in height.

Leaves: 

  • Narrow, lance shaped leaves are 1 - 2 inches long, stalkless with pointed tips.
  • Each leaf is spotted with tiny translucent dots.

Flowers:

  • Yellow star-like flowers have 5 petals with tiny black dots on the margins.
  • Flowers occur in clusters at the ends of stems with 25 – 100 flowers per cluster.

Stems:

  • Reddish stems are single or multiple, smooth, somewhat two-edged, woody at the base, and branching out toward the top of the plant.

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 ten to thirty years.

Where does it grow?
 Infestations spread rapidly on disturbed, well drained sites such as roadways, trails, overgrazed range, logged areas, and similar sites.

What is the distribution in Washington State?

How do I control it?

General control methods:

Cultural: 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 reinfestations

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

Mechanical: Pulling should only be considered an option on new or small infestation sites and 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.

Biological: Two foliage beetles, Chrysolina hyperici and C. quadrigemina were released in California from 1945 to 1946, and established within two years. A root-boring beetle Agrilus hyperici and a leaf bud gall-forming midge Zeuxidiplosis giardi were released in 1950 to help the Chrysolina spp. Recently released and established is the moth Aplocera plagiata.

Are there plants that may be confused with (name of weed)?

 

 

For more information

  • View USDA's plant profile on St. Johnswort here.

More Photos-Click photo to enlarge
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Image of St. Johnswort flower Image of St. Johnswort infestaion in Eastern Washington
A St. Johnswort flower from the back.

Image courtesy of Ben Legler		 St. Johnswort infestation in Stevens County.

Photo by Stevens Co. Noxious Weed Control Board

 Last updated August 25, 2008