WSU Extension

Hortsense

Weeds : St. Johnswort, common (Goatweed, Klamathweed) : Hypericum perforatum
(revision date: 9/3/2015)

Family: Clusiaceae (Guttiferae)
Cycle: Perennial
Plant Type: Broadleaf

Use Integrated Pest Management (IPM) for successful weed management.

Biology
Common St. Johnswort is an erect shrubby plant one to three feet high arising from a taproot. Many stems arise from the base of the plant, each having numerous short, leafy branches. The branches may root where they contact the ground. The oval leaves are opposite and attach directly to the stems. Each leaf has obvious veins and transparent dots which appear as punctures. The showy flowers appear in early summer. Each has five yellow petals with black dots at the margins and a distinctive cluster of many yellow stamens in the center. Seeds have a sticky coating which aids in dispersal. SPECIAL INFORMATION: Common St. Johnswort is toxic to animals. When eaten by white or light-colored animals, a skin irritation in the presence of sunlight can occur. Affected animals may not die due to poisoning, but may refuse to eat resulting in weight loss and eventual death. Skin reactions may also occur in humans. Several insects contribute to biocontrol of this weed. It is commonly mistaken for tansy ragwort. In WASHINGTON, common St. Johnswort is designated as a Class 'C' noxious weed. In OREGON, it is a Class 'B' weed. Management of this species may be required by law in your county. Consult your local Noxious Weed Control Board for more information.
Habitat
St. Johnswort is usually found on dry soils, often in sandy or gravelly areas. It is a common weed on roadsides, grazing lands, waste places, and other areas which are not frequently cultivated. It occurs both east and west of the Cascades.

Management Options

Non-Chemical Management
  • Maintaining a healthy planting or turf area to provide competition will prevent weed establishment.
  • Reduce weed infestation by handpulling weeds.
  • Careful digging is useful to manage weed populations. However, digging can carry undesirable weed seed to the surface and foster further germination.
Select non-chemical management options as your first choice!

Chemical Management

Apply according to label instructions. It is not usually a problem in healthy established turf. Glyphosate products should be applied as spot treatments only! NOTE: Some ingredients listed here are only available in combination. Read the label carefully on combination products to make sure the product is suitable for your specific situation.

Landscape areas
  • glyphosate
  • dichlobenil
Turf areas
  • products containing 2,4-D
Bare ground areas
  • glyphosate
Images
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Caption: St. Johnswort in bloom
Photo by: D.G. Swan
Caption: St. Johnswort flowers
Photo by: D.G. Swan