Common St. Johnswort

Hypericum perforatum

WA C Clusiaceae
Data Maturity Baseline

Washington State Classification

Class C — Widespread, County-Selected Control

Common St.

Quick Reference

Type
perennial herb
Origin
Europe, Asia and Northern Africa

Identification

Growth Habit

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.

Leaves

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.

Flowers

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.

Fruit & Seeds

Flowers form capsules that contain small (1 mm) dark brown seeds.

Impact

Common St. Johnswort spreads easily to new sites. Over-exposure to common St. Johnswort can cause various animal health problems including severe skin lesions and necrosis when their skin becomes hypersensitive to sunlight. Find out more about common St. Johnswort toxicity in our booklet: Protect Your Horses and Livestock From Toxic Plants.

Ecology & Spread

Habitat

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.

Spread Mechanisms

seed vegetative fragments wind bird

Reproduction

Common 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 15,000 to 34,000 seeds per year that are viable for up to 30+ years.

Regional Notes — Puget Sound

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Control Methods

Mechanical

  • 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 / Prevention

  • 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. It is not usually a problem in healthy established turf.

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.