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Ontario Weeds: St. john's wort

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Table of Contents

  1. Name
  2. Other Names
  3. Family
  4. General Description
  5. Stems and Roots
  6. Flowers and Fruit
  7. Habitat
  8. Caution
  9. For more information...

Name: St. John's-wort, Hypericum perforatum L.,

Other Names: Goatweed, Klamathweed, millepertuis perforé, millepertuis commun, herbe Saint-Jean

Family: St. John's-Wort Family (Hypericaceae)

General Description: Perennial, reproducing by seed and by spreading underground roots or rhizomes. It is distinguished by its almost woody base, opposite leaves, bright yellow flowers, and leaves with transparent dots.

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Photos and Pictures

St. John's wort.

St. John's wort.

St. John's wort. A. Plant. B. Pairs of leaves showing transparent dots.
A. Plant. B. Pairs of leaves showing transparent dots.

St. John's wort. A. Top of flowering stem. B. Single leaf showing scattered, transparent dots.
St. John's wort. A. Top of flowering stem. B. Single leaf showing scattered, transparent dots.

St. John's wort. C. Basal crown and root.

St. John's wort. C. Basal crown and root.

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Stems & Roots (2 kinds):
  1. upright flowering stems 40-80cm (16-32in.) high, tough or woody near the base, branching and smooth except for shallow ridges below the base of each leaf;
  2. horizontal basal branches, these produced in late fall or early spring, short, densely leafy, and radiating out from the basal crown of each plant; leaves opposite (2 per node), stalkless, elliptic or linear, smooth-margined and, when viewed against the light, appear to have many small perforations or transparent dots.
Flowers & Fruit: flowers numerous in clusters at ends of branches; each flower, about 2cm (4/5in.) across, has 5 yellow petals with several, small, black dots along the margins, many stamens in 3 clusters and a single pistil in the centre; seedpods splitting from the tip; seeds dark brown about 1mm (1/25in.) long. Flowers from June to August; seedpods frequently persisting all winter.

Habitat: St. John's-wort occurs throughout Ontario in pastures, edges of woodlots, roadsides, abandoned fields, waste areas and occasionally in lawns and flower borders.

Caution: If eaten by livestock, St. John's-wort may cause photosensitization, a condition in which patches of white or light-coloured skin become seriously sunburned under normal exposure to sunlight. Avoid pasturing animals where this weed is abundant or keep them under shade during normal sunny days.

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For more information...

... on weed identification, order OMAFRA Publication 505: Ontario Weeds

... on weed control, order OMAFRA Publication 75: Guide To Weed Control

...on weeds in field crops, contact Mike Cowbrough (mike.cowbrough@omafra.gov.on.ca), Weed Management Specialist (Field Crops), OMAFRA, Guelph
...on weeds in horticultural crops, contact Leslie Huffman (leslie.huffman@omafra.gov.on.ca), Weed Management Specialist (Hort Crops), OMAFRA. Harrow

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