Common St. John's Wort
Hypericum perforatumClusiaceae
Whole Plant
Flower
Fruit
Whole PlantFlowerFruit
Photos (c) Cheryl Comeau Beaton unless otherwise noted

Synonyms:

  • Klamathweed, St. John's wort, Goatweed, Tipton weed

The Plant:

  • A densely branched, erect herbaceous perennial, 1-2 feet tall.

The Leaves:

  • oblong or linear
  • numerous
  • hairless
  • sessile
  • thin
  • entire
  • black or translucent dots

The Stem:

  • hairless
  • two edged (two raised lines that make the stem look pressed flat)

The Flowers:

  • Golden yellow five-petaled flowers with transparent or black dots on petals and numerous prominent stamens. Flowers grow in terminal cymes.

The Fruits:

  • capsule reddish-brown, 3-chambered, ovate, sticky, 5-10 mm long, styles persist
  • seed shiny, black to brown, cylindrical, densely pitted, coated with gelatinous material

The Habitat:

  • fields
  • roadsides, disturbed areas
  • moors

Dispersal:

  • seeds dispersed by water and by sticking to farm machinery, shoes and animals
  • pieces of broken rhizomes can develop into new plants

Key ID Features:

  • golden yellow 5 petaled flowers
  • numerous stamens
  • translucent or black dots on leaves and petals

Similar Species:

  • Dwarf St. Johnswort (Hypericum mutilum) may resemble common St. Johnswort. It is a rare or uncommon erect plant less than 0.6 m tall. It has flower petals about 2 mm long, petals that are equal to or slightly shorter than the sepals, and 1-chambered capsules.

Notes:

  • Under ideal conditions St. Johnswort can be aggressively competitive and spread rapidly by seed and rhizomes. After disturbances such as logging or fire outbreaks of the plant can sometimes occur.
  • On Nantucket, this species tends to exist in small, dense stands.

Growth Form: Herb

Origin: Europe, Asia

Level of Invasiveness for Nantucket: Do not list at this time

Level of Invasiveness for Massachusetts: Not evaluated

Massachusetts Cultivation Restrictions: no




Credits: The Electronic Field Guide to the Invasive Plants of Nantucket (c) 2005-2006 Maria Mitchell Association, EFG Project, UMass Boston