St. Johnswort

St. Johnswort - leaves
St. Johnswort - flowers - closeup
St. Johnswort - flowers

St. Johnswort

Hypericum perforatum

Information sheet PDF

Colorado List C – Control recommended


General

  • Family: St. Johnswort (Clusiaceae)
  • Introduced from Europe
  • Common names: Common St. Johnswort, Klamath weed

Habitat

  • Herbaceous perennial
  • Colony forming
  • Found in pastures and rangeland

PLANT

Vegetation 

  • Mature plants up to 3 feet tall 
  • Leaves 3/8 inch wide and 1 inch long
  • Translucent glands visible when held up to light 
  • Stems with reddish spots, woody base
  • Creeping stems above and below soil surface

Roots 

  • Taproot and rhizomes

Flower 

  • May-July
  • Yellow with small black dots along the edge
  • ¾-1 inch wide
  • Groups of five petaled flowers at the end of branches

Seed 

  • More than 30,000 seeds per plant
  • May remain viable for up to 30 years

Reproduction 

  • Seed, rhizomes, vegetatively 

Control

Biologocal

  •  Leaf feeding beetle (Chrysolina spp.)

Chemical

Cultural

  • Prevention
  • Fertilization to promote grass cover
  • Revegetation of highly disturbed sites
  • Removal - not effective unless done first year plants germinate; otherwise this will fragment the roots and give rise to many more plants.  All plant parts should be bagged and disposed of

Mechanical

  • Burning N/A Not recommended, fire encourages growth
  • Grazing N/A Not recommended.  St Johnswort contains hypericin which is harmful to animals
  • Mowing N/A