St Johns Wort (Hypericum perforatum)

St John's Wort Landscape2Family: Hypericaceae

Growth Habit: An erect plant which grows up to 1.2m in height, averaging 0.6 to 0.9m. The plant tends to be woody near the base. Leaves are stalkless and hairless and have a paler green on the underside and have visible oil glands that give the leaf a perforated appearance when held up to the light.

Type of Plant: St Johns Wort is a perennial, which usually dies down over winter and re-emerges from underground stems in spring.

St John's Wort flowersFlowers: The five petalled yellow flowers grow in clusters. Each flower is approximately 15 to 25mm in diameter with three bundles of long thread-like stamens growing from the centre and appears to have black dots that are actually glands around the edge.

Fruit/Seed: The fruit is a three-celled sticky capsule about 8mm long containing numerous, dark brown or black cylindrical seeds that are released in late summer.

Dispersal: Reproducing from underground stems and seeds.

Distribution: Localised weed of roadsides, poorly managed grazing land, neglected areas and disturbed bushland areas of Tasmania, particularly in the northern Midlands, north-east, central north coast and the south east.

Status: Secondary and Prohibited in Tasmania. It is also an environmental weed, particularly in open grassland-woodland communitites.

Weed Impact:

  • St Johns Wort contains substances known to cause illness in livestock, such as hypericin, that when eaten, livestock becomes very sensitive to sunlight.
  • It is also a valued medicinal plant by some herbalists.
  • It is capable of infesting large areas of grazing land, particularly in drier climates, competing with pastures.

For further information contact the Department of Primary Industry, Parks, Water and Environment, Tasmania.

Control Methods:

  • Grubbing. Isolated plants and small infestations can be removed using hand tools, you must ensure the root system is also removed.
  • Herbicides. Only spray when actively growing. You may two consecutive years of herbicide application to kill the plant due to their deep, extensive root system. Spot spray for smaller patches (when flowering) and boom spray for larger areas (between budding and full flowering). Herbicides registered in Tasmania include 2,4-D, triclopyr and glyphosate.
  • Pasture management. Good autumn and winder pasture cover can suppress new plants. (Cultivation can spread the plant if roots are not brought to the surface to dry out.)
  • Each plant can produce up to 33,000 seeds per year which can remain viable for up to 12 years.

N.B. Always check the herbicide label before use.