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St John's wort (Hypericum perforatum)

Introduced to Australia as an ornamental and for medicinal purposes, St John's Wort is now widespread in temperate areas and invades grazing land, open bushland, roadsides and neglected areas. Dense infestations displace useful vegetation and can make livestock sick, reduce production or cause death.

Key facts

Type: Weed, herb.

Native to: Europe, western Asia and North Africa.

Established in: QLD, NSW, ACT, VIC, TAS, SA.

Other names: Common St John's Wort, Perforate St John's Wort, Klamath Weed, Goatweed.

Identification

  • It is an erect, perennial herb that grows to about 1.2 m tall.
  • Stems are often reddish and have two ridges running along them.
  • The sessile leaves grow in opposite pairs and have small translucent glands visible when held up to the light.
  • The bright yellow flowers have five petals and are 1–2 cm across; small black glands may occur along the petal edges.
  • The sticky, three-celled fruit capsule splits open at the top when mature and contains many small, golden-brown seeds.
  • Look-alike: St Peter's Wort (Hypericum tetrapterum) has similar yellow flowers and a similar growth habit, but has four ridges along its stems rather than the two on St John's Wort.

Where it grows

In Australia it occurs in water catchment reserves, hilly forested areas, roadsides, intermittently grazed areas, railway land, riverbanks and pastures, especially on non-arable land and in temperate regions.

Why it matters

  • In pastures it displaces useful vegetation and, if left to establish, can eliminate almost all other plants.
  • Large, dense infestations have led to the abandonment of formerly productive land in north-eastern Victoria and New South Wales.
  • Hypericin in the plant can cause photosensitisation and affect animal behaviour and organ function; ingestion can result in weight loss, reduced wool and milk production, poor reproductive performance or death. Horses are more susceptible than other livestock.
  • Dry stems left standing in summer can become a fire hazard, and impacts on native habitats are largely unknown.

How it spreads

  • A mature plant can produce up to 30,000 seeds, which can remain viable after being eaten by livestock and can adhere to livestock coats.
  • Wind carries seed short distances, while water currents can spread it, especially in runoff-prone areas and riparian habitats.
  • Lateral roots can form new crowns, and cultivation can spread the plant by scattering root fragments.

Control options

  • Maintain good pasture management that favours desirable species, as St John's Wort is sensitive to competition, particularly when young.
  • Avoid overstocking, which encourages the weed because livestock tend to prefer other species and reduce competition.
  • Herbicides can be used; check the APVMA and contact the relevant local weed management agency for current registered herbicides and approved methods.
  • Established biological control agents known to impact the weed in Australia include the leaf beetles Chrysolina hyperici and Chrysolina quadrigemina, and the aphid Aphis chloris.

Hygiene and washdown

  • It notes that seeds can adhere to livestock coats and spread in water, and that cultivation can scatter root fragments.
  • Inspect and clean vehicles, plant and footwear before leaving an infested area, paying attention to tyres, radiators, belly plates, tracks and floor mats.
  • Wash down at a designated bay where runoff is contained and can be monitored for seedlings.
  • Record each washdown so the client has evidence of clean entry and exit.

Classification by state

  • Australia (national): No national listing. Managed under state and territory law.
  • Queensland: Not prohibited or restricted. Not listed as prohibited or restricted matter in Queensland. Everyone must also take all reasonable and practical steps to minimise its risks (the general biosecurity obligation).
  • New South Wales: Regional priority in 3 of 11 regions. Regional priority weed in Central West, Hunter, North West. The general biosecurity duty applies: anyone dealing with it must prevent, eliminate or minimise the risk it poses.
  • Victoria: Regionally controlled / restricted weed (varies by region). Where it is regionally controlled, landowners must take all reasonable steps to prevent its growth and spread on their land. Where it is a restricted weed, trade in it (including the plant, its seed or contaminated material) is prohibited.
  • South Australia: Not declared. Not declared under the Landscape South Australia Act 2019. Other laws may still apply.
  • Western Australia: Permitted, not declared. Listed as a permitted organism in WA, so it can be brought in, but it is not a declared pest.
  • Tasmania: Declared weed. Property managers are required by law to control declared weeds, and their import and sale are prohibited.
  • Northern Territory: Not declared. Not on the NT A to Z weed list, and Weeds Australia does not list it as declared in the NT. Check the NT declared weeds list before moving or planting it.
  • Australian Capital Territory: Declared pest. It is a declared pest throughout the ACT.

Legal obligations

Every state and territory places a duty on landholders and people working on land to prevent the spread of invasive species, such as the general biosecurity obligation in Queensland and the general biosecurity duty in New South Wales. Declarations and control requirements differ between states and local government areas, so check the current official list before work starts.

  • Queensland: Biosecurity Act 2014 (Qld), administered by Biosecurity Queensland (13 25 23)
  • New South Wales: Biosecurity Act 2015 (NSW), administered by NSW Department of Primary Industries and Regional Development
  • Australian Capital Territory: Biosecurity Act 2023 (ACT), administered by ACT Biosecurity
  • Victoria: Catchment and Land Protection Act 1994 (Vic), administered by Agriculture Victoria
  • Tasmania: Biosecurity Act 2019 (Tas), administered by Biosecurity Tasmania
  • South Australia: Landscape South Australia Act 2019 (SA), administered by the regional landscape board

Sources