St. John’s wort

Scientific name: Hypericum perforatum
  • Key characteristics
  • Biology
  • Impacts
  • Control
  • Further information

  • Hairless perennial plant with erect stems up to 1 m tall, rising singly or in small clusters from r3oots with slender, creeping rhizomes
  • Stems, woody at the base, hollow, and round with two ridges, have black glands along the ridges
  • Upper branches are arranged in opposite pairs at 45 degrees
  • Narrow to oval, stalkless leaves, 1-3 cm long, in opposite pairs along the stems, are covered with many translucent glandular dots, and have black glands along the edges
  • Dense flat-topped clusters of many golden-yellow, star-like flowers (each 8-20 mm across) are produced at the tops of stems between December and March
  • Each flower has 5 petals with black dots on the rims
  • Stems usually die back in autumn, leaving prostrate leafy shoots that can form dense mats
  • Toxic to livestock, especially if eaten in quantity when other food is scarce.

  • Each plant can produce large quantities of seed, but output per plant is variable
  • The fruit is a sticky, 3-celled capsule (5 to 10 mm in length) containing many 1 mm long seeds
  • Seeds require light for germination and seedling growth is much slower than for most other grassland plants
  • This implies that germination and early growth are probably severely restricted or prevented under a strong pasture sward
  • Seeds can survive in the soil for 3 or more years
  • Stems usually die back in autumn, leaving prostrate leafy shoots that can form dense mats
  • In autumn or spring new crowns grow from lateral roots and eventually become separated from the parent plant. This can result in extensive population growth
  • Plants grow in a variety of dry, gravely or poor soils including wasteland, pasture, river beds and banks, roadsides, dunes, open scrub, open grassland, gravel pits, and railway ballast.


  • St John’s wort is native to Europe, western Asia, and North Africa and is now widely distributed through temperate areas, including parts of South America, India, Australia, and South Africa
  • Has been widely cultivated as a garden or medicinal plant, which might be part of the reason for its world-wide distribution
  • Considered a weed in much of its native range, particularly in Turkey, Italy, France, Hungary, and Sweden
  • First recorded in New Zealand in 1869 and is now found in many parts of North and South Islands in tussock grasslands, but also in pastures, riverbeds, waste places, on roadsides, and in many modified open communities.

Similar species

  • Many other Hypericum species are found in New Zealand, including two native species and five introduced species that have been found growing outside their planted range
  • The two native species lack black glandular dots, and are low growing or mat forming plants.


  • The seeds have no special dispersal mechanism but may be spread by wind, water, animals, or by human activity, especially along roadways (e.g. vehicles, metal dumps etc.)


  • In Europe St John’s wort has long been used for a variety of purposes, from treating burns and skin disorders through to depression. It has been used as an ingredient for distilling vodka, and as a source of red, yellow, purple and orange dyes
  • In folklore St John’s wort was thought to possess magical properties, and was used as a charm against storms, thunder, evil spirits, and witches. It was claimed to bring good luck if sprigs of the plant were hung about the house or carried as a charm and, if you slept with them under the pillow, you would dream of a future lover
  • In herbal medicine St John’s wort is commonly recommended as an anti-depressant but health authorities say that it can interfere with other medications and should be used with caution.

  • Displaces better forage species, and although livestock usually ignore it, it is extremely poisonous if consumed
  • Hypericin, the chemical responsible for the toxicity, is found at all growth stages in fresh or dry foliage, and causes photosensitization in animals
  • Horses appear to be more susceptible to hypericin toxicity than cattle, cattle more than sheep, and sheep more than goats
  • For many years St John’s wort was the scourge of livestock farmers, particularly in the South Island, but was one of the early successes of biocontrol.

Non-chemical control

  • Hand-pulling or digging of small populations may be effective only in the early stages of infestation
  • Cutting or mowing often results in regeneration from roots and is rarely effective unless repeated very regularly over long periods
  • Tillage is effective in giving control, and St John’s wort is not a problem in crops

Chemical control

  • Weed-wiping or spraying with glyphosate or metsulfuron-methyl are among the most effective treatments but both materials will kill or damage grasses and other pasture components
  • Picloram granules and triclopyr/picloram sprays are the only registered herbicides for pasture. Note however that both are highly injurious to clovers
  • ALWAYS READ PRODUCT LABELS BEFORE APPLYING: Consult your farm consultant, industry rep or the New Zealand Agrichemical Manual for more information about chemical control.


  • The lesser St John’s wort beetle (Chrysolina hyperici) and the greater St John’s wort beetle (Chrysolina quadrigemina) were imported to New Zealand from Australia in 1943 and 1965 respectively
  • These beetles have been successful in controlling this species in Australia and North America and also in New Zealand
  • Both the adults and the larvae defoliate the plants so flowering and seed production are suppressed. The larvae destroy spring growth almost as soon as it appears
  • Since their import and release in New Zealand the beetles have very effectively destroyed much of the St John’s wort and although isolated infestations of the weed still occur from time to time the beetles quickly build up numbers and suppress the weed again.