Hypericum perforatum
(St John's wort)

H. perforatum is a medicinal plant which is poisonous to livestock, and which is considered a weed even in some of the countries where it is native. Such countries include Turkey, Italy, France, Hungary and Sweden, where it is a weed mostly in poorer pastures, neglected areas and occasionally in crops (Parsons and Cuthbertson, 1992). It has been widely introduced, and has become a serious invasive problem in parts of Canada, USA, Mexico, Argentina, Chile, South Africa, Reunion, Japan, Australia, New Zealand and Hawaii, often because of the damaging photosensitivity it can cause in livestock. Once established in an area it spreads vegetatively and is hard to control without damaging other pasture species. It is invasive in natural ecosystems as well as in agricultural land. This species is considered to have a number of herbal uses and has therefore been spread to many places as a useful medicinal or ornamental plant as well as an accidental hitchhiker on agricultural produce.

H. perforatum was originally distributed in Europe, Western Asia and North Africa, with a broad Eurasian native range extending from the Atlantic seaboard of Europe to China and India. It has now been introduced, accidentally or deliberately, to many temperate parts of the world, most notably North America, Australia, South Africa and New Zealand, in all of which it has become a serious problem as a pasture weed poisonous to livestock. 

The risk of H. perforatum being introduced to yet more temperate countries is clearly high, especially as its small seeds are easily hidden amongst other seed samples or in hay or straw taken from one country to another. In addition this species is also likely to be carried legally or illegally to other countries as a useful medicinal plant. 

H. perforatum grows in humid and sub-humid temperate regions, usually on drier sites. It becomes invasive (weedy) when it invades poorly managed grazing land, sparse bushland, roadsides and neglected areas. In Australia it establishes well on loose soil in areas with a rainfall of above 750 mm (Parsons and Cuthbertson, 1992). Improved pastures are rarely invaded except where they are over-grazed or otherwise mistreated.

In Britain, Stace (2010) describes its habitat as ‘dryish grassland, banks and open woodland’. In Pakistan its habitat is described as ‘In wet or dryish, shaded or open grassy places often near streams or sometimes a weed of cultivation, 1200-3000m’ (Flora of Pakistan, 2015). Flora of China Editorial Committee (2015) says ‘Open woodlands, meadows, grasslands, and steppes, riverbanks, stony and grassy slopes, roadsides, in dry or well-drained habitats; 100-2800m.’

In New Zealand, Webb et al. (1988) say of its habitat ‘A particularly troublesome weed in tussock grasslands, also very common in pastures, roadsides, riverbeds, waste places and in many modified open communities.’ At one time this species was responsible for appreciable harm to livestock, especially sheep in South Island highland tussock grassland, though cattle too sometimes suffered in the same environment (Connor, 1992). In Australia, H. perforatum occurs in grasslands, open Eucalyptus and Callitris woodlands, along river banks, cleared pasture land and forest plantations (Csurhes and Zhou, 2008).

H. perforatum is often mentioned as being found in semi-shade, but this is usually along roads and at the margins of forests: it rarely persists in mature forest stands (Tisdale et al., 1959). Buckley et al. (2003) indicate that plants growing in shaded sites grow more slowly and produce less fruit but live longer. 

In most countries H. perforatum is a problem in poorly managed pastures or rangelands because it can crowd out existing useful pasture species and inhibit the germination and growth of other species. The plant contains hypericin, a red pibment concentrated in the black oil glands, which is toxic to livestock when consumed in quantity.

Elsewhere it is found along roadsides, in waste places, forest clearings and other sites (Crompton et al., 1988). Crompton et al. said it was becoming an increasingly important weed in lowbush blueberry culture in eastern North America.  

In Australia and presumably elsewhere, seeds germinate in autumn, winter or spring and plants rarely flower in their first year. Established plants have two life forms. In winter and spring the plants form low-growing stems 30 cm long and bearing dense foliage which gives a dense, smothering mat. In spring, erect woody stems develop and flowering occurs (in Australia) in late spring (November) and they continue flowering well into summer. These flowering stems die back in late summer but can remain standing for several months or sometimes years (Parsons and Cuthbertson, 1992).

Zohar (2004) explained, quoting a number of studies, that H. perforatum exhibits a high degree of phenotypic and genotypic variation, particularly among populations. Studies comparing the characteristics of populations in California, Oregon and Montana found strong regional differences in chemical (hypericin and pseudohypericin) content and morphological (gland density, leaf area, stem length, leaf length:width ratio) characteristics (Shiflet, 1994;  Walker et al., 2001; Sirvent et al., 2002). The species also shows genetic variation in morphological variation and herbivore resistance (Campbell et al., 1997; Mayo and Roush, 1997; Buckley et al., 2003).

Genetics

Gillett and Robson (1981) found that in Canada haploid chromosome numbers varied from 16 to 18 while diploid numbers were 32 with one report of 48. The majority of studies cited in Missouri Botanical Gardens (2015) give the chromosome count as 2n = 32. 

Reproductive Biology

H. perforatum is a facultative apomict and is also pseudogamous (Pritchard, 1960) meaning that fertilisation is necessary to promote development of the endosperm in the seed but not to fertilise the ovum.

Seed production is prolific and a single plant can produce 33,000 seeds in a season, with some of these seeds remaining viable in the soil for 10 years (Parsons and Cuthbertson, 1992). Tisdale et al. (1959) observed a high degree of variation in field studies in Idaho, with seed production per plant ranging from a few thousand to nearly 100,000. Andersen (1968) reported that seeds maintained a germination rate of 50% after 15 years of dry storage at laboratory temperature. Tisdale et al. (1959) found no loss in germination for 3 years when seeds were buried 1.2 cm deep in the soil.  

In addition to its seed production, H. perforatum reproduces vegetatively by rhizomes, root fragments or lateral root sprouts (Zouhar, 2004). Sometimes this results in several crowns sharing a common rooting system, and also makes the longevity of individual plants next to impossible to determine. Harris (1971) says new St Johnswort plants originate at intervals along underground roots that may extend 90 cm or more from the parent plant. Plants propagated vegetatively severed connections with the parent plant fairly soon after new plants had established (Zouhar, 2004).

In eastern Canada, H. perforatum flowers during summer (late June, July and early August), with seeds reaching maturity in autumn (late August).

Thomson (1922) says that in Europe its flowers are visited by Apis mellifera, Bombus hortorum, B. terrestris, Eristalis tenax and Calliphora erythrocephala, presumably for nectar and pollen.

Physiology and Phenology

Fresh seed does not appear to germinate readily and requires 4-6 months after-ripening; this temporary dormancy may be due to an inhibitor in the sticky material associated with the seed capsules. However this requirement for after-ripening has been disputed by other authors (Crompton et al., 1988). Germination can be enhanced by alternating temperatures, gibberellic acid or potassium nitrate.

Seedling establishment is favoured by bare soil and wet summers (Clark, 1953).

Pritchard (1960) compared populations of H. perforatum from British Columbia, Ontario, California, Britain and New Zealand. He found that populations from British Columbia and California were taller and more aggressive, produced greater biomass, and were, as a consequence, more weedy. Under favourable conditions, plants developed vertical roots, whilst under less favourable conditions rhizomes developed actively. However other authors (Vila et al., 2003; Marron et al., 2004) have disputed these findings, observing that plants from the introduced range were neither uniformly larger nor better interspecific competitors than plants from their native range.

Longevity

Seeds can remain viable in the soil for considerable periods if conditions are suitable for long-term survival (Zouhar, 2004).

Under at least some conditions, seedling mortality can be high and is probably exacerbated by competition from mature plants of the same species or by other plant species (Tisdale et al. 1959; Campbell, 1985). This is at least partly due to the slow growth of H. perforatum seedlings.

The median longevity of individual plants from four sites in Australia with various climatic profiles was 3 to 6 years with an overall range of 1 to 8 years (Briese, 1997). Very few crowns survived longer than 8 years: death of crowns was attributed to drought, fire, defoliation by Chrysolina  beetles, combinations of these, and senescence.

Activity Patterns

Seedlings of H. perforatum are very small and grow slowly so that young seedlings are susceptible to competition for light, nutrients, space and moisture from mature plants of their own and other species (Briese, 1997). In autumn and winter, H. perforatum has a low-growing, very leafy growth form, effectively swamping growth by low-growing or emerging plants of other species. In spring, stems become much taller and woody and carry flowers. Plants produce underground rhizomes and long taproots which allow the plants to be drought-tolerant by comparison with nearby neighbours of other species.

Population Size and Structure

Crompton et al. (1988) describe H. perforatum as a particularly aggressive weed of rangelands characterised by dry summers. Sampson and Parker (1930) said its deep root system can support the plant when water for more desirable plants has been depleted. It forms a dense, spreading canopy up to 1 m tall, and large infestations in western North America covered over 1 million hectares in the 1950s, according to Holloway (1957). These authors also said that in California plants could produce 4300 kg ha^-1 and that vegetative reproduction seemed to be stimulated by grazing, fire and defoliation.

According to Zouhar (2004), crown and stem density of H. perforatum may be affected by many variables. Stem production can be affected by rainfall pattern, insect defoliation, and site factors (Briese, 1997), with St Johnswort population fluctuations and dynamics driven by complex interactions of several stress factors (e.g. shallow, rocky soils, shading, drought, herbivory), and/or positive factors such as good rainfall years. For example, deep soils favour development of vertical roots and long-term crown survival, while shallow soils tend to support H. perforatum populations with more lateral roots, root sprouts, and shorter-lived crowns.

Harris (1971) describes a ‘typical’ St Johnswort near Colville, Washington, as dense, ‘almost to the exclusion of other herbaceous vegetation.’

Nutrition

Several authors have commented that H. perforatum is less common on calcareous soils and seems to prefer soils with slightly acid to neutral pH (Sampson and Parker, 1930; Daubenmire, 1947, cited by Crompton et al, 1988). Sampson and Parker suggested that while H. perforatum is not restricted to acid or alkaline soils, unusually luxurious growth and dense stands are more typically found on soils with pH 5 to 56.5 while very vigorous growth was not observed on strongly alkaline soils.

Associations

Crompton et al. (1988) list the species associated with H. perforatum at two locations in Nova Scotia. Most of the associated species are common weeds found in most places. Sampson and Parker (1930) describe dense St Johnswort stands in California with winter annuals in the understory.

Harris and Clapperton (1997) noted that 31% of the roots of H. perforatum plants growing in Canadian field infestations were infected with vesicular-arbuscular mycorrhizal (AM) fungus. H. perforatum seedlings inoculated with AM fungi were better able to tolerate harsh environmental conditions (Moora et al., 1998).

Environmental Requirements

H. perforatum is a predominantly temperate species with a very wide native range and which can tolerate a broad range of environmental conditions. Its natural range extends from parts of Central Asia with very low winter temperatures, to subtropical environments such as the Canary Islands. The species can tolerate a wide variety of soils, from dry, rocky, shallow soils to deep, fertile soils, with its best performance coming in areas with at least 760 mm of rainfall a year. Soil pH tolerance ranges from 4.3 to 7.6 (Rutledge and McLendon, 1996). It can tolerate drought and disturbance by means of reserves stored in the root crown (Buckley et al., 2003). It does not tolerate saturated soils.

H. perforatum hosts at least 37 species of insects in its native environment (Holloway, 1964), suggesting that at least part of its success in new countries is due to the absence of natural pests. Two leaf-feeding beetles Chrysolina hyperici and C. quadrigemina have been introduced as biological control agents and these have been very successful in some places (New Zealand, British Columbia and California) but less successful in others, like parts of Australia (see Biological Control section, under Prevention and Control). 

Drugs, stimulants, social uses

• Psychoactive

General

• Ritual uses
• Sociocultural value

Materials

• Chemicals
• Essential oils
• Lipids
• Poisonous to mammals

Medicinal, pharmaceutical

• Source of medicine/pharmaceutical
• Traditional/folklore

Other species of Hypericum have similar flowers but the only ones likely to be confused with H. perforatum are those with similar life-forms. Thus H. canariense, native to the Canary Islands but introduced to several other countries as an ornamental, is described by GISD (2015) as ‘similar’ but is a shrub. Dwarf St. John's wort (Hypericum mutilum), also described by GISD (2015) as ‘similar’ is a smaller plant, half the height of H. perforatum, with a one-chambered capsule unlike H. perforatum which has a three-chambered capsule. This species is native to North America but has been spread to other countries. In New Zealand Popay et al. (2010) describe H. pulchrum as similar to H. perforatum, and Webb et al. (1988) report that ‘H. pulchrum may grow in the same locality as H. perforatum, but can be distinguished by the ± terete [round] stems, the even fringe of black glands on the sepals, and the reddish petals, anthers and stigmas.’

In Canada, Crompton et al. (1988) claim that H. scouleri and H. punctatum are similar but point out the important differences. In H. punctatum the black glandular dots are scattered throughout the petals whereas in H. perforatum they are restricted to the margins of the petals. H. scouleri has ovate-elliptical sepals and the black dots on them are elongated more than those sometimes found in H. perforatum.

In Australia, the indigenous species H. gramineum and H. japonicum can co-occur with H. perforatum and be confused with it (Csurhes and Zhou, 2008). Both native species can be distinguished by the absence of oil glands on their leaves and petals, the presence of four longitudinal ridges on the stem (two for St John’s wort) and the stamens not being fused into bundles.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Eradication

Zouhar (2004) stresses that control of H. perforatum infestations requires several strategies designed to affect different plant parts and different stages of the plant’s life history. These strategies will depend be site-specific and long in term as populations can quickly build up again.

Physical/Mechanical Control

Isolated plants should be grubbed before they produce seed, but large areas are best ploughed in the autumn, cultivated whenever regrowth shows, and sown the following autumn in an appropriate pasture mixture (Parsons and Cuthbertson, 1992). Crompton et al. (1988) said that H. perforatum is readily controlled by tillage and is therefore seldom a problem in cultivated crops. Such techniques are obviously limited to small infestations or to areas where tillage is possible.

Biological Control

The introduction of weevils of the genus Chrysolina has led to effective control of H. perforatum in several countries where the weed was a serious problem earlier. Thus it has been very effective in New Zealand, South Africa, the USA and Canada. In South Africa H. perforatum is now considered to be under satisfactory control in the Southwestern Cape (Gordon and Kluge, 1991), thanks to the introduction of Chrysolina quadrigemina in 1960 and of the gall midge Zeuxidiplosis giardi in 1972. In Australia, C. hyperici and C. quadrigemina adequately control the weed under open but not under shaded conditions (Shepherd, 1985). At shaded sites, plants appeared less aggressive than elsewhere and their growth habits differed from those of plants growing at open or semi-shaded sites. C. quadrigemina is now the dominant species and the main insect exerting some control on the plant. In New Zealand C. hyperici was introduced in 1943 and was followed two years later by releases of C. quadrigemina. Syrett et al. (1997) concluded that ‘There are no longer reports of areas in New Zealand where St. John's wort is a problem weed, and we conclude that successful biological control with insects is as least partially responsible for this change.’

In the western USA, where the species was once a problem, the success of the biocontrol agents has led to its replacement with annual grasses or with mixtures of forbs and grasses (Zouhar, 2004).

Chemical Control

Herbicides such as glyphosate can be used to get effective control of small infestations but are rarely the complete or long-term answer to the problem unless competitive species of plants are encouraged by oversowing and by appropriate grazing management. Herbicides are too expensive for use over very large areas of infestations, especially since consideration must also be given to oversowing of desirable species to replace the Hypericum and to prevent its resurgence. Campbell and Nicol (1987) tested a range of herbicides for the control of H. perforatum in Australia but concluded that the species is hard to selectively control with herbicides and that every effort should be made to promote biological control and/or effective grazing management.

IPM

Seedlings of H. perforatum are very susceptible to competition and offer the best stage at which to tackle infestations. This can be done by encouraging competitive species either by using careful grazing management or oversowing with pasture species. Any strategy must consider the presence of long-term soil seed banks. In Australia a programme of seeding pasture species (T. subterraneum and Phalaris spp.) combined with cultivation and fertilization on arable land has been proposed (Campbell and Nicol, 1997; Groves, 1997).

According to these same authors, in New South Wales and Victoria, change in land use from pastures badly invaded with H. perforatum to pine plantations has been used by farmers as an evasion strategy.

22/06/15 Original text by:

Dr. Ian Popay, Hamilton, New Zealand