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Hypericum perforatum
(St John's Wort)
First introduced to Australia from Europe in the 19th Century for its ornamental and medicinal purposes, Hypericum perforatum is now a significant weed of agricultural pastures and bushland, where it out-competes the native plants. In fact, 80% of current infestations occur in natural bushland.
It is a weed in all States and Territories except NT. In Australia it has infested more than 400,000 hectares and is now a prohibited entry. Legislation regarding its control applies in NSW, Tasmania, Victoria and Western Australia.
St John's Wort is a perennial herb that grows to about 1.2 metres tall, with narrow to paddle-shaped leaves that vary from between 5 to 25 mm long by 1.5 to 5 mm wide.
The leaves are dotted with clear oil glands that give them a perforated appearance (hence the species name).
The bright yellow flowers develop in clusters at the end of branches and are about 10 to 20 mm in diameter. It flowers from December to January, after which sticky capsules develop. The plant can produce up to 30,000 seeds annually. Fresh seed is dormant for about 4 to 6 months, but they can also remain viable for up to 12 years. Germination is during spring and autumn.
A number of slightly different forms of this plant exist in Australia, the most common having narrow leaves. The form is very much dependent on habitat and growing conditions.
Spread
H. perforatum is spread primarily by animals, machinery, contaminated fodder, seed or soil, and dumped garden waste. It can also be spread by wind but the spread is less than 10 metres per year, and it suckers but this is also slow at about 30 cm per year.
Control
It's illegal to import or grow this plant.
Preventing its spread is the highest priority for control of St Johns Wort. In heavily infested agricultural land, burning has proven successful.
Hand weeding is not an effective control as the entire root system must be removed - every little bit of it. This is because the plant can reproduce from buds on roots.
Herbicides such as glyphosate are effective against St John's Wort. And in agricultural land, farmers are encouraged to better manage their pastures - so that pasture grasses outcompete the weeds.
Twelve biological control agents have become established in Australia. These include beetles and a gall midge that feed on the plant.
Australian Species
There are several Australian species of Hypericum, which can be confused with the weed, especially as they may co-occur with St John's Wort. Shown here is the Australian Hypericum gramineum (plant and close-up of flower), courtesy of the Australian National Botanic Garden (plant) and University of Tasmania (flower).
There are also several other introduced species of Hypericum.
Black Market
St John's Wort contains the alkaloid hypericin which causes photosensitisation (skin becomes abnormally sensitive to bright sunlight). This results in blisters on weakly pigmented parts of exposed skin. This causes serious problems for livestock.
Unfortunately because of its uses in alternative medicine a black market in this illegal plant has developed. Naturalised plants are often dug up and transferred to pots.
Information sources:
www.weeds.crc.org.au
Environmental Weeds - A field guide for SE Australia, by Kate Blood, published by CH Jerram Science Publishers
www.weeds.crc.org.au/documents/fs1_st_johns_wort.pdf
www.agric.nsw.gov.au/reader/11259
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