St John's Wort: Why a Pretty Garden Herb Became a Regulated Weed
It poisons grazing stock, its seeds stay alive for years, and it's regulated from Tasmania to Alberta. In California, imported beetles brought it under control.
St John’s wort (Hypericum perforatum) opens bright yellow, five-petalled flowers around midsummer, and NC State Extension lists it as tolerant of drought and poor soil. According to NSW WeedWise, it “was brought to Australia in 1875 as a garden plant.”
Today Weeds Australia lists it as a declared weed in five Australian states and territories, a listed noxious weed in US states including Washington, Oregon and Montana, and a prohibited noxious weed in Alberta. Two things put it there. It makes grazing animals dangerously sensitive to sunlight, and one plant can leave seed in the soil for a decade or more.
Why do weed agencies treat it so seriously?
Mainly because of livestock. NSW WeedWise explains that the plant contains hypericin, and animals that eat it “become very sensitive to sunlight.” Stock usually only eat it when other feed is scarce. Intense sunlight worsens the effects, and the page warns that on sunny days, stock without shade “can develop signs of acute poisoning in five hours.” Some animals die.
Hypericin levels are lowest in July and August (winter in NSW), rise as flowering shoots grow in spring, and peak at full flower. Wet weather pushes them higher.
The plant is also hard to shift. NSW WeedWise puts seed output at up to 33,000 a year, viable for 12 years, while NC State says up to 100,000 a year, lasting 10 years in soil. Roots reach about 1 m deep and send out side shoots. NSW says hand-weeding isn’t effective, because the entire root system has to come out to stop regrowth, and cultivating can spread root fragments.
Where is it regulated?
Each list uses its own categories. This table draws on official sources, checked on 8 November 2026.
| Place | Status |
|---|---|
| Tasmania | Declared weed. Importation, sale and distribution are prohibited |
| New South Wales | General biosecurity duty statewide. In the Central Tablelands and Central West, people should not buy, sell, move, carry or release it. In exclusion zones covering most of the Hunter and North West regions, people should not grow it |
| Victoria, ACT and WA | Declared, per the national Weeds Australia profile (updated June 2026). We couldn’t check those governments’ own lists, so confirm with your state |
| Washington State | Class C noxious weed since 1988. Class C weeds “are widespread in Washington or are of special interest to the agricultural industry” |
| Oregon | On the state’s List B of noxious weeds |
| Montana | Priority 2B on the 2019 list, for weeds “abundant in Montana and widespread in many counties” |
| Alberta | Prohibited noxious weed, still listed after the January 2026 revision. The Weed Control Act says these “must be destroyed” |
Rules change and often vary by county, region or catchment. In the US, check with your state department of agriculture or county noxious weed coordinator before planting.
How did beetles bring it under control?
Insects, including these leaf beetles, were first brought from Europe to Australia to control the plant there. According to UC IPM, entomologists from the US Department of Agriculture and the University of California then “obtained from Australia” insects that had come from Europe, checked in quarantine that they didn’t feed on beneficial plants, and released them in California.
UC’s WeedCUT guide dates the first Californian releases of Chrysolina hyperici to 1945 and Chrysolina quadrigemina, the Klamathweed beetle, to 1946. UC IPM reports that by 1957 the weed’s abundance there “was reduced by over 99%,” and that the program still saves ranchers money every year.
The same beetles are now widespread in NSW, where WeedWise notes they can strip every leaf from a stand in spring. There’s a trade-off. In California the beetles have been reported to attack the native Hypericum concinnum and on ornamental Hypericum calycinum, though WeedCUT says they don’t appear to affect those plants’ populations.
Is this St John’s wort, and what now?
Official descriptions give a few reliable cues. Biosecurity Tasmania describes a perennial 30 to 70 cm tall, occasionally 1.2 m, with reddish stems. Its stalkless leaves have small oil glands that make them look perforated when held up to the light, and the bright yellow petals have black glands dotted along their edges. NSW WeedWise adds that flowers are about 20 mm across, with three bundles of long stamens, and the fruit is a sticky capsule.
In poisoned stock, NSW lists early signs such as agitation, head rubbing against posts or trees, panting and weak hind legs. As it worsens, the skin around the forehead, eyes and ears swells and turns red. NC State flags the plant as a problem for horses, dogs and cats too.
For help, NSW suggests your local council weeds officer, Tasmania’s page gives Biosecurity Tasmania’s contact details, and Washington’s board points to county noxious weed coordinators.
What about St John’s wort products?
The US National Center for Complementary and Integrative Health (NCCIH) says St John’s wort can interact “in dangerous, sometimes life-threatening ways with a variety of medicines.” Its list includes birth control pills, some antidepressants, warfarin, some HIV drugs, some heart and cancer medicines, and cyclosporine, which transplant patients take. NCCIH also notes possible severe skin reactions after sun exposure when taken by mouth in large doses or applied to the skin, and says it may be unsafe in pregnancy. Its warnings cover St John’s wort supplements, so ask a pharmacist or doctor before using any form.
Is there a hypericum you can grow instead?
Possibly, but check first, because several garden hypericums have weedy habits of their own.
- Tutsan (Hypericum androsaemum) was introduced to Australia as a garden plant and is declared in Victoria and WA, per Weeds Australia. We couldn’t check those states’ own lists, so confirm locally.
- Aaron’s beard (Hypericum calycinum) is a popular groundcover, but NC State calls it “vigorous to the point where it can swamp out small plants” and highly toxic if eaten.
- In the central and south-eastern US, NC State lists the native cedar glade St John’s wort (Hypericum frondosum), a pollinator shrub about 60 to 120 cm tall (converted from 2 to 4 feet). Its St John’s wort page also suggests native Hypericum prolificum and Hypericum hypericoides.
We didn’t find an official recommendation for a safe ornamental hypericum in Australia, so your state’s declared plant list is the place to start.
What this means for you
- Don’t plant Hypericum perforatum where it’s declared, and check your local weed list before planting any hypericum.
- If you find it on your land, contact your local weeds officer or state agency for advice on control.
- If you keep grazing animals, NSW advises never grazing while the plant is flowering, and shade helps protect stock.
- If you see shiny metallic beetles on it, they may be the biocontrol agents at work, though the plant still needs managing.
- If you take any medicine, talk with your pharmacist or doctor before using any St John’s wort product.
Have you come across St John’s wort on a roadside or paddock near you, and did you know it was a declared weed? Hit reply and tell us.
How we made this: AI tools helped us research and draft this post. We checked every figure, quote and link against its original source before publishing. Figures are as of 8 November 2026.
Grown with intention by Dynamic Nexus Garden.