St. John's wort (Hypericum perforatum)

St John's wort is a herb with bright yellow flowers. It can poison livestock.

Profile

How does this weed affect you?

St John’s wort:

• competes with pastures
• poisons livestock
• can downgrade wool with ‘vegetable fault’
• can reduce property value.

Livestock poisoning

St John’s wort contains a chemical called hypericin. Livestock that eat it become very sensitive to sunlight. Stock will only eat St John’s wort when other feed is scarce. Minor exposure to St John’s wort affects animal health as:

• weight loss
• fewer pregnancies
• stillbirths
• weak young
• cows producing less milk
• fewer lambs and calves surviving weaning.

Intense sunlight worsens the effects of hypericin. Access to shade helps protect animals. On sunny days, stock without access to shade can develop signs of acute poisoning in five hours.

Early symptoms of acute hypericin poisoning include:

• agitation
• rubbing the head against posts or trees
• weak hind legs
• panting
• confusion
• depression.

Some animals develop mild diarrhoea.

As poisoning gets worse, animals get a high temperature. The skin around their forehead, eyes and ears swells and turns red. Head rubbing against hard objects causes wounds and bleeding. Animals can die from acute hypericin poisoning.

Tolerance

Age and the amount of skin protection affect how tolerant animals are to hypericin. More tolerant animals have:

• pigmented skin
• dense wool or hair
• thick, tough skin.

Adults are more tolerant than young animals because they have thicker skin, wool or hair. Suckling young can ingest hypericin from their mother’s milk.

Management

Keep affected stock in full shade for 4 to 7 days. Animals should not show further signs of poisoning when returned to sunlight. If they do, they haven’t excreted all the hypericin from their blood yet and need to go back into the shade.

Keep pregnant and lactating animals out of St John’s wort-infested pastures. Provide shade in infested paddocks to improve stock tolerance to hypericin.

Never graze when St John’s wort is flowering. Levels of hypericin change over the growing season. Hypericin levels:

• are lowest from July to August
• rise rapidly in spring when flower shoots are taller 5 – 10 cm
• continue rising as flowers develop
• are strongest when the plant is in full flower
• decrease towards the end of summer as flowers drop.

The amount of hypericin varies with the type of St John’s wort. See the section on varieties below for more information on how to tell them apart. Hypericin levels increase during wet weather. 

What does it look like?

From a distance an infestation appears:

• yellow from November to January
• dark green, brown and yellow from February to April
• brownish-red in winter.

Leaves are:

• paler green on the underside
• opposite each other on the stem
• spotted with oil glands - leaves can look like they have holes in them when held against a strong light.
• without a stalk.

Flowers are:

• bright yellow
• about 20 mm in diameter
• with five petals
• with three bundles of long thread-like stamens growing from the centre
• present from late October to January.

Fruit are:

• a sticky, three-celled capsule
• about 8 mm long
• split open when ripening.

Seeds are:

• in sticky seed capsules
• small (0.5 – 1 mm)
• cylindrical
• light brown to black
• with a pitted seed coat

Stems are:

Non-flowering stems:

• grow from the crown
• can form tangled thickets
• are present in autumn and winter.

Flowering stems:

• are upright and woody
• sometimes have a reddish tinge
• branch near the top
• have two opposite ridges running the length of the stem
• are produced in spring.

Roots are:

• vertical - growing to about 1 m deep into the soil
• horizontal - producing buds that form new growth above ground.

Varieties

There are two main strains of St John’s wort in NSW - broad-leaf and narrow-leaf strains. The strains have different toxicity levels at different times of the year. See the section on grazing timing below for more information on when to graze each variety.

Measure the leaves to tell them apart. Measure leaves at the 6th node (bump) on the flowering stem when the plant is growing well in spring. The narrow-leaf strain has leaves 7 – 9 mm wide. The broad-leaf strain has leaves 10 – 12 mm wide.

The narrow-leaf strain:

• has more oil glands in the leaves
• is late-flowering
• is tall, growing to 90 cm
• has thin stems
• has small seed capsules.

The broad-leaf strain:

• contains fewer oil glands in the leaves
• is early-flowering
• is short, growing to 60 cm
• has thick stems
• has large seed capsules.

Where is it found?

The heaviest NSW infestations are in the central and southern tablelands and slopes. The narrow-leaf strain is more widespread.

St John’s wort is native to Europe, Asia and North Africa. It was brought to Australia in 1875 as a garden plant. It is used in herbal medicine.

Maps and records

• Recorded presence of St. John's wort during property inspections (Map: Biosecurity Information System - Weeds, 2017-2023)
  These records are made by authorised officers during property inspections under the Biosecurity Act 2015. Officers record the presence of priority weeds in their council area and provide this to the NSW Department of Primary Industries. Records reflect the presence of the weed on the date of inspection.
  
  

What type of environment does it grow in?

St John’s wort grows in pastures, riparian areas, and bushland. It prefers:

• rainfall over 600 mm per year
• locations above 500 m altitude.
• deep soils.

How does it spread?

By seed

The sticky seed capsules stick to animals. Seeds are also carried in the digestive tracts of animals. Wind spreads seed over short distances. Water, machinery, humans, livestock or feral animals spread seed over long distances.

By plant parts

Roots sucker and new plants grow from fragments.  Cultivation can move root fragments.

More information

back to top

Control

Long-term control of St John’s wort needs to consider that:

• new seedlings appear from autumn to spring
• seeds need mild temperatures, light and rainfall to develop
• competition suppresses young seedlings
• most new foliage grows in autumn and winter
• foliage dies off in late spring
• flowers develop in spring to autumn
• new plants do not flower in the first year
• seeds are released autumn to winter
• a plant can produce up to 33 000 seeds per year
• seeds can remain viable for 12 years.

Prevention

To reduce the risk of infestations developing:

• learn to identify the weed and look for it in spring
• minimise stock movements from infested to clean paddocks
• quarantine livestock from a contaminated area for five weeks
• clean vehicles that have passed through infested areas
• buy certified seed
• restrict feeding to flat, easy to monitor areas or paddocks with strong perennial pasture growth.
• check feeding areas and control any St John’s wort plants (it’s best to also manually remove seed heads).
• establish competitive perennial pastures along boundaries of infested paddocks - this acts as a buffer against further spread.

Pasture management

Healthy perennial pastures are the best long-term defence against St John’s wort. Good autumn and winter pasture cover can suppress new St John’s wort plants. To maintain healthy pasture cover:

• grow combinations of winter and summer pastures
• rest pastures between grazing periods

• test soil to check fertility
• control pests
• use fertiliser if needed.

An agronomist can advise on a crop program to support St John’s wort control in arable areas. This can reduce the amount of St John’s wort seed in the soil before you sow a perennial pasture. A typical program might be to:

• spray or plough in summer to reduce weed numbers
• control weeds again in autumn before sowing a winter cereal or forage crop
• repeat for a second year to decrease the number of St John’s wort seeds in the soil
• sow a perennial pasture in the third autumn.

Sowing perennial pastures can help suppress St John’s wort even in less arable country. Use a direct drill/minimum tillage method to reduce soil disturbance. Control annual weeds over summer and again in autumn before sowing pasture. Allow the new pasture to set seed in the first year. Spot spray any new St John’s wort plants.

Grazing

Make sure there are shade trees in St John’s wort-infested paddocks for animals to shelter under. Grazing is usually the only practical way to control St John’s wort in steep areas.

Take care of animals when using grazing as a control strategy. Never graze when St John’s wort is flowering.

Sheep with superfine or fine wool are best suited for controlled grazing of St John’s wort. Superfine merino sheep are more than twice as tolerant to hypericin as medium wool merinos. Use adult merino wethers or dry, non-pregnant ewes with at least four months’ wool growth. Cattle, goats and other stock are less suitable.

Timing

Safe grazing periods are when the flowering stems are dead. The base of the plant will have soft, green shoots that are low in hypericin. Grazing hard in spring can delay flowering stem growth and extend the safe grazing period.

Broad-leaf St John’s wort has a longer grazing period. With sheep, graze:

• broad-leaf from early May to mid-October
• narrow-leaf from early July to mid-September.

In spring, move stock off St John’s wort pastures before flowering stems reach 5 – 10 cm.

Cattle can graze St John’s wort pastures about six weeks earlier than sheep. Cattle can also remain on St John’s wort infested pastures much longer than sheep in spring.

Change grazing periods based on the weather. Shorten during wet years as hypericin levels increase. Grazing can go longer when it’s dry.

Use short periods of intense grazing. Fence in heavy infestations of St John’s wort to encourage stock to intensively graze those areas.

Physical removal

Hand-weeding is not an effective way to control St John’s wort. The entire root system has to be removed to stop new plants from growing.

Cultivating tends to spread the weed unless all the roots are brought to the surface and dried out.

Fire

Burning can destroy seeds on the plant. The plant will regrow from the roots. Fire tends to cause more damage to pasture than to St John’s wort.

Biological control

Eleven biological control agents have been released in Australia. Six have established.

Chrysolina beetles

Chrysolina larvae and beetles feed on the leaves of St John’s wort. Larvae feed on winter growth. Adult beetles attack spring growth. Beetles can form dense infestations that remove all leaves on St John’s wort.

Chrysolina beetles are most effective when beetles and larvae feed in the same or consecutive years. They are only effective in unshaded situations as they mate only in sunlight.

Catch beetles in spring and move them to new infestations. Do not use herbicides when high numbers of Chrysolina beetles are present. Partially defoliated plants are unlikely to absorb enough herbicide to kill them.

Chrysolina hyperici and Chrysolina quadrigemina are black with bronze, dark-blue or purple reflections. They are oval shaped. C. quadrigemina is slightly larger (6.0 – 7.1 mm) than C. hyperici (5.3 – 6.1 mm). Some C. quadrigemina beetles are bluish.

Beetle (Agrilus hyperici)

Adult beetles are bronze coloured and 5 mm x 2 mm in size. They lay eggs in the crown of St John’s wort plants in summer. When they hatch, larvae bore into the roots and kill the plants. This beetle only lives in one or two isolated sites near Mudgee and Tuena.

Gall midge (Zeuxidiplosis giardi)

This small fly lays eggs in the terminal buds. When they hatch, larvae feed on the leaf buds and cause lumps (galls). Populations rarely get large enough to have an impact on St John’s wort infestations. The gall midge helps to control St John’s wort in shady country where other insects are not active.

Green aphid (Aphis chloris)

Green aphid attacks flowering stems of St John’s wort in summer. Populations rarely get large enough to have an impact on St John’s wort infestations.

St John’s wort stunt mite (Aculus hyperici)

The stunt mite affects the narrow-leaf form of St John’s wort. It is too small to see with the naked eye.  The mites feed on the growing tips of plants. Damaged leaves often have yellow streaks or mottling. Rosettes and flowering stems are stunted. Mites can kill narrow-leaf St John’s wort over 2 – 3 years. All life stages are present throughout the year.

Chemical control

Only spray when St John’s wort is actively growing. Try to reduce damage to pastures through herbicide selection and timing. Two consecutive years of spraying is often required to kill plants. The deep, extensive root system can survive the first treatment, and the plant can regrow.

Spot-spraying

Spot-spray isolated infestations when St John’s wort is in flower (November to January). It’s too late once the flowers have turned brown. Cover all the foliage with herbicide.

Boom-spraying

Use boom-sprays between budding and full flowering (November to early January). If the existing pasture can be salvaged use selective herbicides. If the existing pasture cannot be salvaged, use glyphosate in November/December.

Weed wiper

Weed wipers can treat patches of St John’s wort. Graze useful plants below the wiper height before the start of St John’s wort flowering. Treat with the wiper at full flowering.

Herbicide options

See Using herbicides for more information.

---

2,4-D LV ester 680g/L (Estercide® Xtra)
Rate: 3.3–4.7 L/ha
Comments: For use in grass pastures, before flowering, when the plants are less than 40 cm high.
Withholding period: Do not graze or cut for stock food for 7 days after application.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

---

Fluroxypyr 140 g/L + Aminopyralid 10 g/L (Hot Shot™ )
Rate: 700 mL in 100 L of water
Comments: Foliar application from flowering to early seed set
Withholding period: Not required for pastures when used as directed. Do not graze or cut crops for stock food for 7 days after application. See label for export restrictions.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

---

Fluroxypyr 200 g/L (Comet® 200 herbicide)
Rate: 500 mL in 100 L of water
Comments: Spring to mid summer application.
Withholding period: Do not graze failed crops and treated pastures or cut for stock feed for 7 days after application. See label for further information.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

---

Fluroxypyr 200 g/L (Comet® 200 herbicide)
Rate: 3.0 L/ha
Comments: Boom application. Observe withholding period.
Withholding period: Do not graze failed crops and treated pastures or cut for stock feed for 7 days after application. See label for further information.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

---

Fluroxypyr 333 g/L (Starane™ Advanced)
Rate: 300 mL in 100 L of water
Comments: Foliar application from flowering to early seed set. Observe withholding period.
Withholding period: Do not graze failed crops and treated pastures or cut for stock food for 7 days after application. See label for more information.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

---

Glyphosate 360 g/L (Various products)
Rate: 3.0 L/ha
Comments: Apply November to May, flowering to post-flowering.
Withholding period: Nil.
Herbicide group: 9 (previously group M), Inhibition of 5-enolpyruvyl shikimate-3 phosphate synthase (EPSP inhibition)
Resistance risk: Moderate

---

Glyphosate 360 g/L with Metsulfuron-methyl 600 g/kg (Various products)
Rate: 200 mL glyphosate plus 10g metsulfuron-methyl in 100 L of water
Comments: Spray to wet, but not to cause run-off.
Withholding period: Nil.
Herbicide group: 9 (previously group M), Inhibition of 5-enolpyruvyl shikimate-3 phosphate synthase (EPSP inhibition)
Resistance risk: Moderate

---

Picloram 100 g/L + Triclopyr 300 g/L + Aminopyralid 8 g/L (Grazon Extra®)
Rate: 500 mL in 100 L of water
Comments: Foliar application from late spring to early summer, during flowering to early seed set
Withholding period: Where product is used to control woody weeds in pastures there is a restriction of 12 weeks for use of treated pastures for making hay and silage; using hay or other plant material for compost, mulch or mushroom substrate; or using animal waste from animals grazing on treated pastures for compost, mulching, or spreading on pasture/crops.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

---

Picloram 100 g/L + Triclopyr 300 g/L + Aminopyralid 8 g/L (Grazon Extra®)
Rate: 2.0–4.0 L/ha
Comments: Boom spray
Withholding period: Where product is used to control woody weeds in pastures there is a restriction of 12 weeks for use of treated pastures for making hay and silage; using hay or other plant material for compost, mulch or mushroom substrate; or using animal waste from animals grazing on treated pastures for compost, mulching, or spreading on pasture/crops.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

---

Triclopyr 300 g/L + Picloram 100 g/L (Various products)
Rate: 500 mL in 100 L of water
Comments: Late spring to early summer, during flowering to early seed set.
Withholding period: Nil.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

---

Triclopyr 300 g/L + Picloram 100 g/L (Various products)
Rate: 2.0–4.0 L/ha
Comments: Boom spray.
Withholding period: Nil.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

---

Triclopyr 300 g/L + Picloram 100 g/L (Various products)
Rate: 500 mL per 10 L of water
Comments: Gas gun / Splatter gun application. Apply to actively growing bushes.
Withholding period: Nil.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

---

back to top

Biosecurity duty

The content provided here is for information purposes only and is taken from the Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area	Duty
All of NSW	General Biosecurity Duty All pest plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.
Central Tablelands	Regional Recommended Measure* (for Regional Priority - Asset Protection) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should mitigate spread of the plant from their land. A person should not buy, sell, move, carry or release the plant into the environment. Land managers should reduce the impact of the plant on assets of high economic, environmental and/or social value.
Central West	Regional Recommended Measure* (for Regional Priority - Asset Protection) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should mitigate spread of the plant from their land. A person should not buy, sell, move, carry or release the plant into the environment. Land managers should reduce the impact of the plant on assets of high economic, environmental and/or social value.
Hunter An exclusion zone is established for all land in the region, except parts of: MidCoast Local Government Area, Muswellbrook Local Government Area, Upper Hunter Local Government Area.	Regional Recommended Measure* (for Regional Priority - Containment) Entire Hunter Local Land Services region: Land managers should mitigate the risk of the plant being introduced to their land. Exclusion zone: Notify local control authority if found. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Core infestation: Land managers should mitigate spread of the plant from their land. A person should not buy, sell, move, carry or release the plant into the environment. Land managers should reduce the impact of the plant on assets of high economic, environmental and/or social value.
North West An exclusion zone is established for all lands in the region, except the core infestation area comprising the Gunnedah Shire council, Gwydir Shire council, Liverpool Plains Shire council and Tamworth Regional council.	Regional Recommended Measure* (for Regional Priority - Containment) Whole of region: Land managers mitigate the risk of new weeds being introduced to their land. Within exclusion zone: Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found. Within core infestation: Land managers should mitigate spread of the plant from their land. A person should not buy, sell, move, carry or release the plant into the environment. Land managers should reduce the impact of the plant on assets of high economic, environmental and/or social value.
	*To see the Regional Strategic Weeds Management Plans containing demonstrated outcomes that fulfil the general biosecurity duty for this weed click here

back to top

Reviewed 2023