St John's wort is a herb with bright yellow flowers. It can poison livestock.
St John’s wort:
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:
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:
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.
Age and the amount of skin protection affect how tolerant animals are to hypericin. More tolerant animals have:
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.
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:
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.
From a distance an infestation appears:
Non-flowering stems:
Flowering stems:
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:
The broad-leaf strain:
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.
St John’s wort grows in pastures, riparian areas, and bushland. It prefers:
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.
Roots sucker and new plants grow from fragments. Cultivation can move root fragments.
Long-term control of St John’s wort needs to consider that:
To reduce the risk of infestations developing:
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:
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:
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.
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.
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:
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.
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.
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.
Eleven biological control agents have been released in Australia. Six have established.
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.
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.
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 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.
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.
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-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.
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 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.
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
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 |
Reviewed 2023