One of our most serious noxious weeds, St John’s wort (Hypericum perforatum) now infests many properties in the tablelands and slopes of Victoria, New South Wales and Southern Queensland. Little wonder it continues to spread: one plant of St John’s wort can produce 30,000 seeds in a single season. The (sticky) seeds are spread by animals, and although most germinate within the first year, some seeds can remain viable in the ground for ten years or more. So, it only takes one plant to start a major infestation!

NWW9158WX St Johns wort flower detail

NWW9158 St Johns wort flower detail

Early detection and TREATMENT, BEFORE PLANTS SET SEED, is critical.  Unfortunately, this “TIME TO TREAT” window is very small: St John’s wort plants are difficult to find until their bright, yellow-golden flowers start to appear, and then it is a matter of only two or three weeks before the seeds mature.

Because the plants do not all flower at once, continuous surveillance and treatment is necessary to ensure all plants are treated before seed set. Depending on how wet the season is, the flowering period can extend from October right through to April.

NWW9159WX St John's wort working its way through the boundary fence - not a good look!

NWW9159 St John’s wort working its way through the boundary fence – not a good look!

St John’s wort can have a major impact on grazing land. In this example (left) of flowering plants, the seed bank would be enormous. A treatment program planned for this area will face recurring seedling growth for many, many years! Images below show another side of the St John’s wort problem


Apart from its invasiveness, St John’s wort contains the toxin, hypericum. Over-exposure to St John’s wort causes serious health problems for animals, particularly horses, sheep and cattle. Livestock grazing in a paddock containing a heavy infestation of flowering

NWW9154 Skin damage caused by exposure to St John's wort

NWW9154 Skin damage caused by exposure to St John’s wort

St John’s wort can, on a sunny day, develop clinical signs of hypericum poisoning within five hours. Later symptoms include agitation, head-rubbing, hind limb weakness, panting –  leading eventually to raw, weeping areas of skin (ref. NSW DPI Prime­fact 694).

NWW9155WX St John's wort effect on animals

NWW9155WX St John’s wort effect on animals

These graphic photos (left and right) show some of this effect on a horse that had been grazing in a paddock of heavy St John’s wort for about five days. Note that the white parts are mostly affected. [horse images – Ian Davidson, Inverell]

Humans can also be affected by excessive contact with St John’s wort.

(Note: St John’s wort is recognised world-wide for its herbal attributes. It has a medicinal history in Europe going back thousands of Facebook like logoyears. Its proper use and benefit in that area is not denied but, this same plant in Australia is a serious, invasive weed causing pain and misery for animals, as well as financial and emotional pressure for landholders who continue to battle with this weed on their properties. It is a declared noxious weed for a good reason and it is an

NWW9157WX Unique to St John's wort - the "holes" (oil glands) in the leaves - positive ID

NWW9157WX Unique to St John’s wort – the “holes” (oil glands) in the leaves – positive ID

offence for persons to deliberately grow/harvest/sell this plant for what they believe to be medicinal use!)


Apart from the distinctive flowers (shown above), a positive way to identify St John’s wort is by close examination of the leaves.  Hold a leaf up to the light and (if your eyes are good enough) you will see what look like tiny pin holes through every leaf (photo, right).  These are the hypericum oil glands.


Non-Chemical Options: Perennial pastures and grazing management will offer some control – indeed, limited financial resources may necessitate careful and regulated grazing management as a practical option on some heavily infested properties! If grazing can prevent the plants from flowering there is a saving, but stock (especially white sheep or other animals with white markings) should be removed if the plants reach the flowering stage.

Chemical options for St John’s wort:  There is a wide range of effective herbicides suitable for St John’s wort. The key is repeated treatments – 2 or 3 in the one season is the ideal if resources permit. Small and/or isolated infestations should really be “hammered” to prevent further spread. A new plant treated before it first sets seed will seldom come back, but if it does seed there will be plants appearing in that same spot for seasons to come!

According to NSW DPI’s excellent booklet “Noxious and Environmental Weed Control Handbook 5h Edition”, treatments include:

Chemical options Rate: Spot/Boom Comments
Triclopyr 600 g/L + picloram 600 g/L eg Grazon DS® 500 ml in 100 L of water Boom rate: 2.0 – 4.0 L per hectare: Late spring to early summer, during flowering to early seed set. Boom:Apply November to January, use higher rate on dense infestations.
Triclopyr 300 g/L + picloram 100 g/L + Aminopyralid 8 g/L Grazon Extra® 500 ml in 100 L of water Boom rate: 2.0 – 4.0 L per hectare: Foliar application from late spring to early summer, during flowering to early seed set. Apply November to January, use higher rate on dense infestations.
Fluroxypyr 200 g/L eg Starane® 500 mL per 100 L water 3.0 L/ha Spring to mid summer application. Boom appln. Observe withholding period.
Fluroxypyr 333 g/L Starane Advanced® 300 mL per 100 L water 1.8 L/ha Foliar application from flowering to seed set. Observe withholding period. Observe withholding period.
Fluroxypyr 333 g/L eg Starane Advanced® 1.8 L/ha Apply from bud to full bloom. Some regrowth will occur. Treat regrowth the following season for best results. Use at least 200 L water/ha.
Glyphosate 360 g/L Various trade names 3.0 L per hectare Apply November to May, flowering to post flowering. (Suitable isolated plants only)
Aminopyralid 10 g/L + fluroxpyr 140 g/L eg Hotshot® 700 mL in 100 L water Foliar application from flowering to early seed set
2,4-D ester 600 g/L Various trade names 3.7 to 5.3 L per hectare For use in grass pastures, before flowering,when the plants are less that 40 cm high.
Metsulfuron methyl 600 g/L eg Brushoff ® + glyphosate 360 g/L various trade names 10 g metsulfuron methyl PLUS 200 ml glyphosate per 100 L water Spray to wet, but not to cause run-off.

IMPORTANT:  USE OF PESTICIDES – ALWAYS READ THE LABEL Pesticides must only be used for the purpose for which they are registered and must not be used in any other situation or in any manner contrary to the directions on the label.  Never use a herbicide in any way contrary to the label recommendations.

MORE INFORMATION: Detailed St John’s wort information is available on the NSW DPI Weeds Sub-Program website.  Another excellent reference is the book “Noxious Plants of Australia“, by Parsons and Cuthbertson [Inkata Press].

DISCLAIMER:   The information contained in this web site is based on knowledge and understanding at the time of writing. However, because of advances in knowledge, users are reminded of the need to ensure that information upon which they rely is up to date and to check currency of the information with the appropriate officer of North West Weeds or the user’s independent adviser. Les Tanner 6/08/2015