Agronomist Ross Watson, says a multipronged approach is best for St John's wort control

According to Ross Watson, consulting pasture agronomist at Scone, has written several publications on this weed and undertaken many St John's wort herbicide trials over the last 30 years, he considers it to be the number one pasture weed threat, especially in non-arable country.

He has seen a continued explosion of it across the state, including noting it within a few kilometres of almost any farm in the Upper Hunter and surrounding areas.

Mr Watson believes stock trading and movement from all over the country is one high-risk spread aspect. Sticky seed contributes to the spread via livestock and wildlife such as kangaroos.

A most important control strategy Mr Watson stresses is to always be on the lookout for it, especially from October to February.

Key features are bright green plants, 20-80 centimetres tall, bright yellow flowers with five petals, branches and leaves on the stems, always opposite each other.

Early action before it becomes established will likely save enormous pain and cost in future years.

Typically, what happens is year one, the odd plant of wort, in year two, a few patches are here and there, and by year three to five, plants cover extensive areas.

St John's wort can totally crowd out useful pastures, especially in non-arable areas. It can also be toxic to animals and can greatly reduce land value and productivity.

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Wort is rarely a major problem on good arable cropped land that is accessible to boom spraying, and that can be readily treated with registered herbicides and more intensive land management.

Mr Watson says it is best to crop arable paddocks for at least three consecutive years with total control of St John's wort in the fallow.

He advocates glyphosate fallow use plus in-crop herbicide programs during winter. Then returning paddocks to perennial temperate or tropical pastures at the end of the cropping period.

Infestations in non-arable, timbered, and rugged, steep country can be difficult and expensive to control. If the weed is established over large areas, then a more detailed and strategic approach is required.

For many, it will be a case of "managing it" whilst continuing legal requirements to actively control and limit the spread, Mr Watson said.

Ross Watson advisers with major infestations to "seek and spot spray" isolated plants in otherwise clean paddocks first, limit stock and vehicle movements in infested areas and buffer spraying along paddock boundaries.

Changes to grazing management of infested hills may also be needed.

Subdivision, combined with strategic and controlled grazing and resting of paddocks, utilising goats or sheep where appropriate, and encouraging competitive pastures with appropriate fertiliser use, grazing management and strategic use of selective herbicides, should all be considered.

Mr Watson advises caution when considering broadacre aerial spraying, typically with helicopters, in rough tree-covered hill country.

He notes this approach rarely delivers long-term control. Single and follow-up aerial spraying can cause severe damage/death to trees and native vegetation and has a high risk of off-target damage.

Mr Watson says years of research have shown that by far the most effective herbicide in grass-based pastures for spot spraying is products containing triclopyr and picloram plus aminopyralid, commonly marketed as Grazon Extra.

It has excellent knockdown and soil residual activity. Its residual activity is harmful to legumes, but grasses are largely unaffected.

Products like Grazon Extra can be best applied over spring-summer flowering. If spray conditions are good, kill rates can be above 90 per cent.

Mr Watson's research some years ago discovered that products containing fluroxypyr, such as Starane Advanced, are also effective, have a lower cost for broad-acre treatment, are safe on grasses, and have no residual problems with legumes.

Control, while good, does not provide the same high initial control provided by Grazon Extra.

Glyphosate as a spot spray has the disadvantage of leaving bare areas, often reinvaded by other weeds or residual seed of St John's wort.

A range of biological control insect agents, unfortunately, have commonly been disappointing.

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