Getting on top of your thistles and worts

In some areas, saffron thistle was a problem in 2020 but as the season turned out, not such a bad problem in 2021. Because of saffron's seasonal variability, I'm reluctant to advise early herbicide control, such as MCPA or 2,4-D, mid-winter, when it is relatively easy to kill in the rosette stage. However these herbicides can also severely knock winter legumes like sub clover, medics and even kill ones like serradella and biserrula.

I mainly favour leaving herbicide treatment to mid-spring onwards when if control is necessary a relatively low rate of paraquat, at weed early flower stage, is generally very effective at preventing seeding. Weeds tend to desiccate faster than normal, clover has commonly largely completed seeding, and little herbicide damage occurs to other vegetation.

St Barnaby's thistle can be a difficult one to control when infestations become extensive. In our case we were free of it but accidentally gained a property wide infestation, at a low level, via contaminated pasture seed spread across the farm seven years ago. Getting onto it quickly via rigorous scouting and eliminating plants before seeding has largely worked. Surveillance, in case odd plants have been missed, plus odd invasions, will continue to ensure more or less freedom from this pest. Herbicide options, in pastures is more difficult, effectiveness wise, as well as impact on legumes.

Also read: Sprayers learn to recognise weeds in crop

St John's wort is a common invasive perennial weed. If eliminated early large headaches can be avoided. Arable paddocks can be cropped for three years combined with prompt fallow weed control to largely eliminate soil seed banks. Then sow down to a competitive long lasting suitable perennial grass pasture. In our case Premier digit grass is a very competitive pasture and if well managed for many environments can last indefinitely.

Paterson's curse was a pest we worried about several years ago when it appeared to be increasing in density across our property. Again there are good herbicide options for its in-crop control, but in existing pastures most also can adversely impact on pasture legumes. Fortunately for us, and many others, pests capable of controlling Paterson's curse have been introduced (via CSIRO and others) and for many are doing a great control job.

Especially effective are crown weevil (Mogulones larvatus), and root weevil (Mogulones geographicus). Biological control experts last year urged allowing these pests to recover by not herbicide treated Paterson's curse after their reductions in numbers attributed to the 2017-2019 drought. This appears to have happened with a lot of weed damage this past year.

Capeweed is rated by many farmers as one of their most troublesome weeds. It is aggressive and can crowd out valuable pastures like winter legumes, as well as be very competitive in crops. Control options depend on many factors, including level of infestation and spread across the property, arable or non-arable, and if farming is part of the system.

Where infestations are small, and relatively localised, eradication is an option. Eradication has been our aim. Over the last three years a number of small infestation, probably including washed in via storms, are continuing to recede via scouting, spot spraying and hoeing. One more year may see numbers almost eliminated if we can keep up our rigorous scouting program. Capeweed can be killed by various herbicides, a sound strategy where cropping is part of management. Three years preventing seeding goes close to eliminating its soil seed bank. Again aim to follow cropping by a long lasting aggressive perennial pasture.

For most weeds an integrated control approach involving several strategies is best for elimination or good control. On arable land cropping provides excellent opportunities to eliminate most weeds, although reinvasion is always likely.

Next week: Crown rot control in wheat and barley.

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