A unique three-way project between supplier, retailer, and farmer is nearing its final stage. Twelve months ago, PGG Wrightson and Corteva Agriscience recruited farmers from across the country to follow an expert, staged elimination programme for Californian (Cali) thistle to demonstrate it can be permanently controlled in a real-world setting.
Established, dense Californian thistle is one of the most challenging pasture weeds to control. Eradication relies on designing and implementing a successful control programme and requires a well-timed, three-spray approach. Success depends on application timing and follow-up to ensure the entire root system is killed to prevent recovery and re-establishment.
Tordon PastureBoss is the premium herbicide for use at the front end of a Cali thistle control programme. For more than a decade, Tordon PastureBoss has proven to be the most effective herbicide on Californian thistle.
Cali thistle control programmes can be initiated with an application of Tordon PastureBoss when a good portion of thistle shoots reach early bud stage (hard ball stage), typically late November to early January. Ground boom spraying is most effective on dense thistles; and is the preferred option over carpet or wick type applicators early in the control programme.
The rhizomatous root system sends up fresh new shoots in late February or is when a follow-up treatment using a phenoxy herbicide, such as GrassMaster MCPA, should be applied. It is these new shoots that need a second herbicide application is part of the control programme. The timing is when there is adequate green leaf area to absorb the herbicide for maximum translocation down the root system. Ideally, this is when thistles are knee high. Spraying too early, before enough of the shoots have emerged, means there is not enough leaf area for adequate absorption of the herbicide. Appropriate timing is critical for success. While additional shoots may emerge, they are killed free-of-charge by the onset of winter and frost.
The third and final application, to ensure complete control, occurs the following summer. This focuses on spot spraying of any final emerging shoots using Tordon PastureBoss. This follow-up application is crucial to prevent the root system recovering and Cali thistle re-establishment.
Some demonstration sites in the programme have had their three-spray programme, with the last application occuring during summer 2025, while others are due to conclude soon. PGG Wrightson and Corteva held open days for customers at three sites, located Wairarapa, Waikato, and Southland, and will be holding more in Spring 2026 at other locations nationwide.
