Eliminating Californian (Cali) thistle in pasture is possible. In a joint collaboration, PGG Wrightson and Corteva conceived of a carefully constructed three-step control programme to practically fit into a farmer's existing system and eradicate a weed costing the sector millions.
Eighteen months ago, they started working alongside farmers across the country, and in the last year, three of these demonstration farms have successfully completed the programme and prove it is possible to eliminate this prickly, persistent problems.
Setting your battle plan
To win the battle against Cali thistle on your farm, you want to begin planning your well-timed, three-shot applications approach.
Step one: Start with an application of Tordon© PastureBoss© when thistle shoots reach early bud (hard ball) stage, typically late November to early January. The herbicide moves through the plant and down into the root system to prevent growth nodes from producing new shoots.
Step two: In late February or March, apply GrassMaster® to target any shoots that may not have received a sufficient dose in the first application.
Step three: Use Tordon PastureBoss as a spot spray to tidy up any remaining thistles the following summer. Together, this programme is designed to fully exhaust the extensive root system making Cali thistle so difficult to control.
In the spotlight
Masterton: For generations, Simon Campbell and his ancestors have battled Californian thistle on their family property near Masterton. This infestation had a significant impact on pasture production and utilisation. Working with his local PGG Wrightson Technical Field Representative (TFR) Gavin Harris and Corteva Territory Sales Manager (TSM) Alysha Brick, they took an initial shoot count in the targeted paddock, averaging 17 shoots/m2.
Campbell followed steps one and two, applying Tordon PastureBoss and GrassMaster, and now the paddock only contains a few scattered thistles to be treated during the final spot spray this summer.
Miller farm after eliminating Cali thistle
Cambridge: In Cambridge, Andrea Miller also spent years attempting to control the Californian thistles reducing pasture quality on her sheep and beef farm. She began working with her local TFR Simon Dodds and Corteva TSM Ryan Jackson, for a permanent solution.
The initial shoot count, an average of 13 shoots/m2, indicated another high-density infestation. Her control paddock is now free of Cali thistle. Following this success, Andrea plans to roll this programme out across the rest of her farm in stages.
Mataura: The final open day for the 2025/26 season was held in Mataura, at the southern end of the South Island. Susan Riepma's paddock was over run. With an initial average of 25 shoots/m2, Cali thistle out-competed clover, reduced pasture growth, and created grazing challenges on her sheep and beef farm.
Though Susan has only completed the first application of Tordon PastureBoss, shoot numbers have dropped significantly to just two to three shoots/m2. This is often the stage where results appear good enough to stop, but it is also the most critical point in the programme. She will continue with stage two in the Spring which is essential to fully exhaust the root system and achieve permanent control.
Expanding our success
All three sites hosted open days, with strong attendance of 35 to 50 people at each location. PGG Wrightson and Corteva Agriscience are continuing this successful programme and expanding its reach across the country over the next 12 months.
