Paddock of Cali thistle before treatment
Paddock six weeks after application of Tordon PastureBoss.
Cali thistle is a persistent and costly weed in New Zealand pastures. Its aggressive, extensive root system and rapid regrowth is responsible for the loss of some $700 million in pastoral farm revenue each year by reducing pasture availability, pasture utilisation by livestock, and leading to adverse livestock health outcomes. (1)
In 2024, PGG Wrightson and Corteva Agriscience started an 18-month collaboration on best practices to control Californian or Cali Thistle. Together, they set-up control sites on eight dairy and red meat farms across New Zealand to demonstrate best practice in the management and elimination of Cali thistle. Now, in the second growing season, the success of the trial’s ‘three-spray’ programme is a roadmap for affected farmers everywhere.
The three-spray programme
- Summer knockdown: The ‘hard-ball’ phase—before the flower opens but the
foliage is established, and the plant is actively moving sugars from the foliage to the roots— is your first stage. Use a ground boom spray application of Tordon® PastureBoss® herbicide and the plant will translocate the herbicide throughout the plant. Timing of application is typically in late November to early January. - Autumn follow-up: Though the first herbicide application will successfully kill plants above ground, Cali thistle’s creeping root system is still alive underground. After the initial dieback of the shoots, the roots and adventitious buds produce new above ground shoots. Applying a second spray of phenoxy herbicide, like
Grassmaster™ MCPA, kills these shoots and further depletes the plant’s energy reserves before winter, weakening its ability to regenerate. Timing is critical. Spraying too early means the insufficient leaf area will not allow enough absorption of the critical. Spraying the second stage should only occur when thistle regrowth is knee-high which occurs around late-February or early-March. Winter frosts can also kill any subsequent emergence of shoots to further weaken the plant. - Summer spot spraying: The third and final application stage is the following summer. Using Tordon PastureBoss, spot spray any remaining emerging shoots to ensure complete control. This follow-up application is the decisive nail in the coffin to prevent the recovery of the root system and Cali thistle re-establishment.
Alternative spray timing
If you missed the summer window and begin your Cali thistle control programme in autumn, you can still use the three-step programme. Use Tordon PastureBoss in the first application in autumn to provide initial strong knockdown then follow with a phenoxy application, like Grassmaster MCPA, in spring. Then, the following autumn, use Tordon PastureBoss for the third follow-up spot spray application.
