Effective Californian Thistle control requires a 3-shot spray programme approach.
In the battle against one of New Zealand’s most unwanted pasture weeds, Californian thistle is winning more than it should. One of the reasons is that effective Californian thistle control requires a well-timed, 3-shot programme approach. Key sprays often get missed, or not applied at the correct growth stages, thereby getting poor control.
Ian Kirkland, upper North Island territory sales manager for Corteva Agriscience, says it’s no wonder farmers get disheartened when their efforts to control Californian thistle don’t seem to work.
One herbicide application over 12 months is simply not enough to get control of Californian thistle. It is important to have a 3-shot spray programme with well-timed applications, otherwise this persistent species continues to grow and spread, and the infestation is likely to end up back where it started.
The first herbicide application to Californian thistles should be when they have reached the hardball stage, just before the flowers open. This growth stage is usually around November/December and application timing is important as:
1. Most of the shoots have emerged, so you are less likely to miss spraying any dormant shoots.
2. There is sufficient leaf area on the thistle to absorb herbicide into the plant.
3. Spraying at the hardball stage, prior to flowering means the photosynthates (sugars) are being translocated down into the roots which also aids the translocation of chemical into the root system. Spraying at flowering or after flowering means that a lot of the photosynthates (sugars) stay near the top of the plant for flower and seed development and so the chemical is not translocated to the roots where it is most effective.
The shoot growth of Californian thistles sprayed at hardball stage will die off at the top, however the massive underground root system supporting each plant will respond by sending up fresh new shoots. It is these new shoots that need to be hit hard a second time, several weeks after the first initial herbicide. This is normally around February/March, ideally when the thistles get to knee high and there is plenty of green leaf to absorb the herbicide to allow it to be carried down to the roots. Spraying too early means that not enough of the seedlings have emerged and that the leaf area does not allow enough absorption of the herbicide.
Some of the plants not controlled by the first two herbicide applications may still regrow. While some of these may be controlled by the winter frosts, the remaining plants require a third spot spray application in the following November/December.
Ian Kirkland has seen enough paddocks to notice the impact of farmers skipping the second and third sprays, which he describes as the missing link in effective control programmes.
He puts the proliferation of Californian thistles down to a lack of understanding about the plant in general, as well as a plethora of information (and misinformation), opinion and theory about how to control it. “It’s easy for farmers to get confused, and then discouraged. But it’s also easy for reps to start setting them straight with sound advice and the right herbicide”.
“Tordon Pasture Boss is the best herbicide to control Californian thistles, but it’s still not a one-shot application. They can’t just spray and walk away, wave goodbye from the gate.”
The withholding periods of Tordon PastureBoss are relatively short (three days for milk; seven days for meat). Although Tordon PastureBoss will damage clover, pasture production and grazing are already greatly reduced where Californian thistle populations are high, and that productive ground will be re-gained the following season, and the clover will eventually return.
What’s more important than any herbicide is often a change in mindset.
“Farmers get overloaded with information, the message is diluted, and in some cases they end up not even starting a control programme.
“They need confidence in the fact that they can get things turned around, with the right advice and the right approach. And reps are the people best placed to help them achieve this.”
For more detail contact your Corteva Agriscience territory sales manager.
Download the Cali thistle control guide>
