Controlling Californian thistle
One of the most invasive weeds on farms is beginning to rear its ugly head - the Californian thistle or Cali thistle. The reproductive stage does not have to spell disaster if we respond appropriately.
The Cali thistle can cover 60 percent, or more, of a paddock’s grazeable area. It not only takes over grazing area and swamps out clovers, but it can also be the cause of sheep diseases such as scabby mouth.
Biology of Cali thistle
The thistle spends springtime building a massive network of rhizomes, or underground runners, to send new plants up through the soil. In early summer, they develop a seed head. The thistle slows, sending reserves to its roots via the phloem while it flowers and sets seed. Late summer is the quiet phase. While it may send up some new runners, the plant concentrates on extending its invasive underground root system in preparation for repeating the cycle the following spring.
As Cali thistle moves through its reproductive stages, the stem quickly elongates and produces a hard ball, containing the developing flower, on top. This is the best stage, when it’s most vulnerable, to apply herbicide with the active aminopyralid. This is most effective because the thistle carries it back to its massive root system. Once attacked, expect the root system to defend itself by sending up new runners. A successful counteroffensive is a second application of herbicide, usually MCPA, in the autumn for the knockout punch. Repeat this process the following year for best results.
Treatment aftermath
The downside of an aggressive, but effective, eradication programme is the loss of surviving clover in the pasture. However, in the long term, thistle invasion is a far bigger threat to clover. A second potential issue is soil residue. Should a soil residue develop, crops and clover may have a withholding period before reimplantation. Residue issues are easily overcome with good planning and systematic planting approach:
» Grass - immediately
» Brassica crops – after three months
» Clover and chicory – after 12 months
» Fodder beet (and other sensitive crops) – 18 months
On dairy farms
Aminopyralid, the active ingredient in the herbicide, is excreted in cow faeces and broken down in the soil through microbial activity. While it is known to negatively impact clovers in high concentrations, local effluent ponds often contain low concentrations of Aminopyralid, so this isn't a large concern across New Zealand. The withholding period, for all grazing animals, is seven days and three for milk.
For advice on controlling Cali thistle on your farm, contact your local PGG Wrighton Technical Field Representative for an on-farm consultation. PGG Wrighton Agronomists are also available to help with your planting programme.
