October - Californian Thistle

Californian thistleCalifornian Thistle (Cirsium arvense)

A perennial thistle with far-creeping root system, each plant sending up many aerial shoots in the spring, which seed and die in the autumn. Shoot emergence is not uniform which means that it often appears that the herbicide is not working. Cali's are spread mainly by rhizomes - the root system. Seedling germination and survival is very low. They often form extensive patches that are very difficult to control.

The key to successful Cali thistle control is timing and follow-up. The best time to target a Cali thistle is at the hard-balled stage,  just before the flower bud bursts. At this stage of growth the sap flow in the plant is in the downward direction and you get the maximum amount of herbicide carried down into the root with it. Because the emergence of the aerial shoots is not uniform, some people choose to top the plants early in the season and allow them to regrow to the desired stage, therefore having more shoots of the plant at the optimum stage for treatment with herbicides.

Californian thistle root systemBecause there is such an extensive root system under the ground it is difficult for any herbicide to move to the outer reaches of the entire system and it is very likely that those sections of root not reached by the herbicide will survive and send up aerial shoots again. By re-treating those re-emerging shoots the following season you will be continuing to kill off the remainder of the root system that survived the first treatment. Leave it and not treat in the second season, and the sap will begin to work it’s way back into the partially dead section of the plant and the plant will regenerate and you will be back to where you started.

When a paddock is cropped (eg) brassica crops and maize and the ground worked, this provides an excellent opportunity for cali thistle control because the root system has been chopped up and therefore the herbicide need only travel through smaller sections of root.

CONTROL
  • New pasture and clover selective boom spraying option - Select* herbicide
  • (NB. Pasture-Kleen* is not good on Cali thistle.)
  • Spot-spraying with a knapsack - Versatill* or Tordon* Gold herbicides
  • Weed wiper - Versatill or Tordon Gold herbicides

*Registered trademarks of Dow AgroSciences

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