Mowing an option?

 On some farms mowing the thistle is an effective option but when and how often is critical to success. He says the best time for a first mowing is when the thistle is at its most vigorous between November and January.

Bourdot says that a mowing in December followed by a second in either January, February, March or April, and repeated each year, should cause the thistle population to go into decline. He says the best results are achieved when it’s raining and that farmers can see a video about this method of control at the PestWeb website.

“We know from field experiments what farmers have done throughout the country – that if they mow Californian thistle in the rain they get 30% better control of the thistle than if they mow in the dry. Mowing in the dry is also effective. But we know if you mow in the rain you get better control. One farmer told me he’s been mowing or breaking down his thistle by towing a gate behind a tractor and macerating them during wet weather. He told me he has got the rust fungus in that paddock and he has got very good control.”

Bourdôt says he thinks by running the gate over the thistles in autumn, when the spores of the rust fungus that cause the lethal systemic disease in the thistle are present in large numbers, the disease was spread to thistle shoots throughout the paddock. So we have the combined effect of defoliation by the gate on root production and the facilitation of disease caused by a naturally occurring pathogen of the thistle.