Californian Thistle Control

Mowing is an effective way of controlling Californian thistle, but it is even more effective when it is done in the rain.

Californian thistle is one of the most common thistles in New Zealand and is very troublesome in pastures. It is the root system, not the seeds, which are the problem. Seedlings rarely contribute to the multiplication of Californian thistle in an established pasture, although they may be involved in the initial establishment of the weed.

Once a seedling has established, it soon begins to form creeping roots and shoot buds, and these roots continue to grow and form buds throughout spring, summer and autumn. They may spread as much as two metres.

During winter when the foliage and flower stems have died, about one in five of the buds is released from dormancy to form subterranean shoots. These grow up to the soil surface where they remain until the last spring frost, after which they emerge to form the rosette and later, the flowering shoots of the thistle patch.

Now, instead of the original seedling, the patch of shoots can cover a circular area as wide as four metres.

The research conducted by Dr Graeme Bourdôt from AgResearch in Lincoln could save farmers a lot of money, including on herbicides and loss of pasture production.

The important message is that the creeping root system drives the population’s growth in the pasture, not the thistle seeds. Seeds play no role whatsoever in an established pasture.

The more foliage the thistle grows, the more roots it produces, and the more shoots the roots will produce the following spring, and the bigger the population will be.

Over-wintered bud numbers directly relate to the root mass. Many farmers believe the roots of Californian thistle survive and get larger for years and years. Research shows that the creeping roots are ephemeral; they are replaced each year with new roots.

The creeping roots produced each year are only to get the thistle through the winter so killing the root system is the way to achieve good control of the thistle. Stopping the seed has no effect at all on the population.

Research has shown defoliation is the way to control this thistle, and this can be done by mowing, hard grazing or by herbicide.

While there are herbicides with label claims for Californian thistle control, all of them will damage the clovers in the pasture.

The key, regardless of the defoliation method, is to remove as much of the above ground vegetation as possible for as long as possible, to minimise the root formation.

Mowing stops photosynthesis in its tracks and stops production of the roots.

For a long time farmers have been saying that mowing in the rain controls the thistle. In 2005 and 2006 Beef + Lamb New Zealand funded a basic survey of the diseases that occurred in Californian thistle throughout New Zealand. Hundreds of populations of thistle were surveyed.

Several interesting fungal pathogens were noted and one of these was Verticillium dahliae. It causes disease in quite a few crops, including potatoes. This fungus spreads by rain. Its spores are released from inside the stem of the infected plant and splashed and dispersed under wet conditions. The penny dropped. Maybe this was the rain pathogen.

The Verticillium was everywhere, and more than half of the 124 populations of the thistle that were visited had the fungus. It was widespread. Maybe this was the causal agent for what farmers have been saying.

Graeme says “We went back to Beef + Lamb and said we think we are on to something, and asked them to fund a further set of experiments to get farmers to mow in the dry and mow in the rain – which they did.

We simply got a bunch of 12 farmers throughout New Zealand to mow paddocks either in the dry or in the rain, and technicians did measurements a year later to measure the changes in Californian thistle numbers.

First of all, we proved beyond doubt that mowing in the rain is better than mowing in the dry. While mowing in the dry does reduce the thistle population, it is highly variable between farms but on average, if you mowed Californian thistle in the wet you would get 30% better control then if mown in the dry.

In full-scale paddock experiments we had a 34% reduction when mown in the dry in one year on average across nine farms and a 6% reduction the following year across 12 farms.

Mowing in the rain increased these reductions by 21 and 32% respectively, which is roughly a 30% better control than mowing in the dry.

Mowing twice gives better control, and mowing three times gives even better control.

If you could completely prevent foliage growth for a year you would eradicate the thistle in one year.

What we weren’t able to prove is that the reduction in numbers from mowing in the wet was due to the Verticillium fungus.

So the jury is still out on what the biological mechanism is. It is almost certainly a pathological effect of some sort.

I have had conversations with farmers who said they mowed their thistle in the rain and the weed disappeared and never came back. They may well have had the right combination of fungi in their thistle.

However in our research we found a whole heap of other fungi, including one of great interest. We call it the P fungus, and it could be the one. We have been able to collect it and apply its spores back on Californian thistle and get quite devastating effects. We hope to see if it has potential as a myco-herbicide. If we do decide to go ahead with the new P fungus, we have to find a way around spore storage issue, and we are looking at potential ways to enhance the longevity of spores.

Whatever is the case, mowing in the rain is a simple, low-cost technique which can be used by farmers to fight this destructive pasture weed.

The research was formally published in October 2011 in Biocontrol Science and Technology.

Thanks to Beef + Lamb New Zealand for funding this work and the initial survey which led to the research.”