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Controlling California Thistle Organically

by Mark Hill
Zucchini crop

The following advertisements are not placed by Organic Pathways and are not necessarily organic

Controlling the California thistle without the use of herbicide is one of the biggest headaches organic farmers face, yet the means of organically managing this tenacious plant is very simple says AgResearch scientist Dr Graeme Bourdot.

The species occurs throughout New Zealand, being particularly prevalent in wetter areas like Southland. There has been no direct national study of the current status of this thistle on NZ farms, but there is no question California thistle is one of the most serious and troublesome weeds of both pastures and crops. In a survey in Southland, farmers indicated that pasture weeds resulted in an opportunity cost of $4,300 per farm, and much of this was result of a loss of grazing due to the Cali thistle.

The Cali has been a problem in New Zealand since the very earliest days of agriculture, the mid-1800s. It's not native to California, the species comes from Europe and no-one seems to know why it is called California thistle in New Zealand - it's called Canada thistle in the USA!

"To manage the Cali thistle without herbicide use means we have to thoroughly understand the biology of the plant, particularly the processes which regulate the population size," Graeme says, pointing to an uprooted male thistle specimen. The California thistle is dioecious with male and female plants, and only the female plant produces seed. The female plant's seed head is much longer and more urn-shaped than the seed head of the male plant. The female flower heads produce copious amounts of white fluffy pappus in the late summer.

"Our research shows seed plays no role in the ecology of this species in pastures. We have been monitoring California thistle for over five years in several sheep grazed pastures, and we have never seen a seedling, and this result holds true in overseas research. Seed is not important for thistle population growth in pasture ecosystems."

The other part of the plant, hidden underground, is the creeping roots system and this is very, very, important, Graeme said: "This is what we really have to get a handle on. These roots produce many adventitious buds, and a small proportion of these buds each spring give rise to the new crop of shoots, after the winter-dormant phase."

The shoot population which comes up is directly proportional to the amount of the over-wintering roots: "If you are able to halve the root system going into the dormant phase, you would expect to have only half as many thistle shoots coming up the following spring. It's as simple as that."

The over-wintering roots biomass is controlled by the photosynthetic opportunity which the plant experiences over the preceding growing season: "The more photosynthesis which takes place over the growing season the more root biomass is going to grow, and the greater the shoot population will be in the following spring."

The key way to limit that photosynthetic opportunity is simply by mowing the thistle. "It's a pretty simple technology, with nothing fancy about it. Several strategic mowings, or grazing or whatever, means you reduce the plant's photosynthetic opportunity, you reduce the root biomass."

Graeme says an experiment conducted with mowing the thistle to ground level in November, another mowing in January and another in March. "This resulted in an 80% reduction in the root biomass, and an 80% reduction in the shoot population. If you project that out over the following three years, simple mathematics show you will have less than 1% of the shoot population you would have had if you had done nothing. Our experiments have shown if you do this often enough you can reduce a very dense thistle population down to a very low level. The mowing/defoliation is a very simple tool I would be recommending to organic growers, and provided it is adhered to over several years, it's a very powerful tool."

Graeme said another way to manipulate the thistle's photosynthetic opportunity is through using crop competition, getting the canopy of a useful crop plant up ahead of the thistle in the spring. "Calis come through the ground in late August, and are not growing much until the end of September. If you get a dense crop canopy over the thistle within this time frame you would expect a very high level of reduction in the thistle's photosynthetic opportunity, therefore a big reduction in the thistle's root biomass, and therefore a lower shoot population in the following year.

"Experiments have shown rye corn is a very strong competitor against weeds. It's very tall growing and produces the sort of canopy we're looking for to reduce the thistle's photosynthetic opportunity. These are two powerful ways to get a very high level of control of the California thistle over a period of three years."

Another approach is to try and exhaust the roots system currently present: "Root fragmentation is the way this is done. Most of the adventitious buds on the roots system never do anything - only about 2% of the buds on the roots system in an undisturbed situation will ever form shoots. If you want to try and reduce the roots system present, the simple approach is to fragment the system, break up the 'apical dominance' that inhibits bud growth, inducing shoots to grow from the roots system, and use up some of its reserves."

The key here is to do this fragmentation when the roots system reaches a minimum dry weight. Chop up the roots then and you stimulate a lot of the buds to grow, which wouldn't, if left undisturbed. They consume the carbohydrate in the roots system and the whole system starts to collapse, using up its reserves. The shoots grow through to the surface and begin to photosynthesise to gain more reserves. The point seems to be when the regenerating plant has eight to ten leaves, and the plant is then starting to make a net gain, producing new roots.

"This is when you come back with the next fragmentation. If this is done religiously for several years over the summer/autumn months, it will bring the California thistle down to a very low level. Studies in the United States have shown this very clearly - cultivation was done at three weekly intervals over the entire summer/autumn period, and in the third year there was virtually no California thistle regrowth."

How deep should cultivation be to fragment the roots system this way? After much sampling on the Templeton/Wakanui soils, Graeme and his research team has found by far the bulk of the thistle's root system lies between 100mm to 200 mm below the surface.

Although this root fragmentation system certainly works, Graeme says this would be much more damaging to the soil system, the soil structure and biota, and the farmer will have the paddock out of production for several years. "Farmers probably wouldn't want to lose production this way and they would have to ask themselves, is California thistle worth this much? The most powerful tool the farmer has in an organic pasture system is defoliation (via mowing)."

The following advertisements are not placed by Organic Pathways and are not necessarily organic