Californian thistle is one of the most common weeds on NZ farms, and one of the hardest to kill off, but it is possible to do it without the use of chemicals.

The trick with this persistent weed is to be persistent yourself: it typically takes three years to achieve a significant (eg 90%) reduction of thistle biomass using any technique.

Unlike most other weeds, Californian thistle’s trick to being so successful is its rhizomatous root system, not its spread by seed, so the key to killing it is to focus on destroying that root system.

DEEP RIPPING/SUBSOILING TO CRUSH THE UNDERGROUND RHIZOME
This requires the use of specialist equipment but it basically crushes the root system, killing the plant immediately. Although no research has been done yet, it is believed to be the single most effective way to get rid it.

THE USE OF GOATS 
Goats are more partial to thistle than cattle and sheep, they have the edge if an infestation is on steep terrain where mowing or subsoiling isn’t possible, and they can be profitable too.

Using goats does require special management: they will eat more palatable species first, and they won’t eat all of it in one go, so it takes longer.

They also need to be in there eating well before plants go to seed or they can actually encourage seeds to germinate (passing them in their manure). Only meat goats should be used due to risks with parasites in dairy goats.

MOWING IN THE RAIN
This is a farmer-developed technique that has been validated by AgResearch. The theory is that wet conditions allow for greater transfer and infection of a range of fungal diseases into the cut stems which harm or kill the plants.
You can more about the specific strategy which requires careful timing here and here.

USING NOVEL MACHINES SUCH AS COMBCUT
This is a specialist cutting system that is considerably faster than toppers or mulchers, and could be redesigned for use behind quad bikes or utes. The advantage is the CombCut uses considerably less fuel to do the same job as a mulcher or topper.

PASTURE AND CROP COMPETITION
Research shows Cali thistles struggle when under heavy competition from other plants, but especially ones that compete with them at root level. The best options are chicory and lucerne, or mixes of green manure cover crops like triticale, rye and vetch.

REGULAR MOWING
This method also hits the plant’s root system, especially when done regularly. The plants need green leaves to feed the root system – if this is consistently lost in a short timeframe, the root system runs out of the energy it needs to keep growing.

 

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