Californian thistle

Scientific name: Cirsium arvense

Female flowerhead

Male flowerhead

Thistles invading a maize crop

Thistle seeds

Californian thistle seedlings

Green thistle beetle life cycle

Green thistle beetle larvae

  • Key characteristics
  • Biology
  • Impacts
  • Control
  • Further information

  • Perennial with creeping roots and buds that give rise to aerial shoots
  • Slender flower stems up to 1 m tall with terminal flower head clusters
  • The purple or mauve (rarely white) flower heads occur in the summer
  • Male and female flower heads occur on separate plants
  • The male flower head is shorter and less urn-shaped than the female, and lacks the fluffy white pappi (in late summer early autumn) that act as parachutes for dispersing the seeds from the female flower head
  • Leaves are hairless, or have cobwebby hairs, green above and paler underneath, with shallow notches between triangular spiny lobes
  • Occurs as distinct patches of either male or female plants or as coalesced patches of both sexes.

Origin

  • Native to Eurasia, Californian thistle began its invasion of agricultural land in New Zealand in the late 1800s. It now occurs on most farms throughout the country.

Seeds

  • Germination occurs in disturbed soils in the summer
  • Most of the seeds produced are viable and fall in the vicinity of the parent plant. The few that remain attached to their pappus, after release from the seed head, can be carried long distances on the wind
  • Many of the seeds are eaten by birds and other animals (such as mice) in autumn and winter. Survivors will persist in the soil
  • Germination requires light and relatively high soil temperatures, which explains why seedlings frequently occur in a summer cultivated crop seedbed and why they rarely occur in an undisturbed pasture.

Shoot and root development

  • After seedling establishment, Californian thistle begins to develop an extensive creeping root system that supports many adventitious buds
  • The creeping roots grow throughout the growing season and may spread and give rise to aerial shoots 2 metres or more from the parent plant
  • The aerial shoots die back in the autumn and during the winter some of the buds on the surviving creeping root fragments form shoots that grow up to the soil surface
  • The shoots emerge from the soil in early spring after the last frost and form leafy rosettes that produce new creeping roots as they develop. These new roots replace last season’s overwintered roots
  • Only about 1 in 5 of the buds on a creeping root develops into a new shoot, the others remaining dormant to provide a reservoir from which the plant can regenerate if the roots are fragmented
  • Fragmentation of the creeping roots can be caused by many factors (disease, soil displacement during flooding, grazing, mowing, herbicides, excavation machinery etc.). This breaks bud dormancy and allows the root fragments to develop into shoots
  • The shoots are soft and palatable when young, but are avoided by most classes of stock once they begin to flower.

Benefits

  • Provides nectar for bees and other insects.

Impact on stock

  • Californian thistle reduces pasture availability; sheep, and to a lesser extent cattle, avoid grazing infested areas in a paddock
  • Reduces the stock carrying capacity of grazing land
  • Seed heads can contaminate wool
  • Increases susceptibility of stock (especially young sheep) to scabby mouth. Abrasions to the skin, around the lips and mouth, caused by the thistle’s spines, create entry points for the parapoxvirus that causes scabby mouth disease
  • Deer are particularly prone to scabby mouth and can suffer from severe and even fatal dermatitis as a result. Harvested velvet infected by the virus becomes worthless. However, the virus does not affect the next year’s velvet growth
  • Californian thistle infestations can make stock handling more difficult.

Economic impact

  • The direct costs of Californian thistle to farmers in Otago and Southland (for herbicide, topping and scabby mouth vaccine) in 1992 where estimated at $21 million
  • Taking into account the peak losses in productive grazing area of 4, 6, 6, 12 and 6% for beef, dairy, deer, sheep and sheep/beef farms respectively, the losses in productive potential due to this thistle are likely to exceed $500,000 million per year.

Defoliation

  • A Californian thistle infestation can be reduced substantially in pasture by a 2-year defoliation programme, with three defoliations (removing all shoots to ground level) per growing season, and virtually eradicated by a 4-year programme
  • A single defoliation per year is likely to have only a short-term cosmetic effect and unlikely to cause decline in the population according to recent research
  • For greatest success from mowing or grazing, Californian thistle should be defoliated as close to the ground as possible
  • Defoliation may be achieved by mowing, hard rotational grazing or with an herbicide
  • Mowing is about 30% more effective when done during rainfall
  • The key, regardless of the defoliation method, is to remove as much of the above ground vegetation as possible for as long as possible. This will minimise root formation and, as a result, minimise the number of shoots that will emerge in the following growing season.

Pasture competition / grazing management

  • Vigorous intact pastures without gaps will resist the invasion of Californian thistle
  • Keep soil fertility at recommended levels
  • In grazing systems where the thistle is avoided (e.g. sheep), a lax or deferred grazing regime (e.g. long rotational grazing or hay) is likely to be a more effective control strategy than an intensive grazing regime (e.g. set stocking or continuous grazing)
  • High-intensity/low-frequency rotational cattle grazing systems, where the thistle is eaten and the neighbour species have time to regrow and compete between grazings, may also be effective
  • Competitive exclusion of the thistle is unlikely under any grazing system if the weed is already densely established and has excluded other pasture species. Under these conditions, re-sowing the pasture will be necessary before implementing a grazing regime to competitively exclude the thistle:
    • Apply a knockdown herbicide pre-cultivation to kill existing thistle plants, after which an annual ryegrass can be sown
    • Apply a further knockdown herbicide in the spring. A summer crop can then be sown, followed by a third knockdown herbicide in autumn before a new perennial pasture is established
    • Sow a pasture cultivar that is well adapted to the environment (drought tolerant, pest-resistant etc.).

Herbicides

  • Several herbicides have label claims for the control of Californian thistle in pastures. They are given in the Table below using their active ingredient names.

 

Herbicides with label claims for controlling Californian thistle in a pasture. REFER TO PRODUCT LABELS BEFORE APPLYING
Active ingredient Application time – method Grass damage Clover damage
2, 4-D Post-emergence at any stage – boom spray No Yes
MCPA Post-emergence at any stage – boom spray No Yes
MCPB Post-emergence at seedling stage – boom spray No No
MCPB/MCPA Post-emergence at any stage – boom spray No Yes
glyphosate At any stage – spot or weed wiper only Yes Yes
clopyralid Post-emergence at any stage – spot or weed-wiper No Yes
aminopyralid Post-emergence at any stage- spot or weed-wiper No Yes
dicamba Post-emergence at any stage- spot or weed-wiper No Yes
triclopyr + picloram At any stage – spot treatment only No Yes

 

Herbicide application methods

  • Boom spraying. Apply the herbicide through a properly calibrated boom with flat fan nozzles to give even application over the whole area – all thistle shoots must be treated
  • ULV/Controlled droplet. These applicators also give blanket coverage but are more suited to rougher terrain or where drift from boom spraying is a major concern
  • Rotary carpet or wick wipers. These applicators apply a small dose of concentrated herbicide to tall plants that make contact with the wiper. They are good for selective control of Californian thistle using non-selective herbicides. Grazing beforehand is recommended so tall-growing grass isn’t touched by the herbicide
  • Spot application. Ideal for small populations and small infestations. This method minimises pasture damage and amount of herbicide used. With the use of weed-wipers or spot application more potent herbicides can be used in pasture.

Biocontrol

A number of insects have been released in New Zealand as potential “classical” biocontrol agents for Californian thistle. So far, only one of these, the green thistle beetle, Cassida rubginosa, is showing any promise at controlling populations of the thistle:

  • Released in pastures in Otago and Southland in spring 2007, the beetle is establishing well at many sites, including some in the North Island
  • The beetle’s larvae and adults both eat the leaves and can completely defoliate entire shoots causing them to die
  • Its potential as a bio-control agent for Californian thistle in hill country pastures is the subject of a current Sustainable Farming Fund project. Updates on this project are available on the MPI website
  • Although this beetle prefers Californian thistle, it will also feed on other thistles. Its potential in this regard is the subject of a research programme funded by AgResearch

In addition, Californian thistle in New Zealand is host to many pathogenic fungi some of which are probably responsible for the greater control of the thistle when mown during rainfall. Others have unrealised potential as “biological herbicides”.