Growth Habit: An erect thistle, growing to 90 cm high with a strong spreading rhizomatous root system. Above ground growth dies off over winter, replaced by new rosettes and shoots from buds along the underground rhizomes in spring.
Plants are usually one sex only (dioecious) thus bearing only male or only female flowers. Most infestations are made up of male-only or female-only plants, although some infestations comprise plants of both sexes.
Type of Plant: Herbaceous perennial.
Flowers: Flower heads grow to 25mm in length and approximately 15mm in diameter. The florets are a soft pink to mauve colour. They are borne either solitary or in groups up to four, from both the apex and axils of the plant.
Fruit/Seed: Fertile seed needs both male and female flowers, which are borne on separate plants. Flowers need to be within approximately 100 metres of each other to allow successful insect pollination.
Dispersal: Most dispersal occurs by the spreading root system, unless you have an infestation with both male and female plants then the plant will spread both vegetatively and via seed. Infestations have been known to expand outwards by 13 metres in a single year by rhizome growth alone. Any soil cultivation and movement may transfer rhizome fragments to new areas and begin new infestations.
Distribution: Californian Thistle is widespread throughout Tasmania, although most infestations are small and subject to control practices.
Status: Californian Thistle is a declared ‘Secondary Weed’ under the Noxious Weeds Act 1964. Landholders may be required to control Californian Thistle by an Enforcement Notice or to comply with prescribed measures contained in a regulation prepared under the Noxious Weeds Act.
- Californian Thistle is a very competitive weed in cropping areas as the regular soil cultivation readily breaks up the rhizome system and spreads it.
- The plant also interferes with harvesting of the crop.
- Infestations in grazed areas are rarely eaten by stock due to the sharp spiny leaves, allowing them to grow and multiply unchecked.
Control Methods: Before attempting to control Californian Thistle it must be appreciated that a continuous program extending for a minimum of at least two to three years is required. Established stands cannot be eliminated in one season. These heavier infestations which can survive for many years take longer to eradicate because of the enormous energy reserves stored in their root systems.
- Cultivation, by itself is not a practical method for controlling Californian Thistle. However, it can be used as the first step in an integrated control program especially where large, well-established infestations are involved. If done incorrectly, cultivation may actually assist the spread and proliferation of Californian Thistle, therefore land holders should carefully plan any cultivation works in areas where the plant is present. Refer to the Department of Primary Industries, Water and Environment for a complete description of the techniques involved in successful use of cultivation against Californian Thistle.
- Mowing or Slashing. Since seed plays a relatively insignificant role in the spread of this thistle, mowing or slashing the plants serves little purpose.
- Grazing. This is probably one of the least effective methods for the control of this thistle. Sheep and goats are known to graze the young shoots in spring but it is likely that conventional grazing alone will not significantly halt its spread.
- Cover Crops. Seedlings of this species are suppressed by shade created by competing crops species and death occurs when light intensity falls below 20% of full daylight. Cover crops may be used in conjunction with other techniques as part of an integrated weed management program.
- Chemical Control. Several herbicides are registered for use on Californian Thistle in Tasmania, including glyphosate, clopyralid and MCPA. Established infestations will not normally be eradicated from just one application of any product. Repeat applications, used in conjunction with other practices such as cultivation, may be required over several years to successfully eradicate an infestation. Contact the Department of Primary Industries, Water and Environment for a complete listing and details of the registered herbicides.
N.B. Always check the herbicide label before use.