What is Californian thistle?
- Californian thistle is a weed of pastures and crops.
- Californian thistle is a
declared weed under the Tasmanian Weed Management Act 1999. The importation, sale and distribution of Californian thistle are prohibited in Tasmania.
How to identify Californian thistle
Images top & above left: Californian thistle flowers,
(photo above left, © Kiowa Fenner).Image above right: Californian thistle rosette.
- Californian thistle is a perennial (long-lived) plant. Over winter the top growth dies off leaving only the root system. The roots remain alive from year to year and actively spread through the soil.
- In spring the roots produce rosettes (whorls of leaves close to the ground), which send up a branched stem to about 1 metre in height. The stems are usually hairless and there are no wings or other outgrowths from the stem as occur in slender, cotton and nodding thistle.
- The bracts which surround the flower heads are green with purple tips and although tapering to a point are not spined. Each flower head contains a large number of rose-purple to lavender florets smelling strongly of honey.
- Male and female flowers are borne on separate plants. Infestations that have either all male or all female plants spread by vegetative growth only. If male and female plants are found within the same infestation, viable seed is produced and the infestation spreads both vegetatively and by seeding.
- For help in identifying thistles in Tasmania, see
Identifying Thistles in Tasmania and search the
Dennis Morris Weeds and Endemic Flora database for Californian thistle illustrations. If you are still in doubt about the thistle you are dealing with, contact Biosecurity Tasmania on 03 6165 3777 for help.
Californian thistle in Tasmania
- Californian thistle occurs in most parts of Tasmania.
- Californian thistle competes with pasture species and a heavy infestation can significantly reduce production. Stock avoid grazing around the rosettes and shoots, further reducing productivity of infested areas. Dense infestations of flowering stems may totally exclude stock.
- Californian thistle also competes with crop species and can interfere with harvesting.
What is the legal status of Californian thistle in your area?
Detailed management and control guidelines for Californian thistle can be found in the Californian Thistle Control Guide. Refer also to Herbicides for Californian Thistle Control
. For more information see
DPIPWE's Weed Links and Resources
Herbicides for Californian Thistle Control
Statutory Management Plan for Californian Thistle
Weed Links and Resources
Other useful links:
Californian Thistle Control Guide
- Plan your control program, this will save time and money in the long-run;
- Consider the impact of your control methods on off-target species, especially if herbicides are used;
- Ensure machinery and equipment is washed down between sites or prior to contractors leaving site;
- Get in early - for new infestations, eradicate before the plants reach the flowering stage: once plants begin seeding, control becomes more difficult and expensive;
- Carefully time your use of herbicide for best results (see
Herbicides for Californian thistle control for more information);
- Coordinate your control program with neighbouring landholders where your weed problem crosses property boundaries;
- Revisit and regularly inspect the site and ensure follow-up is undertaken;
- Expect your control program to last at least two to three years: established infestations cannot be eliminated in one season;
- Use an integrated control program which includes repeated cultivation followed by establishing shading crops or vigorous pasture and follow-up herbicide treatment of any regrowth.
- Don't introduce thistles to thistle-free areas (e.g. by failing to wash down machinery and equipment between sites);
- Don't start your control program without first planning your approach;
- Don't allow thistle's to flower and set seed before treatment;
- Don't rely on one attempt at removal - follow-up is essential.
Spread of Californian thistle
- Californian thistle spreads vegetatively by budding from the extensive rootstock and from seed. Californian thistle seed has a well developed pappus (fine hairs that act as a parachute) and can be spread by wind for over a kilometre.
- Movement of root fragments on cultivation implements and on the feet of livestock is the most common method of spread over short distances.
Avoiding the introduction of Californian thistle
- When areas infested with Californian thistle are being cultivated, great care must be taken not to increase the problem by spreading root segments.
- Care must also be taken not to start new infestations by transferring pieces of root on implements or tractor tyres.
- Begin work on the least affected part of the paddock and finish in the most heavily infested area. Clean implements before they are taken onto clean land.
- See the
Washdown Guidelines for Weed and Disease Control for detailed information on how to wash-down equipment and personnel to reduce the chance of spreading Californian thistle.
- Mowing and slashing the above-ground parts of Californian thistle are not effective as the root system is not removed and seed plays a relatively minor role in spread.
- Cultivation alone will not control Californian thistle. However, cultivation is important in an integrated control program, especially where large, well-established infestations are involved.
- Cultivation is used to smash up the root system and force the root buds to germinate.
- For the initial working, use a tool with deep penetrating blades such as a chisel plough and work to the depth to which the thistle roots have penetrated.
- Subsequent cultivations can be done with shallower working implements which sever the growing stems and drag the root fragments to the surface.
- The first cultivation should be carried out in the spring before thistle growth and while the ground is moist to allow deep working. Later cultivations should be carried out during the spring, summer and early autumn, to destroy the tops of the thistles before they flower.
- If used alone, between three and five cultivations may be needed in a season.
- Cultivation can be followed by a forage crop (such as an autumn cereal) or pasture.
- Californian thistle seedlings are suppressed by crops which reduce the sunlight below 20% of full daylight.
- Following cultivations, the area can be sown down to permanent pasture in the autumn and a herbicide follow-up program used. Care must be taken to ensure a strong and rapid pasture establishment.
- In some crops such as lucerne and berry fruits, Californian thistle can be difficult to control. The weed should be eliminated before these crops are planted.
- Grazing is not effective in controlling Californian thistle. Sheep and goats will graze young shoots in spring but grazing alone will not eradicate or stop the spread of Californian thistle.
- A number of herbicides are registered for use on Californian thistle in Tasmania. See
Herbicides for Californian Thistle Control for more information.
- Although the top growth of Californian thistle is susceptible to many herbicides, long term control is difficult for this deep rooted perennial weed.
- Care should be taken to differentiate between newly-establishing seedlings and rosettes that have re-grown from root stock, as the timing for herbicide treatment are different.
- For newly establishing seedlings, treatment should be applied as soon as germination is complete.
- For rosettes that have regrown from root stock, delay treatment until the plants have grown flowering stems and are at the bud stage. At this point root reserves are at a minimum and the plant begins to move nutrients (and herbicide) down into the roots. A practical indication of timing is to spray as soon as the first flowers are seen in an infestation, usually between December and January.
Herbicides for Californian Thistle Control