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Common and Scientific Names
Slender thistles are native to Europe and North Africa. The range of C. pycnocephalus extends to Asia Minor and Pakistan while that of C. tenuiflorus extends northwards to Britain and Scandinavia. They are a problem in many areas of the world. Both species were present in Victoria during the 1880s and now occur throughout much of the State. Slender thistles are troublesome weeds in pastures and wastelands, favouring areas of winter rainfall and soils of moderate to high fertility. The two species often occur together in mixed populations. Description
Management Program Some control methods described in this note are only effective if used in combination with other control options as part of a long-term management program. If used in isolation, these methods do not effectively destroy the plant, allowing it to re-shoot or continue to grow. Authorised officers from DPI or DSE may direct landowners to undertake specific control activities to ensure methods are used that are capable of destroying plants and preventing their spread. Where directed to do so, landowners must use the method or methods as directed by the authorised officer. In most cases the landowner will be able to choose from a variety of options appropriate for use in their particular situation. Priorities for controlling different infestations must be worked out when planning a slender thistle management program. Clean areas should be kept free of the weed and managed to prevent infestation. Lightly infested areas should be cleaned up as soon as possible to prevent spread. Extensive infestations are best quarantined and tackled progressively as part of pasture improvement programs, in conjunction with other control techniques. Any technique which kills the seedlings during the autumn germination period will give control until the next autumn. A pronounced autumn break with good rains generally leads to more uniform mass germination than a weak break with scattered rainfall, and thus allows for more effective control. Continuous and vigilant monitoring and control is the best approach to thistle management. Manual Control Isolated plants can be chipped or grubbed. Remove as much of the taproot as possible to prevent regrowth. Cultivation Ploughing is effective in killing young plants. Deep ploughing before flowering, followed up with repeated cultivations and pasture improvement will give the best results. Slashing or Mowing These techniques are usually not effective because cut plants can regrow from the base and cut flowering stems may still produce fertile seed. Pasture Management Careful grazing management is necessary to minimise bare ground, which assists seedling establishment. Presence of a strong, competitive, perennial grass pasture giving good continuous ground cover during the autumn germination period will effectively limit thistle populations. Perennial grasses suppress slender thistle seedlings, reducing establishment. Phalaris and cocksfoot are suitable pasture species. When cattle are replaced by sheep, slender thistles may become more dominant. Applications of superphosphate encourage slender thistles. Contact your local DPI Catchment and Agriculture Services staff for pasture management advice. Grazing Deferred grazing, in which stock are excluded from infestations during autumn and winter, allows the rosettes to become erect and the spines to soften. Stock will often eat such plants in preference to other species. Deferred autumn grazing increases the suppressive effects of competition by more valuable pasture species: growth of thistles is systematically suppressed by shading. Goats readily consume slender thistle flower heads and have been used at the flowering stage to reduce seed production. Fencing must be adequate to restrict these animals to the chosen control area. Keep in mind that browsing animals eat a range of vegetation and may destroy desirable plants as well as thistles. Spray-grazing can be used at the rosette stage (see below). Chemical Control The Australian Pesticides & Veterinary Medicine Authority (APVMA) is responsible for the assessment and registration of agricultural and veterinary chemicals in Australia. As chemical products are registered on a daily basis and renewal of these registrations are undertaken each financial year, there is much change in the registration status of products each year. The information is available from APVMA (external link). The Chemical Standards Branch (CSB) of the Department of Primary Industries provides information on agricultural chemicals registered in Victoria and their uses. Enquiries will be referred through the Customer Service Centre on 136 186. Information can also be obtained by visiting the CSB website. Under Victorian legislation there are controls on the use of agricultural chemicals. It is the responsibility of the user to be familiar with these controls. These responsibilities are outlined in Information Note AG0520: “Responsible use and handling of farm chemicals”. Farm chemicals are registered for specific uses. Each chemical has a ‘product label’, which documents the approved use and the approved rate of use within each State of Australia. This label is important in determining the appropriateness of chemical use. Choose only products registered for use on slender thistles in your particular situation. Read the product label carefully and follow all label instructions. Your chemical retailers can provide information on registered chemical products that are available in their store. They can also supply a ‘material safety data sheet’ which outlines the health and safety issues associated with use of a product. Legal use of some restricted chemicals requires the user to possess an Agricultural Chemical User Permit (ACUP). Other chemicals have restrictions on their use in Agricultural Chemical Control Areas (ACCAs). Information on ACUPs, ACCAs and other chemical information can be found at the CSB website. Herbicidal control is most effective in autumn on young seedlings and rosettes. Control is much more difficult after plants have bolted. Spray-grazing Spray-grazing involves the use of sub-lethal rates of phenoxy herbicides on the actively growing rosette stage, followed by heavy grazing (preferably at more than five times the normal stocking rate) until the weed is eaten. The herbicide increases the attractiveness of the weed to stock by mobilising sugars and changes its growth form so that the leaves are more erect and accessible. Using low rates of herbicide minimises the impact on the legume component of a pasture. Weed Wiping Weed wiping using wicks, carpet or other materials can be used against slender thistles once they have started to ‘bolt’ or send up flowering heads. There should be a minimum height difference of 15 cm between the top of the thistles and the pasture so that the weeds can be wiped without contacting the pasture.
Rust infection starts on new seedlings in autumn and builds up to epidemic proportions by spring, covering most of the leaves and flowering stems. Infection reduces the vigour and subsequent seed set of plants and makes them more susceptible to competition from desirable pasture species. Rust infection reduces plant height, plant mass and the production of viable seed. The rust has spread considerable distances and control appears to be most effective in wetter summers. The thistle receptacle weevil, Rhinocyllus conicus, released in Victoria to suppress spear and variegated thistles, and the thistle rosette weevil, Trichosirocalus horridus, released in Victoria for control of spear thistle, attack slender thistles in New Zealand. Specific strains of these insects suitable for control of slender thistles have not been released in Australia. Further Advice
Slender thistle suppression with the slender thistle rust fungus. Information Notes: Biological Control LC0146, Frankston, Keith Turnbull Research Institute. Code, G. (1998) Weed Control in Pastures. Rutherglen, Department of Natural Resources and Environment. Parsons, W.T. and Cuthbertson, E.G. (1992) Noxious Weeds of Australia. Melbourne, Inkata Press. Woodburn, T.L., Briese, D.T. and Corey, S. (1996) Thistle management. Proceedings of a workshop held at CSIRO Division of Entomology, Canberra on 12-13 June 1996. Plant Protection Quarterly 11 Supplement 2, pp. 231-292. Acknowledgements Prepared by Ian Faithfull. El Bruzzese. Brad Roberts of Keith Turnbull Research Institute (KTRI) provided information and comment. Updated by Melanie Martin, DPI, October 2006. Chemical information from the Chemical Standards Branch, August 2006. Updated by Simon Bonwick, DPI, August 2007. The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication. |
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© The State of Victoria, 1996 - 2008.
This document was published on 26/10/2008.