Common and scientific names
Slender thistle, shore thistle
Carduus pycnocephalus L. (slender thistle) Carduus tenuiflorus Curt. (winged slender thistle) Family Asteraceae (daisy family)
Origin and distribution
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 oftenoccur together in mixed populations.
Erect annual herbs, commonly 60 to 100 cm high but up to 2m, reproducing by seed. Seed germinates in the 6 weeks following the autumn break. Seedlings develop into rosettes and remain in the rosette stage over winter. Flowering stems are produced in early spring and flowering continues from September to December. Plants die in early summer after flowering, but dead stems can remain standing for months.
Stems - flowering stems are single or multiple from the base, branched, strongly ribbed and slightly woolly. Spiny wings occur along most of the length of flowering stems.
Leaves - rosette leaves 15 to 25 cm long, stalked and deeply lobed with numerous spines on each lobe. Stem leaves shorter, deeply lobed with spines along the margin and with the base forming wings along the stem.
Flowers - pink or purple, in slender heads 2 cm long or less, lacking stalks; surrounded by numerous spiny bracts (modified leaves at the base of flower). Heads in groups at the ends of the branches or in the leaf axils.
Seeds - two types in each head: inner seeds and outer seeds. Inner seeds, comprising about 85% of seeds, occur in the central part of the head and have longitudinal striations. They are 4 mm long and grey brown in C. tenuiflorus and 5 mm long and yellowish fawn in C. pycnocephalus. Outer seeds are the same size but are grey in colour. All seeds have a group of plumes (the pappus) about three times as long as the seed for wind dispersal.
Roots - branched, slender or stout tap root.
Slender thistles are found mainly in areas with an annual rainfall exceeding 500 mm. Seedlings establish preferentially on bare and disturbed sites such as stock yards, sheep camps, rabbit burrows and heavily grazed annual pastures. They are competitive weeds in improved pastures, outperforming subterranean clover and ryegrass, and significantly reduce pasture production, but tend to occur irregularly from year to year. Slender thistles are avoided by stock because of their spines and this encourages their spread in heavily grazed pastures. The spines can cause mechanical injury to stock and dogs. Slender thistles are potentially toxic to stock due to high levels of nitrate. Their spines and dead leaves contribute to vegetable fault in wool. The pollen produced in late springis a useful bee forage.
Table 1. Features used to distinguish the two species of slender thistle.
|Carduus pycnocephalus||Carduus tenuiflorus|
|Stalks of rosette leaves always green||Stalks of rosette leaves often purplish or with red/purple flecks|
|Leaves shiny, green, with lighter coloured areas at base of leaf spines (usually)||Leaves bluish-green. No lighter coloured areas at base of spines on leaves (usually)|
|Wings discontinuous on the stems||Wings continuous on the stems|
|Stems usually without wings beneath the flower heads||Stems with wings to the base of the flower heads|
|Flower heads in clusters of 3 or 4||Flower heads in clusters of 3 to 8|
|Inner spiny bracts on flower heads are shorter than the adjacent florets||Inner spiny bracts on flower heads are as long or longer than the adjacent florets|
Slender thistles are dispersed solely by seed which can be carried for long distances in the wind. Seeds have a parachute of barbed hairs (the pappus) which aids wind dispersal and attaches to clothing and animal coats, particularly the fleece of sheep. Goldfinches and other granivorous birds eat the seed but they remove the husk before consumption so do not disperse viable seed. Seed is spread in contaminated hay, silage and grain and on farm machinery, and can be dispersed in water.
Prescribed measures for the control of noxious weeds
- Application of a registered herbicide
- Physical removal
Important information about prescribed measures for the control of noxious weeds.
Other management techniques
Changes in land use practices and spread prevention may also support slender thistle management after implementing the prescribed measures above.
Biological control is a long term program which is best used on large, chronic infestations with a low priority for control due to inaccessibility, remoteness or low threat of spread. The single agent released to control slender thistle in Victoria, the rust fungus, Puccinia carduipycnocephali, will assist in integrated management of slender thistles.
The slender thistle rust has occurred in Australia since early this century. It infects both species of slender thistle and is common, but this particular strain of rust has had little effect on slender thistle infestations. Two new strains selected in Europe by the CSIRO Division of Plant Industry, have been released at numerous sites in Victoria since 1995. Strain IT2 from Italy and strain FR3 from France, are extremely virulent on Australian types of C. pycnocephalus and C. tenuiflorus respectively, although each strain will attack the other species. Thorough host specificity studies have demonstrated that the rust presentsno danger to other species of plants.
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.
- contact your local landcare or friends group for further assistance and advice.
- Call the DEPI Customer Service Centre on 136 186.
- Visit the Weeds Australia website at: www.weeds.org.au
Slender thistle suppression with the slender thistle rust fungus. Landcare 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.