Carduus pycnocephalus
(Italian thistle)

Pictures

Picture	Title	Caption	Copyright
Title Habit Caption Carduus pycnocephalus (slender or Italian thistle); general habit of flowering plants, showing the tall, slender nature of the flower stems. Copyright ©Trevor James/Hamilton, New Zealand-2014	Habit	Carduus pycnocephalus (slender or Italian thistle); general habit of flowering plants, showing the tall, slender nature of the flower stems.	©Trevor James/Hamilton, New Zealand-2014
Title Habit Caption Carduus pycnocephalus (slender or Italian thistle); general habit of flowering plants, showing the foliage and tall, slender nature of the flower stems. Copyright ©Trevor James/Hamilton, New Zealand-2014	Habit	Carduus pycnocephalus (slender or Italian thistle); general habit of flowering plants, showing the foliage and tall, slender nature of the flower stems.	©Trevor James/Hamilton, New Zealand-2014
Title Flowers Caption Carduus pycnocephalus (slender or Italian thistle); cluster of three flower heads at the top of a stem, showing the very narrow ‘wings’. Copyright ©Trevor James/Hamilton, New Zealand-2014	Flowers	Carduus pycnocephalus (slender or Italian thistle); cluster of three flower heads at the top of a stem, showing the very narrow ‘wings’.	©Trevor James/Hamilton, New Zealand-2014

Identity

Preferred Scientific Name

• Carduus pycnocephalus L.

Preferred Common Name

• Italian thistle

International Common Names

• English: compact-headed thistle; Italian plumeless thistle; Plymouth thistle; sheep thistle; shore thistle; slender thistle; slender winged thistle; slender-flower thistle; winged slender thistle; woolly thistle
• Spanish: Cardo de clavero
• French: Chardon a epines vertes; Chardon a trochets

Local Common Names

• Egypt: lisan el-kalb
• Germany: knaulköpfige Distel
• South Africa: Corsican thistle
• Sweden: gyttertistel

EPPO code

• CRUPY (Carduus pycnocephalus)

Summary of Invasiveness

Carduus pycnocephalus is a thistle that is native to the Mediterranean region and some other countries further north or east. It has been introduced, presumably accidentally, to the USA, Australia, New Zealand and some other countries in Europe, Asia, Africa and South America. In many of the countries where it has become naturalized it is regarded as a legally-defined noxious plant or pest plant, depending on the current terminology; it also causes problems in some countries where it is considered a native species. It can form dense infestations in some places where it can smother other, smaller plants and, where it occurs in grazed pastures, can limit the access of livestock and also cause them physical damage, as well as contaminating wool. In this way it has become a problem in the USA, Australia, New Zealand, Pakistan, Iran and Europe (Pitcher and Russo, 1988).

Taxonomic Tree

• Domain: Eukaryota
•     Kingdom: Plantae
•         Phylum: Spermatophyta
•             Subphylum: Angiospermae
•                 Class: Dicotyledonae
•                     Order: Asterales
•                         Family: Asteraceae
•                             Genus: Carduus
•                                 Species: Carduus pycnocephalus

Notes on Taxonomy and Nomenclature

Carduus pycnocephalus is very similar to C. tenuiflorus and at least in some populations distinguishing between the two species (Webb et al., 1988) can be extremely difficult and intergrades can occur. The two species frequently hybridize but the seed set of hybrids is low (Olivieri, 1985).

Description

Annual or very rarely biennial, herbaceous, from a stout taproot; stems 15-200 cm tall, erect, glabrous to sparsely tomentose, narrowly and discontinuously winged, the wings spinose, tomentose, branched above the lower third, branches erect to ascending. Basal leaves 6-15 cm long, oblanceolate, deeply 4-10-lobed, the base tapered; cauline leaves alternate, decurrent, sinuate to pinnately lobed, margins spinose, upper surfaces loosely tomentose, becoming glabrous, lower surfaces densely tomentose. Heads discoid (all corollas radial and salverform), 17-22 mm long, 10-20 mm wide, cylindrical to subcylindrical, sessile to stalked, solitary or 2-5 in terminal clusters. Outer phyllaries ovate-lanceolate, loosely tomentose, margins membranous, apices acuminate, terminating in a straight spine; inner phyllaries narrower, scarious. Corollas 10-14 mm long, pink to rose-purple, sometimes white. Achenes 4-6 mm long, tan to brown, sometimes shiny, transversely wrinkled, tubercled above; pappus 10-20 mm long, composed of flat, minutely barbed, white bristles. (Description slightly modified from Wilken and Hannah, 1998).

Distribution

In addition to the countries listed in the Distribution table, Pitcher and Russo (1988) claimed that the species was present in South Africa, but it is not listed as an alien invasive plant there (Environment News South Africa, 2013), and it is listed as a banned import (Department of Environmental Affairs and Tourism, South Africa, 2009).

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

History of Introduction and Spread

Presumably C. pycnocephalus was carried to many countries by the dispersal of European populations in the 19^th century, when people took with them hay and straw for livestock, seed for new pastures, and bedding and packing materials that contained vegetable matter.

Old herbarium records in the USA indicate that Carduus pycnocephalus was introduced on ships’ ballast into several east-coast ports (e.g., Philadelphia, Mobile) in the 19^th century; the lack of subsequent collections suggests that conditions were unsuitable for it to become permanently established (Flora of North America, 2014). Although now listed as present in several states, some on the east coast (USDA-NRCS, 2013), it is usually of limited or very limited distribution, and only seems to be of concern in California and parts of Oregon.

Introductions

Risk of Introduction

Thanks to modern phytosanitary precautions the ongoing spread of this species to more countries ought to limited. However, spread within countries will no doubt continue, possibly helped by changing global climates.

Habitat

In its native habitats in southern and western Europe, C. pycnocephalus occurs in disturbed places (Olivieri et al., 1983). Similarly in the countries to which it has been introduced and become naturalised it is invariably recorded as found in disturbed places where competition from other plants has been reduced, sometimes temporarily, although the causes of the disturbance may vary. In California Bossard and Lichti (2000) describe it as infesting areas below 3000 feet (1000 m) in chaparral and oak savanna and also in meadows, pastures and ranges, on roadsides and in disturbed wildland areas.

In Australia, the Department of Environment and Primary Industries, Victoria (2014) report that 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.  

The species occurs in similar places in New Zealand, where its populations in pastures fluctuate from year to year, along with those of other annual thistles. In general this fluctuation is influenced by the condition of pastures the previous late summer and autumn. If conditions then were such that perennial pasture species were weakened or dormant (as a result of drought, overgrazing or insect attack) then autumn rain favours rapid establishment of quick-germinating and fast-growing annual species like thistles and barley grass (Critesion murinum [Hordeum murinum]), which can then dominate pastures in the following summer (Popay et al. 1981; Edmonds and Popay, 1983; Harradine, 1985). Webb et al. (1988) describe its habitat in New Zealand as ‘waste land, pasture, riverbeds, roadsides, railway yards, tussock grassland’.

Habitat List

Hosts/Species Affected

Pasture species can be replaced, their growth inhibited and accessibility to grazing livestock obstructed by high populations of C. pycnocephalus rosettes and flowering plants (Kelly and Popay, 1985).

Biology and Ecology

Genetics

According to the Flora of North America (2014) ‘The only published chromosome counts for Carduus pycnocephalus from North American material are from California specimens (A. M. Powell et. al. 1974). Published chromosome counts (2n = 18, 31, 32, 54, 60, 64, 80) for C. pycnocephalus from a variety of Old World localities indicate that this is a complex species in need of further investigation.’

In Egyptian material Kamel (1999) found that material of this species was hexaploid with a somatic chromosome number of 2n=54 and basic number of x=9. The same author reported that other workers had found 2n=18, 32, 60 and 64 for this species.

Reproductive Biology

As reported by several authors, C. pycnocephalus is bisexual, self-compatible and pollinated by a diverse range of insects (Bendall, 1975; Evans et al., 1979; Olivieri et al., 1983). The achenes (seeds) in the centre of the seed head receptacle, which are cream-coloured and striated and carry a pappus, are shed from the seed head and dispersed by air currents, possibly for some distance with the help of the pappus. Others, at the outer edges of the receptacle, are darker, unstriated and without a pappus; these remain within the head and fall with the plant as it dies and desiccates. Those that are dispersed are more likely to germinate readily when conditions of moisture, light and temperature are right, whereas those remaining within the head are more likely to remain dormant for some time before becoming able to germinate (Olivieri et al., 1983). Both kinds of seeds are mucilaginous, which may help either dispersal or germination or both (Wilken and Hannah, 1998).

Germination usually occurs with the first substantial rain of autumn and some seeds may germinate as late as early winter (Kelly, 1988). However, as with the seeds of many weed species in grasslands, a pasture cover at the time of germination may inhibit or prevent germination, possibly for as long as a few years.

Seedling mortality can be high at some sites (Kelly, 1988). Plants overwinter as rosettes and, in dense infestations, a high proportion of the ground can be covered by these. Rosettes begin to ‘bolt’ (that is, the flower stalks start to elongate) in mid-spring and flowering takes place at the end of autumn or early in summer (Kelly. 1988).  Plant size, which relates to the number of seed heads per plant and therefore to the total number of seeds per plant (number of seeds per head is usually no more than 15 – Olivieri et al., 1983) is very variable. Kelly (1988) found that the number of seeds per plant averaged 72 in one year and 276 the next.  

Longevity

The plants themselves are strictly winter annuals but, at least under some circumstances, seeds can survive in the soil for 3 years or more (Olivieri et al., 1983).

Population Size and Structure

Rosettes can blanket the ground in winter, preventing the germination and establishment of other species (Bossard and Lichti, 2000). 

Nutrition

Groves (1989) said that the growth of C. pycnocephalus is promoted by rich soils and Bendall (1975) found that adding nitrogen promoted plant growth. In both Australia and New Zealand where this species often grows on stock ‘camps’ (areas of pasture in which stock congregate and rest) the pasture cover is highly disturbed and the area fertilized with dung and urine – perfect conditions for the thistles to thrive.

Associations

In their native France and also where they have been introduced - Australia and New Zealand - C. pycnocephalus and C. tenuifolius are often found growing together. In such circumstances distinguishing between the two species can be difficult because they can interbreed, although the offspring are not very fertile (Olivieri, 1985; Webb et al., 1988).

Climate

Natural enemies

Notes on Natural Enemies

According to Pitcher and Russo (1988) all major parts of C. pycnocephalus are damaged by one or more insect species in southern Europe, whereas in southern California the thistles are relatively free of insect damage. Species that have been used or considered as biological control agents include the seed head weevil Rhinocyllus conicus [or Curculio conicus], the crown weevil (Trichosirocalus horridus [or Ceutorhynchus horridus]), and the fungal rust Puccinia cardui-pycnocephali [P. calcitrapae]. For more information see the ‘Prevention and Control’ section.

Means of Movement and Dispersal

Natural Dispersal (Non-Biotic)

The central seeds of the seed head carry a pappus which facilitates their movement by wind and air currents. According to Bossard and Lichti (2000), seeds are moved an average of 23 m from the parent plant and up to 108 m in strong winds.

Vector Transmission (Biotic)

Seed heads (no doubt containing seeds) sometimes become detached from the parent plant and can be observed clinging to the wool of grazing sheep – an excellent way of transferring seeds around the countryside. In New Zealand birds, especially goldfinches (Carduelis carduelis) prefer to feed on seeds of weeds, including thistles (Troup, 2012). This may reduce seed numbers, but some spillage of seed may also occur, leading to further spread.

Accidental Introduction

In the past, accidental transport of seeds to Australia, New Zealand, the USA and possibly South Africa probably occurred with the movement of animals and their feed and bedding material from Europe to the new colonies. Modern phytosanitary regulations and associated inspections militate against further accidental introduction to most countries.

On a more local scale, seed-contaminated hay or vehicles could easily transport seed to other localities.

Intentional Introduction

This is most unlikely since the species has no redeeming features.

Impact Summary

Economic Impact

C. pycnocephalus is a competitive weed in pastures and can cause injury to livestock, as well as being potentially toxic (Department of Environment and Primary Industries, Victoria, 2014). Kelly and Popay (1985) estimated the costs of lost pasture production due to thistles (mainly Carduus nutans with some C. pycnocephalus and Cirsium arvense) in a paddock in New Zealand and found that control with herbicide would have been cost-effective in some seasons but not in all, due to the fluctuation in thistle populations. However, in some seasons the loss of pasture production from C. pycnocephalus alone would no doubt be substantial on affected farms.

Fragments of thistle leaves and seed heads can get caught up in wool, which increases wool cleaning costs and makes life less pleasant for shearers (Australian Wool Testing Authority, 2002). Such fragments are also present in hay bales, reducing the value of the hay, as well as spreading seed.

Many farmers in both New Zealand and Australia regularly treat their pastures with broadleaf herbicides almost as an ‘insurance’ against thistle infestations. This is aimed at ameliorating at least some of the losses caused by this and other thistles. However, as Harrington and Hewage (1997) discovered, this can lead to development of herbicide resistance which in turn adds extra costs and worries to the farmers.

Cal-IPC (2004) say that in California C. pycnocephalus increases fire frequency and movement into the overstory of island scrub oak chaparral. Fire is carried into the oak overstory by the C. pycnocephalus midstory. This may or may not increase the threat compared to native vegetation or annual grasses in the same area, but C. pycnocephalus tend to grow taller and be a better fire ladder than other species.

Environmental Impact

Impact on Habitats

Many of the habitats in which C. pycnocephalus occurs have already been disturbed by direct or indirect human activity so these thistles are unlikely to cause any additional damage, although they may delay recovery of the habitat to a less damaged state.

Impact on Biodiversity

Again, damage to biodiversity has already been caused by the factors which disturbed the environment in the first place so the thistles are unlikely to cause any further damage.

Risk and Impact Factors

• Proved invasive outside its native range
• Has a broad native range
• Pioneering in disturbed areas
• Fast growing
• Has high reproductive potential
• Has propagules that can remain viable for more than one year

• Modification of fire regime
• Negatively impacts agriculture
• Damages animal/plant products

• Competition - shading
• Produces spines, thorns or burrs

Uses

Although thistles are usually thought of as useless in terms of their feed value, at some times in the past they have been seen as useful in hard times (for more information see under ‘Grazing’ in the ‘Prevention and Control’ section).

Similarities to Other Species/Conditions

C. pycnocephalus is very similar in general appearance to C. tenuiflorus (often called slender thistle) but differs in having narrow discontinuous wings along the stems, 4-10-lobed leaves (as opposed to 12-20-lobed leaves in C. tenuiflorus), 3-5 heads in clusters (against 5-20), stems leafless just below the heads, and fruits that are mucilaginous or sticky (Healy, 1982). Keil and Turner (1993; cited in Wilken and Russo, 1998) and Webb et al. (1988) suggested that the two species hybridize so that many populations (at least in New Zealand) contain a continuum of plants showing intermediate characters. Olivieri (1985) found that hybrids have intermediate morphology and isozyme patterns.

Prevention and Control

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

Farmers and other land managers should always take care, if they have a thistle-free property, to ensure that any agricultural machinery (such as tractors, ploughs, mowers or haymaking machinery) has not previously been used on thistle-infested properties or that it has been carefully cleaned beforehand. Similarly pasture or other seed brought onto the property should be of high quality and properly tested for contamination with weed seeds.

Control

Physical/mechanical control

Hand pulling of isolated thistles well before seed set has been used in some places (PIER, 2013); the root must be severed at least 10 cm below ground to prevent regrowth (Bossard and Lichti, 2000). Isolated plants can be chipped or grubbed, a common practice in both Australia and New Zealand, but as much of the taproot as possible should be removed to prevent regrowth (Harrington, 2014; South Gippsland Landcare Network, 2014).

Although slashing or mowing is not usually recommended because C. pycnocephalus plants can regrow from the base (Bossard and Lichti, 2000) many farmers use this as a method of making the farm look tidy.

Cultivation, usually used for larger areas of thistles, will eventually destroy the thistles, but it needs to be repeated until the seedbank is depleted (up to ten years) (Bossard and Lichti, 2000). According to the Department of Primary Industries, Parks, Water and Environment, Tasmania (2014), repeated cultivation followed by the establishment of a vigorous crop or improved pasture species can control thistles, but only if appropriate care of the subsequent pasture is taken.

Movement control

As mentioned under ‘Prevention’, care should be taken not to move thistle seed to unaffected farms or to unaffected parts of the same farm on agricultural machinery, vehicles or livestock.

Biological control

According to Picher and Russo (1988) all major parts of C. pycnocephalus are damaged by one or more insect species in southern Europe, whereas in southern California the thistles are relatively free of insect damage. The seed head weevil Rhinocyllus conicus [or Curculio conicus] has been introduced into several countries (Canada, USA, Australia, New Zealand) for control of one or more thistle species, including C. pycnocephalus. The larvae feed on the receptacle and developing achenes of thistle species and certainly destroy many seeds, but often enough survive to maintain populations of thistles (Popay et al., 1984; Pitcher and Russo, 1988). A crown weevil (Trichosirocalus horridus [or Ceutorhynchus horridus]) has also been introduced to several countries as a biocontrol agent for thistle species and may be effective against C. pycnocephalus.

The fungal rust Puccinia cardui-pycnocephali [P. calcitrapae], already present in many countries where C. pycnocephalus or C. tenuifolius or both are present, has also been considered as a possible biocontrol agent, but its effects seem less than lethal although more virulent strains may be more damaging (Olivieri, 1984).

Chemical control

2,4-D or MCPA have long been used as overall sprays for the control of C. pycnocephalus and other thistles, although the finding of herbicide resistance in the species suggests that they will become less useful in the future (Harrington and Hewage, 1997). Clopyralid is now commonly used to control thistles, although it damages clovers where these are important for herbage production in grazed pastures. Other herbicides like picloram and triclopyr are also used for killing thistles, but they too kill clovers and other broadleaf plants.

IPM

Bendall (1974) described a method of controlling C. pycnocephalus in pastures, using grazing management. In autumn, after thistle seedlings start to appear, grazing animals are removed and the pastures left alone for about 6 to 10 weeks, when the pasture grasses are 10-15 cm tall. The area is then heavily grazed with sheep, which will then selectively graze the tender, etiolated thistles, killing 90 to 95% of them.

Grazing

Although C. pycnocephalus is usually considered to be an unpalatable and useless thistle, it has not always been considered so in some places. Thomson (1922) reported a Mr D Petrie as saying ‘Wherever it is plentiful it affords a very considerable bulk of highly nutritious feed. There are experienced runholders who reckon it little inferior to rape. The young plants that shoot up after the earliest autumn rain form the main and almost the sole winter feed in the desert lowlands. The old dry and withered stems are almost completely eaten out at this season of scarcity.’ This referred to the seasonally dry New Zealand province of Central Otago.

Sheep and especially goats have often been used to control thistles, including C. pycnocephalus. These become more attractive to goats after they start flowering, when goats preferentially eat the flower and seed heads; this limits seed production since most of the seeds are digested, with very few surviving passage through the gut (Harrington et al., 2011).

The use of grazing by sheep in conjunction with allowing the grass to grow is described above under ‘IPM’.

Gaps in Knowledge/Research Needs

The cytology and taxonomy of C. pycnocephalus may repay further study.

References

Australian Wool Testing Authority, 2002. Testing the Wool Clip. Kensington, Victoria, Australia: Australian Wool Testing Authority, 26 pp. http://www.awtawooltesting.com.au/index.php/en/component/edocman/?task=document.download&id=35

Batra SWT; Coulson JR; Dunn PH; Boldt PE, 1981. Insects and fungi associated with Carduus thistles (Compositae). Technical Bulletin, Science and Education Administration, United States Department of Agriculture, No. 1616. 100 pp.

Bendall GM, 1974. Slender thistles in pasture: control by grazing management. Tasmanian Journal of Agriculture, 45(1):62-63.

Bendall GM, 1975. Some aspects of the biology, ecology and control of slender thistle, Carduus pycnocephalus L. and C. tenuiflorus Curt. (Compositae) in Tasmania. Journal of the Australian Institute of Agricultural Science, 41(1), 52-53.

Biological Records Centre, 2013. Online Atlas of the British and Irish flora. Wallingford, UK: Biological Records Centre. http://www.brc.ac.uk/plantatlas/

Bossard C; Lichti R, 2000. Carduus pycnocephalus. In: Invasive Plants of California's Wildlands [ed. by Bossard, C. C. \Randall, J. M. \Hoshovsky, M. C.]. Berkeley, California, USA: University of California Press. http://www.cal-ipc.org/ip/management/ipcw/pages/detailreport.cfm@usernumber=24&surveynumber=182.php

Cal-IPC (California Invasive Plant Council), 2004. Cal-IPC Plant Assessment Form for Carduus pycnocephalus. Berkeley, California, USA: California Invasive Plant Council. http://www.cal-ipc.org/paf/site/paf/284

Cheeseman TF, 1883. The naturalized flora of the Auckland Provincial District. Transactions of the New Zealand Institute, 15:268-298.

Council of Heads of Australasian Herbaria, 2013. Australia's virtual herbarium. Australia: Council of Heads of Australasian Herbaria. http://avh.ala.org.au

Department of Environment and Primary Industries; Victoria, 2014. A-Z of weeds. Melbourne, Victoria, Australia: Department of Environment and Primary Industries. http://www.depi.vic.gov.au/agriculture-and-food/pests-diseases-and-weeds/weeds/a-z-of-weeds

Department of Environmental Affairs and Tourism South Africa, 2009. National Environmental Management: Biodiversity Act, 2004. List 2: List of Prohibited Alien Species. South African Government Gazette, 52(32090). 76-99. https://www.environment.gov.za/sites/default/files/gazetted_notices/nemba_alienspecies_g32090gen349.pdf

Department of Primary Industries; Parks; Water and Environment; Tasmania, 2014. Weeds index. Hobart, Tasmania, Australia: Department of Primary Industries, Parks, Water and Environment. http://dpipwe.tas.gov.au/invasive-species/weeds/weeds-index

Dunn PH, 1976. Distribution of Carduus nutans, C. acanthoides, C. pycnocephalus and C. crispus in the United States. Weed Science, 24(5):518-524

Edmonds DK; Popay AI, 1983. Effect of pasture competition on the survival and flowering of nodding thistle. In: Proceedings of the thirty-sixth New Zealand weed and pest control conference [ed. by Hartley, M.J.\Popay, A.J.]. Palmerston North, New Zealand: New Zealand Weed and Pest Control Society, 89-92.

Environment News South Africa, 2013. Alien Invasive Plants List For South Africa. South Africa: Environment News South Africa. http://www.environment.co.za/weeds-invaders-alien-vegetation/alien-invasive-plants-list-for-south-africa.html

Evans RA; Young JA; Hawkes R, 1979. Germination characteristics of Italian thistle (Carduus pycnocephalus) and slender flower thistle (Carduus tenuiflorus). Weed Science, 27(3):327-332.

Flora of North America Editorial Committee, 2014. Flora of North America North of Mexico. http://www.efloras.org/flora_page.aspx?flora_id=1

Goeden RD, 1974. Comparative survey of the phytophagous insect faunas of Italian thistle, Carduus pycnocephalus, in southern California and southern Europe relative to biological weed control. Environmental Entomology, 3(3):464-474.

Groves RH; Kaye PE, 1989. Germination and phenology of seven introduced thistle species in southern Australia. Australian Journal of Botany, 37(4):351-359

Harradine AR, 1985. Dispersal and establishment of slender thistle, Carduus pycnocephalus L. as affected by ground cover. Australian Journal of Agricultural Research, 36(6):791-797.

Harrington K, 2014. New Zealand Weeds. Palmerston North, New Zealand: Massey University. http://www.massey.ac.nz/massey/learning/colleges/college-of-sciences/clinics-and-services/weeds-database/weeds-database_home.cfm

Harrington KC; Beskow WB; Hodgson J, 2011. Recovery and viability of seeds ingested by goats. New Zealand Plant Protection [New Zealand Plant Protection Society's Annual Conference, Rotorua, New Zealand, 9-11 August 2011.], 64:75-80. http://www.nzpps.org/journal/abstract.php?paper=640750

Harrington KC; Hewage N, 1997. Resistance of slender winged thistle to MCPA. In: New Zealand Weed and Pest Control Conference Proceedings, 50. 538.

Healy AJ, 1982. Identification of Weeds and Clovers (third edition). Featherston, New Zealand: Editorial Services Limited, 299 pp.

Holle B von, 2010. Invasional meltdown. In: Encyclopedia of Biological Invasions [ed. by Simberloff, D. \Rejmanek, M.]. Berkeley and Los Angeles, California, USA: University of California Press, 360-363.

Kamel KA, 1999. Karyological studies on some taxa of the Asteraceae in Egypt. Compositae Newsletter, 33:1-18.

Keil D; Turner C, 1993. Carduus. In: The Jepson manual: higher plants of California [ed. by Hickman, J.]. Berkeley, California, USA: University of California Press, 220.

Kelly D, 1988. Demography of Carduus pycnocephalus and C. tenuiflorus. New Zealand Natural Sciences, 15:11-24. http://www.science.canterbury.ac.nz/nzns/issues/vol15-1988/kelly.pdf

Kelly D; Popay AI, 1985. Pasture production lost to unsprayed thistles at two sites. Proceedings, New Zealand Weed and Pest Control Conference Hastings, New Zealand; New Zealand Weed and Pest Control Society, 115-118

Nesom G, 2009. Notes on non-native Asteraceae in Texas. Phytologia, 91(2):325-332. http://biodiversitylibrary.org/page/28763446

Nesom GL, 2004. Asteraceae from wool mill sites in South Carolina, including new records for North America. SIDA, 21(2):1215-1223.

Olivieri I, 1984. Effect of Puccinia cardui-pycnocephali on slender thistles (Carduus pycnocephalus and C. tenuiflorus). Weed Science, 32(4):508-510.

Olivieri I, 1985. Comparative electrophoretic studies of Carduus pycnocephalus L., Carduus tenuiflorus Curt. (Asteraceae), and their hybrids. American Journal of Botany, 72(5):715-718.

Olivieri I; Swan M; Gouyon PH, 1983. Reproductive system and colonizing strategy of two species of Carduus (Compositae). Oecologia,, 60(1):114-117. http://www2.mnhn.fr/oseb/spip/IMG/pdf/1983Oecol.pdf

Parsons WT; Cuthbertson EG, 1992. Noxious Weeds of Australia. Melbourne, Australia: Inkata Press, 692 pp.

PIER, 2013. Pacific Islands Ecosystems at Risk. Honolulu, Hawaii, USA: HEAR, University of Hawaii. http://www.hear.org/pier/index.html

Pitcher D; Russo MJ, 1988. Element stewardship abstract for Carduus pycnocephalus: Italian thistle. Arlington, Virginia, USA: The Nature Conservancy, 9 pp. http://www.invasive.org/gist/esadocs/documnts/cardpyc.pdf

Popay AI; Hartley MJ; Crouchley G, 1981. The effect of different pasture cultivars on barley grass invasion. In: Proceedings of the New Zealand Weed and Pest Control Conference, 34, 107-110. http://www.nzpps.org/journal/34/nzpp_341070.pdf

Popay AI; Lyttle LA; Edmonds DK; Phung HT, 1984. Incidence of the nodding thistle receptacle weevil on nodding and slender winged thistles. In: Proceedings of the Thirty-Seventh New Zealand Weed and Pest Control Conference. Russley Hotel, Christchurch, August 14th to 16th, 1984 [edited by Hartley, M.J.; Popay, A.J.; Popay, A.I.]. Hastings, New Zealand; New Zealand Weed and Pest Control Society, 28-32

Powell AM; Kyhos DW; Raven PH, 1974. Chromosome numbers in Compositae. X. American Journal of Botany, 61:909-913.

Robbins W, 1940. Alien plants growing without cultivation in California. Berkeley, California, USA: University of California, 128 pp. [Agricultural Experiment Station Bulletin 637.]

South Gippsland Landcare Network, 2014. Thistles. Leongatha, Victoria, Australia: South Gippsland Landcare Network, 1 p. [SGLN Weed Fact Sheet #4.] http://sgln.org.au/files/3613/6356/7445/weed_fact_sheet_4_Thistles.pdf

Texasinvasives.org, 2014. Invasive plant database. http://texasinvasives.org/plant_database

Thomson GM, 1922. The naturalisation of animals & plants in New Zealand. Cambridge, UK: Cambridge University Press, 607 pp.

Troup C, 2012. Introduced land birds - Finches. In: Te Ara - the Encyclopedia of New Zealand. Wellington, New Zealand: Manatu Taonga Ministry for Culture and Heritage. http://www.TeAra.govt.nz/en/introduced-land-birds/page-12

USDA-ARS, 2013. Germplasm Resources Information Network (GRIN). Online Database. Beltsville, Maryland, USA: National Germplasm Resources Laboratory. https://npgsweb.ars-grin.gov/gringlobal/taxon/taxonomysearch.aspx

USDA-NRCS, 2013. The PLANTS Database. Baton Rouge, USA: National Plant Data Center. http://plants.usda.gov/

Wagner WL; Herbst DR; Lorence DH, 2014. Flora of the Hawaiian Islands website. Washington DC, USA: Smithsonian Institution,. http://botany.si.edu/pacificislandbiodiversity/hawaiianflora/index.htm

Washington State Noxious Weed Control Board, 2014. Noxious weeds. Olympia, Washington, USA: Washington State Noxious Weed Control Board. http://www.nwcb.wa.gov/

Webb CJ; Sykes WR; Garnock-Jones PJ, 1988. Flora of New Zealand, Volume IV: Naturalised pteridophytes, gymnosperms, dicotyledons. Christchurch, New Zealand: Botany Division, DSIR, 1365 pp.

Wilken D; Hannah L, 1998. Carduus pycnocephalus L. (Asteraceae): Italian Thistle (Channel Islands National Park Service Species Literature Review). http://tinyurl.com/qhwyr4h

Contributors

09/04/13: Original text by:

Ian Popay, consultant, New Zealand, with the support of Landcare Research.

Distribution Maps