Carduus pycnocephalus
(Italian thistle)

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).

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).

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.

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’.

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).

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).

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.

Animal feed, fodder, forage

• Forage

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.

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’.

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

09/04/13: Original text by:

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