star thistle

"Basket flower" and spelling variants redirect here. In Australia, this usually means the unrelated plant Adenanthos obovatus.

"Centaurea" is also the name of the racing horse that won the Australasian Oaks and Storm Queen Stakes in 1985.

Centaurea
Centaurea pullata
Scientific classification
Kingdom:	Plantae
(unranked):	Angiosperms
(unranked):	Eudicots
(unranked):	Asterids
Order:	Asterales
Family:	Asteraceae
Subfamily:	Carduoideae
Tribe:	Cynareae
Subtribe:	Centaureinae
Genus:	Centaurea L.
Species
About 350-600, see text
Synonyms
Cyanus L. Plectocephalus D.Don in R.Sweet (but see text)

Centaurea (pronounced /ˌsɛntɔːˈriːə/ "Cen-tau-ré-a")^[1] is a genus of at least some 350, if not 500 to 600 species of herbaceous thistle-like flowering plants in the family Asteraceae. Although the genus' distribution is Holarctic, most are native to the Palaearctic, where the Middle East and surrounding regions are particular species-rich. Common names for this genus are starthistles, knapweeds, centaureas and the more ambiguous "bluets"; a vernacular name used for these plants in parts of England is "loggerheads". The Plectocephalus group – possibly a distinct genus – is known as basketflowers. "Cornflowers" is used for a few species, but that term more often specifically means C. cyanus (sometimes also called "Basket Flower"). And while one sometimes finds the name "centauries", this properly refers to the unrelated plant genus Centaurium.^[2]

Description and ecology

Knapweeds are robust weedy plants. Their leaves, spiny in some species, are usually deeply divided into elongated lobes at least in the plants' lower part, becoming entire towards the top. The "flowers" (actually pseudanthium inflorescences) are diverse in colour, ranging from intense blues, reds and yellows to any mixture of these and lighter shades towards white. Often, the disk flowers are much darker or lighter than the ray flowers, which also differ in morphology and are sterile. Each pseudanthium sits atop a cup- or basket-like cluster of scaly bracts, hence the name "basketflowers". Many species, in particular those inhabiting more arid regions, have a long and strong taproot.

Common Knapweed (C. nigra), perhaps the single most abundant Centaurea species of England

Centaurea tchihatcheffii, a highly distinctive and rare knapweed endemic to Turkey

Certain knapweeds have a tendency to dominate large stretches of landscape together with a few other plants, typically one or two grasses and as many other large herbs. The Common Knapweed (C. nigra) for example is plentiful in the mesotrophic grasslands of England and nearby regions. It is most prominently found in pastures or meadows dominated by Cock's-foot (Dactylis glomerata) as well as either of Crested Dog's-tail (Cynosurus cristatus) and False Oat-grass (Arrhenatherum elatius). It is also often found in mesotrophic grassland on rendzinas and similar calcareous soils in association with Glaucous Sedge (Carex flacca), Sheep's Fescue (Festuca ovina), and either Tor-grass (Brachypodium pinnatum) and Rough Hawkbit (Leontodon hispidus), or Upright Brome (Bromus erectus). In these grasslands, Greater Knapweed (C. scabiosa) is found much more rarely by comparison, often in association with Red Festuce (F. rubra) in addition to Cock's-foot, False Oat-grass.

Due to their habit of dominating ecosystems under good conditions, many Centaurea species can become invasive weeds in regions where they are not native. In parts of North America, Diffuse Knapweed (C. diffusa), Spotted Knapweed (C. maculosa) and Yellow Starthistle (C. solstitialis) cause severe problems in agriculture due to their uncontrolled spread. The seeds are typically transported by human traffic, in particular the tires of all-terrain vehicles. The two knapweeds are harmful mainly because they are strongly allelopathic, producing powerful toxins in their roots that stunt the growth of plants around them not adapted to this^[3]. Yellow Starthistle, meanwhile, is inedible to most livestock due to its spines and apparently outright poisonous to horses and other equines. However, efficient methods of biological control by insect pests of these weeds have been developed; the knapweeds can also exploited to their detriment by targeted grazing. Controlled burning may also be used, though the timing is important to avoid the plants having seeded already, and neither allowing sufficient time for them to regrow from the rootstock^[4].

Yet other species of Centaurea – mostly ones that occur between Italy and the Caucasus – are endemics of a single island or valley, and some of these are endangered. The Akamas Centaurea (Centaurea akamantis) of Cyprus is almost extinct, while the western Caucasus endemics C. leptophylla and C. straminicephala are at least very rare and C. hedgei and C. pecho from the same region are certainly not abundant either. The last four species would be adversely affected by the proposed Yusufeli Dam, which might actually destroy enough habitat to push the two rarer ones over the brink of extinction.

Mating pair of Knapweed Fritillaries (Melitaea phoebe), male at the left.
These butterflies can spend their entire lives living off a patch of Brown Knapweed (C. jacea).

Centaurea are copious nectar producers, especially on high-lime soils. The high nectar yield of the genus makes it very attractive to insects such as butterflies – including the endangered Karner Blue (Lycaeides melissa samuelis) which visits introduced Spotted Knapweed – and day-flying moths – typically Zygaenidae, such as Zygaena loti or the Six-spot Burnet (Z. filipendulae). The larvae of some other Lepidoptera species use Centaurea species as food plants; see List of Lepidoptera that feed on Centaurea. Several of these are used in biological control of invasive knapweeds and starthistles.

Larvae of several true weevils (Curculionidae) of the subfamily Lixinae also feed on Centaurea. Some genera – such as Larinus whose larval food is flowerheads – have many species especially adapted to particular knapweeds or starthistle and are used in biological control too. These include the Yellow Starthistle Flower Weevil (L. curtus) for Yellow Starthistle, Lesser Knapweed Flower Weevil (L. minutus) for Diffuse Knapweed and Blunt Knapweed Flower Weevil (L. obtusus) for Spotted Knapweed. Broad-nosed Seedhead Weevil (Bangasternus fausti) larvae eat Diffuse, Spotted and Squarrose Knapweed (C. virgata ssp. squarrosa), while those of the Yellow Starthistle Bud Weevil (B. orientalis) do not seem to live on anything other than Yellow Starthistle and occasionally Purple Starthistle (C. calcitrapa). But perhaps most efficient in destroying developing Yellow Starthistle seedheads is the larva of the Yellow Starthistle Hairy Weevil (Eustenopus villosus). Knapweed Root Weevil (Cyphocleonus achates) larvae bore into the roots of Spotted and to a lesser extentely Diffuse Knapweed, sometimes killing off the entire plant.

Also used in biological control are Tephritidae (peacock flies) whose larvae feed on Centaurea. Knapweed Peacock Fly (Chaetorellia acrolophi) larvae eat Spotted Knapweed and some other species. The Yellow Starthistle Peacock Fly (C. australis) has an initial generation each year which often uses Cornflower (C. cyanus) as larval food; later generations switch to Yellow Starthistle. The flies are generally considered less efficient in destroying the growing seedheads than the weevils, but may be superior under certain conditions; employing flies and weevils in combination is expensive and does not noticeably increase their effect.

Use by humans

Though the overall effect of the entire genus on humans is probably detrimental, this is merely due to the considerable damage done by a handful of species when these are allowed to spread uncontrolled. On a per-species basis, most centaureas are inoffensive and at least pleasant-looking plants. As noted above, many species of Centaurea are popular food for insects that may otherwise attack crops; particular in small-scale farming it is often advisable to allow non-invasive species (like Cornflower, C. cyanus, in Europe) to grow around fields. In such a beetle bank, they will draw pests away from crops, and at the same time by their robust structure provide shelter for invertebrate and small vertebrate predators that will keep overall pest populations low^[5].

Yellow Starthistle (C. solstitialis), an invasive weed that yields a fine honey

The abundant nectar produced by Centaurea flowers attracts many pollinators. This is another reason for the success of the highly invasive species, but as it holds true for the entire genus it is not decisive. Yet it can be exploited to the farmer's advantage in combination with biological control of these weeds: In particular the Yellow Starthistle (C. solstitialis) as well as Spotted Knapweed (C. maculosa) are major honey plants for beekeepers. Monofloral honey from these plants is light and slightly tangy, and one of the finest honeys produced in the USA – due to its better availability, it is even fraudulently relabeled and sold as the scarce and expensive Sourwood honey of the Appalachian Mountains. Placing beehives near stands of Centaurea will cause increased pollination. As most seedheads fail however when biocontrol pests have established themselves, the plants will bloom ever more abundantly in an attempt to replace the destroyed seedheads, to the point where they exhaust their resources in providing food for the pests (seeds), bees (pollen) and humans (honey). Output of allelopathic compounds is also liable to be reduced under such conditions – the plant has to compromise between allocating energy to reproduction and defense. This renders the weeds more likely to be suppressed by native vegetation or crops in the following years, especially if properly timed controlled burning^[4] and/or targeted grazing by suitable livestock are also employed: while Yellow Starthistle and perhaps other species are toxic to equines, some other livestock may eat the non-spiny knapweeds with relish. In Europe, Common Knapweed (C. nigra) and Globe Knapweed (C. macrocephala) are locally important pollen sources for honeybees in mid-late summer.

Arctiin from C. imperialis destroys cancer cells

8-Hydroxyquinoline has been identified as a main allelopathic compound produced by Diffuse Knapweed (C. diffusa); native North American plants are typically sensitive to it, while those of Eastern Europe and Asia Minor usually have coevolved with the knapweed and are little harmed if at all, aided by native microorganisms that break down or even feed on the abundantly secreted compound^[3]. Thus, 8-hydroxyquinoline is potentially useful to control American plants that have become invasive weeds in the Diffuse Knapweed's native range. Arctiin, found in C. imperialis, has shown anticancer activity in laboratory studies. And Spotted Knapweed as well as other species are rich in cnicin, a bitter compound found mainly in the leaves and often used to flavor the digestif amaro.

Some species are cultivated as ornamental plants in gardens. As regards other aspects of popular culture, Cornflower (C. cyanus) is the floral emblem of Östergötland province (Sweden) – where is it called blåklint, literally "blue mountain" – and of Päijänne Tavastia region in Finland, where it is known as ruiskaunokki ("rye-beaks") or ruiskukka ("rye-flower"). It is also the national flower of Estonia – its local name rukkilill meaning "rye-lily" –, Belarus where it is called vałoška (Belarusian: валошка), and one of those of Germany where it is called Kornblume ("cornflower"). The origin of the name "caltrop" for the ancient low-tech area denial weapon is probably in some way connected with the Purple Starthistle (C. calcitrapa) and its spiny seeds. This plant is attested to by the colloquial name "caltrop" at a time when the weapons were still called by their Roman name tribulus^[6]. Lastly, the color cornflower blue is named after C. cyanus.

Systematics and taxonomy

As namesake member of the subtribe Centaureinae of tribe Cynareae, the knapweeds are probably most closely related to genera such as Carthamus (distaff thistles), Cnicus (Blessed Thistle) or Crupina, and somewhat less closely to most other thistles. The monotypic Cnicus seems in fact to properly belong in Centaurea.^[7]

Research in the late 20th century shows that Centaurea as traditionally defined is polyphyletic. A number of 19th- and 20th-century efforts to reorganize the genus were not successful, and it is not yet clear what the consequences of the recent research will be for classification of this genus and other related genera. The type species C. centaurium stands somewhat aparrt from the main lineage of knapweeds and thus taxonomic the consequences of a rearrangement might be severe, with hundreds of species needing to be moved to new genera. It has thus been proposed to change the type species to one of the main lineage to avoid this problem. What seems certain however is that the basketflowers – presently treated as a section Plectocephalus – will be reinstated as a distinct genus in the near future.^[2]

Selected species

Centaurea horrida

Globe Knapweed (C. macrocephala)

Centaurea napifolia

Centaurea nervosa

Centaurea pseudophrygia

Centaurea pulcherrima

Centaurea triumfettii

Species presently placed in Centaurea include:

Formerly placed here

Plant species placed in Centaurea in former times include:

• Acroptilon repens – Russian Knapweed (as C. repens)
• Cheirolophus crassifolius – Maltese Rock-centaury (as C. crassifolia, C. spathulata)
• Femeniasia balearica (as C. balearica)

Footnotes

References

• Emery, S.M. & Gross, K.L. (2005): Effects of timing of prescribed fire on the demography of an invasive plant, spotted knapweed Centaurea maculosa. Journal of Applied Ecology 42(1): 60-69. doi:10.1111/j.1365-2664.2004.00990.x (HTML abstract)
• Hierro. J.L. & Callaway, R.M. (2003): Allelopathy and exotic plant invasion. Plant and Soil 256(1): 29–39. doi:10.1023/A:1026208327014 PDF fulltext
• Keil, David J. (2006): 21. Plectocephalus. In: Flora of North America North of Mexico Vol. 19 (Magnoliophyta: Asteridae, part 6: Asteraceae, part 1). Oxford University Press. ISBN 0195305639 HTML fulltext
• Keil, David J. & Ochsmann, J. (2006): 24. Centaurea. In: Flora of North America North of Mexico Vol. 19 (Magnoliophyta: Asteridae, part 6: Asteraceae, part 1). Oxford University Press. ISBN 0195305639 HTML fulltext
• Panero, J.L. & Funk, V.A. (2002): Toward a phylogenetic subfamilial classification for the Compositae (Asteraceae). Proceedings of the Biological Society of Washington 115(4): 909–922. HTML abstract
• Vivanco, J.M.; Bais, H.P.; Stermitz, F.R.; Thelen, G.C. & Callaway, R.M. (2004): Biogeographical variation in community response to root allelochemistry: Novel weapons and exotic invasion. Ecol. Lett. 7(4): 285–292. doi:10.1111/j.1461-0248.2004.00576.x PDF fulltext Supplementary material
• Wäckers, Felix; van Rijn, Paul & Bruin, Jan (2005): Plant-Provided Food for Carnivorous Insects - a protective mutualism and its applications. Cambridge University Press, UK. ISBN 139780521819411

Further reading

This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (January 2009)

Wikimedia Commons has media related to: Centaurea

• Flora Europaea: Centaurea
• USDA Plant Profile: Centaurea
• Flora of China: Centaurea species list
• Flora of Chile: Centaurea (pdf)
• Mabberley, D.J. 1987. The Plant Book. A portable dictionary of the higher plants. Cambridge University Press, Cambridge. 706 p. ISBN 0-521-34060-8.
• Robbins, W.W., M. K. Bellue, and W. S. Ball. 1970. Weeds of California. State of California, Dept. of Agriculture. 547 p.