Common names for this genus arecentaury,centory,starthistles,knapweeds,centaureas and the more ambiguousbluets; a vernacular name used for these plants in parts of England isloggerheads (common knapweed). ThePlectocephalus group – possibly a distinct genus – is known asbasketflowers.Cornflower is used for a few species, but that term more often specifically means eitherC. cyanus (the annual cornflower) orCentaurea montana (the perennial cornflower). The common namecentaury is sometimes used, although this also refers to the unrelated plant genusCentaurium.[2]
The name is said to be in reference toChiron, the centaur ofGreek mythology who discovered medicinal uses of a plant eventually called "centaury".[3]
Knapweeds are robustweedy 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" (actuallypseudanthiuminflorescences) are diverse in colour, ranging from intense blues, reds and yellows to any mixture of these and lighter shades towards white. Often, thedisk flowers are much darker or lighter than theray flowers, which also differ inmorphology and aresterile. Each pseudanthium sits atop a cup- or basket-like cluster of scalybracts, hence the name "basketflowers". Many species, in particular those inhabiting morearid regions, have a long and strongtaproot.
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 herbaceous plants. Thecommon knapweed (C. nigra) for example is plentiful in themesotrophic grasslands of England and nearby regions. It is most prominently found in pastures or meadows dominated bycock's-foot (Dactylis glomerata) as well as either ofcrested dog's-tail (Cynosurus cristatus) andfalse oat-grass (Arrhenatherum elatius). It is also often found in mesotrophic grassland onrendzinas and similarcalcareous soils in association withglaucous sedge (Carex flacca),sheep's fescue (Festuca ovina), and eithertor-grass (Brachypodium pinnatum) andrough hawkbit (Leontodon hispidus), orupright brome (Bromus erectus). In thesegrasslands,greater knapweed (C. scabiosa) is found much more rarely by comparison, often in association withred fescue (Festuca rubra) in addition to cock's-foot and false oat-grass.
Due to their habit of dominating ecosystems under good conditions, manyCentaurea species can becomeinvasive weeds in regions where they are not native. In parts of North America,diffuse knapweed (C. diffusa),spotted knapweed (C. maculosa) andyellow starthistle (C. solstitialis) cause severe problems inagriculture due to their uncontrolled spread. Theseeds are typically transported by human traffic, in particular thetires ofall-terrain vehicles. The two knapweeds are harmful mainly because they are stronglyallelopathic, producing powerfultoxins in theirroots that stunt the growth of plants around them not adapted to this.[4] Yellow starthistle, meanwhile, is inedible to mostlivestock due to its spines and apparently outrightpoisonous tohorses and otherequines. However, efficient methods ofbiological control by insectpests of these weeds have been developed; the knapweeds can also exploited to their detriment bytargeted 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.[5]
Yet other species ofCentaurea – mostly ones that occur between Italy and theCaucasus – areendemics of a single island or valley, and some of these areendangered. TheAkamas Centaurea (Centaurea akamantis) ofCyprus is almostextinct, while the western Caucasus endemicsC. leptophylla andC. straminicephala are at least very rare andC. hedgei andC. pecho from the same region are certainly not abundant either. The last four species would be adversely affected by the proposedYusufeli Dam, which might actually destroy enoughhabitat to push the two rarer ones over the brink of extinction.
Centaurea are copiousnectar producers, especially on high-limesoils. The high nectar yield of the genus makes it very attractive toinsects such asbutterflies – including theendangeredKarner blue (Plebejus melissa samuelis) which visits introduced spotted knapweed – and day-flyingmoths – typicallyZygaenidae, such asZygaena loti or thesix-spot burnet (Z. filipendulae). Thelarvae of some otherLepidoptera species useCentaurea species as food plants; see List of Lepidoptera that feed onCentaurea. Several of these are used in biological control of invasive knapweeds and starthistles.
Also used in biological control areTephritidae (peacock flies) whose larvae feed onCentaurea.Knapweed peacock fly (Chaetorellia acrolophi) larvae eat spotted knapweed and some other species. Theyellow starthistle peacock fly (C. australis) has an initialgeneration each year which often usescornflower (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.
Despite the negative agricultural and environmental impacts of the more aggressiveCentaurea species, there are many ways in which they benefit humans as well. For instance, due to their moderate to high nectar production, which can occur over a comparatively long duration, many species ofCentaurea are popular food sources forinsects that may otherwise attack certaincrops.[citation needed] It may be advisable for some types of farms to allow certain Centaurea species, such ascornflower (C. cyanus) in a European setting, to grow adjacent to fields. These areas are known asbeetle banks, though they support and attract a diversity of beneficial life beyond beetles. When certain Centaurea species are present, some pests may be drawn away from crops, and predatory insects andarachnids that feed upon pest insects will be better-supported by these more naturalized areas. They additionally have the beneficial aspect of supportingpollinators, unlike many field crops such asmaize. Moreover, being untreated withpesticides and providing morediversity, plants growing in more wild areas adjacent to farms produce more insects that attract and support birds which can also feed on pests that would harm crops. Insect production is especially high for beetle banks that have enough plants that serve in the role of host plant for immature insects, rather than just in the roles of adult food and/or shelter provision.
Some plants which are considered invasive or problematic in certain areas can have beneficial qualities that outweigh their negative qualities from a human and/or human agricultural point of view, although this sometimes requires some human management – particularly if adequate biological control has not been established for the more aggressive species. An example is wild parsnip,Pastinaca sativa, which produces florets that feed predatory (and other beneficial) insects as well as large tubular stems that provide winter shelter for native bees, wasps, and other organisms that can be beneficial for agriculture. The plant is considered invasive in some areas of the United States and is also often considered undesirable due to its ability to cause contact skin irritation. However, it also serves as a host plant for the black swallowtail butterfly, helps to bring nutrients up from soils with its deep taproot, and possesses evergreen foliage even in climate zones such as US zone 6. This foliage increases soil warmth and moisture which can be beneficial for certain types of life. Perhaps the most dramatic example of a generally disliked plant's beneficial qualities being usually overlooked is the often-despised ragwort,Jacobaea vulgaris, which topped the list by a large amount for nectar production in a UK study, with a production per floral unit of (2921 ± 448μg).[6] This very high nectar production, coupled with its early blooming period, makes the plant helpful for the establishment of bee colonies in spring — a period that is often not well-served by commercial flower meadow seed mixes.[7] It also has the situationally-beneficial quality of being a springephemeral, as well as an annual that lacks difficult-to-combat roots. Plants that provide necessary structural supports forinvertebrate and smallvertebratepredators can help to keep overall pest populations low.[8]
The abundantnectar produced byC. solstitialis flowers attracts manypollinators. This is another reason for the success of the (situationally) highlyinvasive species. Due to genetic differences related to evolutionary adaption, not all members ofCentaurea produce the same amount of nectar. Growing conditions, such as climate and soil, can have a very strong impact, even if the plants grow and flower. For instance, cornflower plants,Centaurea cyanus, produced 33% less seasonal nectar thanCentaurea nigra in a UK study.[6]C. nigra also ranked higher than ragwort in another UK study, although ragwort was still in the top 10 for yearly nectar production.[7] The strong nectar production of certain members of the genus can be exploited to the farmer's advantage, possibly in combination withbiological control. In particular, theyellow starthistle (C. solstitialis) as well asspotted knapweed (C. maculosa) are majorhoney plants forbeekeepers.Monofloral honey from these plants is light and slightly tangy, and one of the finest honeys produced in the United States – due to its better availability, it is even fraudulently relabeled and sold as the scarce and expensivesourwood honey of theAppalachian Mountains. Placingbeehives near stands ofCentaurea 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 ofallelopathic 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 timedcontrolled burning[5] and/ortargeted grazing by suitablelivestock are also employed. While yellow starthistle and perhaps other species aretoxic toequines, 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.
8-Hydroxyquinoline has been identified as a main allelopathic compound produced bydiffuse knapweed (C. diffusa); native North American plants are typically sensitive to it, while those ofEastern Europe andAsia Minor usually havecoevolved with the knapweed and are little harmed if at all, aided by nativemicroorganisms that break down or even feed on the abundantly secreted compound.[4] Thus, 8-hydroxyquinoline is potentially useful to control American plants that have become invasive weeds in the diffuse knapweed's native range.
Arctiin, found inC. imperialis, has shownanticancer activity in laboratory studies. The roots of the long-lostC. foliosa, anendemic ofHatay Province (Turkey), are used in folk medicine, and other species are presumably too. A South Italian variety[verification needed] of thepurple starthistle (C. calcitrapa) is traditionally consumed by ethnic Albanians (Arbëreshë people) in theVulture area (southern Italy); e.g. in the Arbëreshë communities inLucania the young whorls ofC. calcitrapa are boiled and fried in mixtures with other weedy non-cultivated greens. According to research by the Michael Heinrich group at the Centre for Pharmacognosy and Phytotherapy (School of Pharmacy, University of London) "theantioxidant activity [...] of the youngwhorls ofCentaurea calcitrapa, both in theDPPH and in thelipid peroxidation inhibition assays, [is] very interesting and [the] species should be investigated phytochemically and biochemically focusing on these properties". Extracts fromC. calcitrapa were furthermore found to have significantxanthine oxidase (XO)-inhibiting activity.[9]
Spotted knapweed as well as other species are rich incnicin, abitter compound found mainly in the leaves and often used to flavor thedigestifamaro. In westernCrete,Greece a localvariety[verification needed] ofC. calcitrapa calledgourounaki (γουρουνάκι "little pig") also has its leaves eaten boiled by the locals. In the same island an endemic local species,C. idaea calledkatsoula (κατσούλα),tsita (τσίτα) oraspragatha (ασπραγκάθα), has its leaves eaten boiled by the locals too.[10]
Some species are cultivated asornamental plants ingardens. As regards other aspects ofpopular culture,cornflower (C. cyanus) is thefloral emblem ofÖstergötland province (Sweden) – where is it calledblåklint, literally "blue mountain" – and ofPäijät-Häme region inFinland, where it is known asruiskaunokki ("rye-beaks") orruiskukka ("rye-flower"). It is also the national flower ofEstonia where its local namerukkilill means "rye-lily",Belarus where it is calledvałoška (Belarusian:валошка), and one of those of Germany where it is calledKornblume ("cornflower"). The origin of the name "caltrop" for the ancient low-techarea denial weapon is probably in some way connected withC. 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 theirRoman nametribulus.[11] Lastly, thecolorcornflower blue is named afterC. cyanus. Cornflower is also used as a cut flower.
As namesake member of thesubtribeCentaureinae oftribeCardueae, the knapweeds are probably most closely related to genera such asCarthamus (distaff thistles),Cnicus (blessed thistle),Crupina (crupinas) orNotobasis (Syrian thistle), and somewhat less closely to most otherthistles. ThemonotypicCnicus seems in fact to properly belong inCentaurea.[12]
Research in the late 20th century shows thatCentaurea as traditionally defined ispolyphyletic. 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. Thetype speciesC. centaurium stands somewhat apart from the main lineage of knapweeds and thus thetaxonomic 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 lineages to avoid this problem. What seems certain however is that the basketflowers – presently treated as asectionPlectocephalus – will be reinstated as a distinct genus in the near future. Therock-centauries (Cheirolophus), formerly usually included inCentaurea, are now already treated as separate genus.[2]
^Blackwell, Laird R. (2006).Great Basin Wildflowers: A Guide to Common Wildflowers of the High Deserts of Nevada, Utah, and Oregon (A Falcon Guide) (1st ed.). Guilford, Conn.:Morris Book Publishing, LLC. p. 57.ISBN0-7627-3805-7.OCLC61461560.
Emery, S.M. & Gross, K.L. (2005): Effects of timing of prescribed fire on the demography of an invasive plant, spotted knapweedCentaurea maculosa.J. Appl. Ecol.42(1): 60-69.doi:10.1111/j.1365-2664.2004.00990.x (HTML abstract)
Pieroni, A.; Janiak, V.; Dürr, C.M.; Lüdeke, S.; Trachsel E. & Heinrich, M. (2002):In vitro Antioxidant Activity of Non-cultivated Vegetables of Ethnic Albanians in Southern Italy.Phytother. Res.16(5): 467–473.doi:10.1002/ptr.1243PDF fulltext
Stavridakis, Kleonikos G. (Κ. Γ. Σταυριδάκης) (2006):Wild edible plants of Crete - Η Άγρια βρώσιμη χλωρίδα της Κρήτης [English and Greek]. Rethymnon Crete.ISBN960-631-179-1
Wäckers, Felix; van Rijn, Paul & Bruin, Jan (2005):Plant-Provided Food for Carnivorous Insects - a protective mutualism and its applications. Cambridge University Press, UK.ISBN978-0-521-81941-1Preview atGoogle Books
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