Editing Centaurea (section)

==Use by humans==
Despite the negative agricultural and environmental impacts of the more aggressive Centaurea 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 of ''Centaurea'' are popular food sources for [[insect]]s that may otherwise attack certain [[crop]]s.{{Citation needed|date=December 2017}} It may be advisable for some types of farms to allow certain Centaurea species, such as [[cornflower]] (''C. cyanus'') in a European setting, to grow adjacent to fields. These areas are known as [[beetle bank]]s, 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 and [[arachnid]]s that feed upon pest insects will be better-supported by these more naturalized areas. They additionally have the beneficial aspect of supporting [[pollinator]]s, unlike many field crops such as [[maize]]. Moreover, being untreated with [[pesticide]]s and providing more [[Biodiversity|diversity]], 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).<ref name=Nectar>{{cite journal |last1=Hicks |first1=DM |last2=Ouvrard |first2=P |last3=Baldock |first3=KCR |title=Food for Pollinators: Quantifying the Nectar and Pollen Resources of Urban Flower Meadows |journal=PLOS ONE |volume=11 |issue=6 |pages=e0158117 |date=2016 |doi=10.1371/journal.pone.0158117 |bibcode=2016PLoSO..1158117H |pmc=4920406 |pmid=27341588 |doi-access=free}}</ref> 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.<ref name=Pollinators>{{cite web |title=Which flowers are the best source of nectar? |publisher=Conservation Grade |url=http://www.conservationgrade.org/2014/10/which_flowers_best_source_nectar/ |access-date=2017-10-18 |date=2014-10-15 |archive-date=2019-12-14 |archive-url=https://web.archive.org/web/20191214024659/http://www.conservationgrade.org/2014/10/which_flowers_best_source_nectar/ |url-status=dead}}</ref> It also has the situationally-beneficial quality of being a spring [[ephemeral]], as well as an annual that lacks difficult-to-combat roots. Plants that provide necessary structural supports for [[invertebrate]] and small [[vertebrate]] [[predator]]s can help to keep overall pest populations low.<ref>Wäckers ''et al.'' (2005)</ref>

[[File:Yellow Star Thistle.jpg|thumb|left|[[Yellow starthistle]] (''C. solstitialis''), an [[invasive weed]] that yields a fine [[honey]]]]
The abundant [[nectar]] produced by ''C. solstitialis'' flowers attracts many [[pollinator]]s. This is another reason for the success of the (situationally) highly [[invasive species]]. Due to genetic differences related to evolutionary adaption, not all members of ''Centaurea'' 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 than ''Centaurea nigra'' in a UK study.<ref name=Nectar /> ''C. nigra'' also ranked higher than ragwort in another UK study, although ragwort was still in the top 10 for yearly nectar production.<ref name=Pollinators /> The strong nectar production of certain members of the genus can be exploited to the farmer's advantage, possibly in combination with [[biological control]]. In particular, the [[yellow starthistle]] (''C. solstitialis'') as well as [[spotted knapweed]] (''C. maculosa'') are major [[honey]] plants for [[beekeeper]]s. [[Monofloral honey]] from these plants is light and slightly tangy, and one of the finest honeys produced in the United States &ndash; due to its better availability, it is even fraudulently relabeled and sold as the scarce and expensive [[sourwood]] honey of the [[Appalachian Mountains]]. Placing [[beehive]]s 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 &ndash; 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]]<ref name=emerygross2005/> and/or [[targeted grazing]] by suitable [[livestock]] are also employed. While yellow starthistle and perhaps other species are [[toxic]] to [[equine]]s, some other livestock may eat the non-spiny knapweeds with relish. In Europe, [[common knapweed]] (''C. nigra'') and globe knapweed (''[[Centaurea macrocephala|C. macrocephala]]'') are locally important pollen sources for honeybees in mid-late summer.

[[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 [[microorganism]]s that break down or even feed on the abundantly secreted compound.<ref name=hcv0304/> Thus, 8-hydroxyquinoline is potentially useful to control American plants that have become invasive weeds in the diffuse knapweed's native range.

[[File:Arctiin.svg|thumb|right|[[Arctiin]] from ''[[Centaurea imperialis|C. imperialis]]'' kills [[cancer]] cells in culture]]
Arctiin, found in ''C. imperialis'', has shown [[anticancer]] activity in laboratory studies. The roots of the long-lost ''[[Centaurea foliosa|C. foliosa]]'', an [[endemism|endemic]] of [[Hatay Province]] (Turkey), are used in folk medicine, and other species are presumably too. A South Italian variety{{Verify source|date=April 2011}}<!-- might be ssp, form or even cultivar --> of the [[purple starthistle]] (''C. calcitrapa'') is traditionally consumed by ethnic Albanians ([[Arbëreshë people]]) in the [[Vulture area]] (southern Italy); e.g. in the Arbëreshë communities in [[Lucania]] the young whorls of ''C. 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]]) "the [[antioxidant]] activity [...] of the young [[Whorl (botany)|whorls]] of ''Centaurea calcitrapa'', both in the [[DPPH]] and in the [[lipid peroxidation]] inhibition assays, [is] very interesting and [the] species should be investigated phytochemically and biochemically focusing on these properties". Extracts from ''C. calcitrapa'' were furthermore found to have significant [[xanthine oxidase]] (XO)-inhibiting activity.<ref>Pieroni ''et al.'' (2002).</ref>

Spotted knapweed as well as other species are rich in [[cnicin]], a [[bitter (taste)|bitter]] compound found mainly in the leaves and often used to flavor the [[digestif]] [[amaro (drink)|amaro]]. In western [[Crete]], [[Greece]] a local [[variety (botany)|variety]]{{Verify source|date=April 2011}}<!-- might be ssp, form or even cultivar --> of ''C. calcitrapa'' called ''{{lang|grc-Latn|gourounaki}}'' ({{lang|grc|γουρουνάκι}} "little pig") also has its leaves eaten boiled by the locals. In the same island an endemic local species, ''[[Centaurea idaea|C. idaea]]'' called ''{{lang|grc-Latn|katsoula}}'' ({{lang|grc|κατσούλα}}), ''{{lang|grc-Latn|tsita}}'' ({{lang|grc|τσίτα}}) or ''{{lang|grc-Latn|aspragatha}}'' ({{lang|grc|ασπραγκάθα}}), has its leaves eaten boiled by the locals too.<ref>Stavridakis (2006)</ref>

{{color swatch|#6495ED|[[Cornflower]] blue|left}}
Some species are cultivated as [[ornamental plant]]s in [[garden]]s. As regards other aspects of [[popular culture]], [[cornflower]] (''C. cyanus'') is the [[floral emblem]] of [[Östergötland]] province (Sweden) &ndash; where is it called ''blåklint'', literally "blue mountain" &ndash; 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]] where its local name ''rukkilill'' means "rye-[[lily]]", [[Belarus]] where it is called ''vałoška'' ({{langx|be|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 ''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 [[Ancient Rome|Roman]] name ''tribulus''.<ref>In reference to their resemblance to the spiny seeds of the [[puncture vine]], later named ''Tribulus terrestris''.</ref> Lastly, the [[color]] [[cornflower blue]] is named after ''C. cyanus''. Cornflower is also used as a cut flower.