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.

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 fescue (Festuca rubra) in addition to cock's-foot and 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.

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.

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.

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.

The high nectar yield of the genus makes it very attractive to insects such as butterflies – including the endangered Karner blue (Plebejus melissa samuelis) which visits introduced spotted knapweed – and day-flying moths – typically Zygaenidae, such as Zygaena loti or the six-spot burnet (Z. filipendulae).

Broad-nosed seedhead weevil (Bangasternus fausti) larvae eat diffuse, spotted and squarrose knapweed (C. virgata ssp.

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 insects that may otherwise attack certain crops.

[citation needed] 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.

Moreover, being untreated with pesticides and providing more 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.

This renders the weeds more likely to be suppressed by native vegetation or crops in the following years, especially if properly timed controlled burning[5] and/or targeted grazing by suitable livestock are also employed.

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.

A South Italian variety[verification needed] 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 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".

[9] 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.

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").

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.