Plectocephalus D.Don in R.Sweet (but see text)
Centaurea (pronounced /ˌsɛntɔːˈriːə/ "Cen-tau-ré-a") is a genus of between 350 and 600 species of herbaceous thistle-like flowering plants in the family Asteraceae. Members of the genus are found only north of the equator, mostly in the Eastern Hemisphere; the Middle East and surrounding regions are particularly 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.
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.
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.
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