Centaurea cyanus (Cornflower)

Centaurea cyanus, commonly known as cornflower or bachelor's button, is an annual flowering plant in the family Asteraceae, native to Europe. In the past it often grew as a weed in cornfields (in the broad sense of "corn", referring to grains, such as wheat, barley, rye, or oats), hence its name. It is now endangered in its native habitat by agricultural intensification, particularly over-use of herbicides, destroying its habitat. It is also, however, through introduction as an ornamental plant in gardens and a seed contaminant in crop seeds, now naturalised in many other parts of the world, including North America and parts of Australia.

Centaurea cyanus
 Kingdom:     Plantae
 Clade:  Angiosperms
 Clade:  Eudicots
 Clade:  Asterids
 Order:  Asterales
 Family:  Asteraceae
 Genus:  Centaurea
 Species:  C. cyanus

Cornflower is an annual plant growing to 40–90 cm tall, with grey-green branched stems. The leaves are lanceolate, 1–4 cm long. The flowers are most commonly an intense blue colour, produced in flowerheads (capitula) 1.5–3 cm diameter, with a ring of a few large, spreading ray florets surrounding a central cluster of disc florets. The blue pigment is protocyanin, which in roses is red. It flowers all summer.

Centaurea cyanus is native to temperate Europe, but is widely naturalized outside its native range. It has been present in the British Isles as an archaeophyte (ancient introduction) since the Iron Age. In the United Kingdom it has declined from 264 sites to just 3 sites in the last 50 years. In reaction to this, the conservation charity Plantlife named it as one of 101 species it would actively work to bring 'back from the brink'. In Co. Clare (VC H9), Ireland, Centaurea cyanus is recorded in arable fields as very rare and almost extinct, while in the north-east of Ireland it was abundant before the 1930s.

It is grown as an ornamental plant in gardens, where several cultivars have been selected with varying pastel colours, including pink and purple. Cornflower is also grown for the cutflower industry in Canada for use by florists. The most common colour variety for this use is a doubled blue variety such as 'Blue Boy' or 'Blue Diadem'. White, pink, lavender and black (actually a very dark maroon) are also used but less commonly. Cornflowers germinate quickly after planting.

Light requirements: full sun. Water requirements: high-average water daily. Soil pH requirements: neutral (6.6-7.5) to mildly alkaline (7.6-7.8).

The edible flower of the cornflower can be used for culinary decoration, for example to add colour to salads. Cornflowers have been used historically for their blue pigment. Cornflowers are often used as an ingredient in some tea blends and herbal teas.

Cornflower seed is one of the favourite foods of the European goldfinch.

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