Centaurea cyanus, commonly known as cornflower or bachelor's button (among other names), 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.
C. cyanus is now endangered in its native habitat by agricultural intensification, particularly by over-use of herbicides. However, it is now also naturalized in many other parts of the world, including North America and parts of Australia.
Description
Centaurea cyanus is an annual plant growing to tall, with grey-green branched stems. The leaves are lanceolate and long. The flowers are most commonly an intense blue colour and arranged in flowerheads (capitula) of 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. Fruits are approx. 3.5 mm long with 2–3 mm-long pappus bristles. It flowers all summer.
Taxonomy Centaurea cyanus was given its scientific name in 1753 by Carl Linnaeus. It is classified in the genus Centaurea as part of the Asteraceae family. The species has no accepted subspecies, but has one among its 21 synonyms.
Table of Synonyms Name Year Rank Notes Centaurea cyaneum 1880 species = het., not validly publ. Centaurea cyanocephala 1886 species = het. Centaurea cyanus subsp. coa 1949 subspecies = het. Centaurea cyanus var. denudata 1927 variety = het. Centaurea hoffmanniana 1899 species = het. Centaurea hortorum 1887 species = het. Centaurea lanata 1832 species = het. Centaurea pulcherrima 1838 species = het., not validly publ. Centaurea pulchra 1834 species = het. Centaurea rhizocephala 1873 species = het. Centaurea segetalis 1796 species ≡ hom., nom. illeg., superfl. Centaurea umbrosa 1856 species = het. Cyanus arvensis 1794 species = het. Cyanus cyanus 1768 species = het. Cyanus dentato-folius 1782 species = het., opus utique oppr. Cyanus segetum 1769 species ≡ hom. Cyanus vulgaris 1800 species = het. Jacea segetalis 1840 species = het. Jacea segetum 1779 species = het. Leucacantha cyanus 1917 species = het. Setachna cyanus 1867 species = het. Notes: ≡ homotypic synonym ; = heterotypic synonym
Genetics Centaurea cyanus is a diploid flower (2n = 24).Martin E., Dinç M., Duran A. (2009). "Karyomorphological Study of Eight Centaurea L. Taxa (Asteraceae) from Turkey". Turkish Journal of Botany 33:97–104. The genetic diversity within populations is high, although there could be a future decline in diversity due to population fragmentation and intensive agriculture.Le Corre V., Bellanger S., Guillemin J.-P., Darmency H. (2014). "Genetic diversity of the declining arable plant Centaurea cyanus: Population fragmentation within an agricultural landscape is not associated with enhanced spatial genetic structure". Weed Research 54:436–444. In general, Centaurea cyanus is a self-incompatible species. However, selfing still occurs occasionally, but results in inbreeding depression.
Names The genus name, Centaurea, is derived from the Greek (kéntaura) meaning "centaur". The species name, cyanus, is Botanical Latin also taken from the Ancient Greek κύανος (kúanos) meaning "dark blue" as a reference to the color of the flowers.
The common name bachelor's button is an allusion to the shape of the flowers. Originally the bachelor's buttons was applied to the double form of the common buttercup (Ranunculus acris), but it has been applied to various round flowers. The name cornflower is often used for this species in particular, but is also used for other weedy species that grow among corn in the sense of any grain crop such as the common corncockle. The name bluebottle continues in common use, having been coined in the form blewbothel around 1450.
Other names include blue cap, a rare English regional name, blue blobs, blue bonnet, bluet, cornbottle, boutonierre flower, ragged robin, and gogglebuster. Though the name ragged sailor is applied to the species, it is also sometimes used for chicory (Cichorium intybus). The name hurtsickle or hurt-sickle, dates to the late 1500s. This name came from farmers believing that it would blunt their sickles when cutting fields.
Distribution and habitat Centaurea cyanus is native to temperate Europe, but is widely naturalized in many other parts of the world, including North America and parts of Australia through introduction as an ornamental plant in gardens and as a seed contaminant in crop seeds.
It has been present in Britain and Ireland 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 County Clare (VC H9), Ireland, C. cyanus is recorded in arable fields as very rare and almost extinct, while in northeast Ireland, it was abundant before the 1930s.
Ecology Weed in arable crops Centaurea cyanus is considered a noxious weed in arable crops, especially cereals and rapeseed. In winter wheat, one plant per m2 can cause a yield loss of up to 30 kg / ha. Centaurea cyanus produces around 800 seed per plant, which are either shed shortly before the harvest of cereals, or they are threshed together with the cereal grains, contributing to the further spread of the species by the harvesting machinery and contaminated seed. The occurrence of Centaurea cyanus strongly decreased during the last decades due to improved seed cleaning, more intensive nitrogen fertilization and herbicide use. However, Centaurea cyanus has become more common in cropland due to an increase in crop rotations dominated by winter cereals and rapeseed and the use of more selective herbicides with a low effectiveness against Centaurea cyanus. In addition, the emergence of resistance against the herbicide class of sulfonylureas has been reported recently. Due to its strong roots, Centaurea cyanus is difficult to control mechanically in spring.
Fodder for insects and birds
The pollen of Centaurea cyanus is used by several different insect species. Insects of the orders Hymenoptera and Diptera are particularly attracted by the flower.Carreck N., Williams I. (2002). "Food for insect pollinators on farmland: insect visits to flowers of annual seed mixtures". Journal of Insect Conservation 6:13–23. As Centaurea cyanus is a self-incompatible species, it needs external pollination. The nectar of Centaurea cyanus is very sweet with a sugar content of 34%. Due to its high sugar production of up to 0.2 mg sugar per day and flower, the species is highly appreciated by beekeepers.Helmut Horn, Cord Lüllmann: Das große Honigbuch, Kosmos, Stuttgart, 3. Auflage, 2006, , S. 31
The seeds of Centaurea cyanus are one of the favourite foods of the European goldfinch.
Control of insect pests Centaurea cyanus was found to produce volatiles attracting Microplitis mediator, which is a major parasitoid of the cabbage moth (Mamestra brassicae), which is the most important pest of cabbage (Brassica oleracea) in central Europe. Planting Centaurea cyanus in cabbage fields as a companion plant was thus suggested as an alternative to the widespread use of insecticides to control Mamestra brassicae. Field experiments showed that planting Centaurea cyanus in cabbage fields at a density of 1 plant / m2 can result in a significant increase in parasitation of Mamestra brassicae larvae, predation of Mamestra brassicae eggs (e.g. by carabid beetles or spiders) and ultimately cabbage yield.
Cultivation
Several cultivars of Centaurea cyanus with varying pastel colours, including pink and purple, have been selected for ornamental purposes. The species is also grown for the cut flower industry in Canada for use by florists. Doubled blue cultivars (such as 'Blue Boy' or 'Blue Diadem') are most commonly used for this purpose, but white, pink, lavender and black (actually a very dark maroon) cultivars are also used, albeit to a lesser extent.
Breeding goals As for all ornamental plants, important goals of Centaurea cyanus breeding include the induction of phenotypic variation (e.g. in flower coloration, size and shape, foliage characteristics or plant height), higher flower yield, resistance to pests and diseases as well as tolerance to abiotic stress (e.g., extreme temperatures, drought or salinity).
Soil and climate requirements Centaurea cyanus requires full sun and neutral (pH 6.6–7.5) to mildly alkaline (pH 7.6–7.8), moist and well-drained soil. However, Centaurea cyanus is quite tolerant to drought once established.
Sowing For summer-blooming plants, sowing should be executed in late spring. In moderate climates, however, it is also possible to sow Centaurea cyanus in early autumn. In this case, plants will already start to flower in the following spring. Recommended spacing between plants is approx. 20 to 30 cm. Centaurea cyanus can germinate from up to 10 cm depth, but the best result is obtained at 1 cm sowing depth. Germination occurs quickly after sowing.
Fertilization and cultural practices High phosphorus fertilization in mid-summer will increase flower production. Mulching is recommended to prevent drying out of the soil and exposure of the root system to the sun.
Pests and diseases In general, Centaurea cyanus is not very susceptible to pests and plant diseases. However, it may be affected by stem rot and stem rust if grown too tightly or by powdery mildew. Furthermore, aphids and leafhoppers can cause relevant damage to Centaurea cyanus.
Seed harvesting Seeds are harvested either by hand or, in an agricultural setting, with a seed harvesting machine. On average there are 97,000 seeds in a pound of cornflower seeds.
Hand collecting can be time-consuming and yields are rather low.
A seed harvesting machine is more efficient than collecting the seeds by hand, but it is costly. The main principle of such a machine is that it brushes the ripe seeds off the plant and creates a cross flow fan action that generates sufficient air velocity to hold and gather the seeds into the seed bunker.
Pruning Deadheading will encourage the plant to produce more blooms. Cornflowers are often used for ornamental purposes and by cutting them, up to their third leaves, they will produce more blooms and grow a bigger stem.
Uses Culinary
The flowers of Centaurea cyanus can be eaten raw, dried or cooked.Rop O., Mlcek J., Jurikova T., Neugebauerová J., Vabkova J. (2012). "Edible Flowers-A New Promising Source of Mineral Elements in Human Nutrition". Molecules 17:6672–83. Dried petals are used in foods, including in spices. Their main purpose is to add colour to food. There are cheeses or oils that contain raw petals. Petals can also be added to salads, drinks, and desserts for garnishing purposes in raw or dried form.
Dried petals are also used in teas and other beverages. Blue cornflower petals are sometimes one of the ingredients in Lady Grey tea.
Medicine Centaurea cyanus contains a wide range of pharmacologically active compounds, such as flavonoids, anthocyanins and aromatic acids.Al-Snafi A. (2015). "The pharmacological importance of Centaurea cynus - A review". International Journal of Pharmacy Review and Research 5:379–384. Especially the flower head finds application in herbal medicine, but leaves and seeds are also used for pharmacological purposes, albeit to a lesser extent.