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 Food grain, such as wheat, barley, rye, or oats), hence its name. It is now endangered in its native habitat by agriculture, particularly by over-use of . However, Centaurea cyanus is now also naturalised 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.
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
inflorescence (capitula) of 1.5–3 cm diameter, with a ring of a few large, spreading ray
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.
Genetics
Centaurea cyanus is a
Ploidy 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,
self-pollination still occurs occasionally, but results in inbreeding depression.
Distribution and habitat
Centaurea cyanus is native to
temperate Europe, but is widely naturalized outside its native range.
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 the County Clare (vice-county H9) in 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 m
2 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
dominated by
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
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
.
[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
to control
Mamestra brassicae. Field experiments showed that planting
Centaurea cyanus in cabbage fields at a density of 1 plant / m
2 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
of
Centaurea cyanus with varying
, 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 fall. 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.
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
. 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
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
,
and
.
[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.
In particular, extracts from the flower heads have anti-inflammatory properties used in the treatment of minor ocular .[Garbacki N., Gloaguen V., Damas J., Bodart P., Tits M., Angenot L. (1999). "Anti-inflammatory and immunological effects of Centaurea cyanus flower-heads". Journal of ethnopharmacology 68:235–241.] Antioxidant properties are high due to Vitamin C and Phenolic acid compounds.[Lockowandt L., Pinela J., Roriz C., Pereira C., Abreu R., Calhelha R., Alves M., Barros L., Bredol M., Ferreira I. (2019). "Chemical features and bioactivities of cornflower (Centaurea cyanus L.) capitula: The blue flowers and the unexplored non-edible part". Industrial Crops and Products 128:496–503.] Furthermore, extracts of the flower head and vegetative parts of the plant were shown to have Stomach effects due to their content of quercetin, apigenin and caffeic acid derivates.[Pirvu L., Dragomir C., Schiopu S., Mihul S. (2012). "Vegetal extracts with gastroprotective activity. Part. I. Extracts obtained from Centaurea cyanus L. raw material". Romanian Biotechnological Letters 17:7169-7176.]
Pigment
The blue color of
Centaurea cyanus is due to
protocyanin, an
anthocyanin pigment that is also found in roses.
[Shiono M., Matsugaki N., Takeda K. (2005). "Structure of the blue cornflower pigment". Nature 436:791–791.] Different anthocyanins derived from
Centaurea cyanus are used as natural additives in food products, such as yoghurts.
Phytoremediation
Centaurea cyanus has been evaluated for
phytoremediation of soils contaminated with
lead. Inoculation of the contaminated soil with
Glomus spp. (fungus) and
Pseudomonas spp. (bacterium) would significantly enhance the biomass production and lead uptake of
Centaurea cyanus.
In culture
In folklore, cornflowers were worn by young men in love; if the flower faded too quickly, it was taken as a sign that the man's love was not returned.
The blue cornflower was one of the national symbols of Germany. This is partly due to the story that when Queen Louise of Prussia was fleeing Berlin and pursued by Napoleon's forces, she hid her children in a field of cornflowers and kept them quiet by weaving wreaths for them from the flowers. The flower thus became identified with Prussia, not least because it was the same color as the Prussian military uniform. After the unification of Germany in 1871, it went on to become a symbol of the country as a whole. For this reason, in Austria the blue cornflower is a political symbol for pan-German and rightist ideas. It was worn as a secret symbol identifying members of the then-illegal Nazi Party in Austria in the 1930s. Members of the Freedom Party wore it at the openings of the Austrian parliament since 2006. After the last general election 2017 they replaced it with the edelweiss.
It was also the favourite flower of Louise's son Kaiser Wilhelm I. Because of its ties to royalty, authors such as Theodor Fontane have used it symbolically, often sarcastically, to comment on the social and political climate of the time.
The cornflower is also often seen as an inspiration for the German Romanticism symbol of the Blue Flower.
Due to its traditional association with Germany, the cornflower has been made the official symbol of the annual German-American Steuben Parade.
The blue cornflower has been the national flower of Estonia since 1969 and symbolizes daily bread to Estonians. It is also the symbol of the Estonian Conservative People's Party.
It is also the symbol of the Finland National Coalition Party, and the Liberal People's Party of Sweden, where it has since the dawn of the 20th century been a symbol for social liberalism.
It is the official flower of the Swedish province of Östergötland and the school flower of Winchester College and also of Dulwich College, where it is said to have been the favourite flower of the founder, Edward Alleyn.
In France the bleuet de France is the symbol of the 11 November 1918 armistice and, as such, a common symbol for veterans (especially the now defunct poilus of World War I), similar to the Remembrance poppies worn in the United Kingdom and in Canada.
The cornflower is also the symbol for motor neurone disease and amyotrophic lateral sclerosis.
Cornflowers are sometimes worn by Old Harrovians, former pupils of the British Harrow School.
A blue cornflower was used by Corning Glass Works for the initial release of Corning Ware Pyroceram cookware. Its popularity in the United States, Canada, United Kingdom and Australia was so high that it became the symbol of Corning Glass Works.
In paintings
File:Vincent van Gogh - Wheat Field with Cornflowers.jpg|Vincent van Gogh, Wheat Field with Cornflowers, 1890
File:Левитан Васильки.jpg|Isaac Levitan, Cornflowers, 1894
File:Group portrait with cornflowers by Igor Grabar, 1914.jpg|Igor Grabar, Group Portrait with Cornflowers, 1914
File:Osipov-Cornflowers-7oci10bw.jpg|Sergei Osipov, Cornflowers, 1976
File:Stamps_Ukrainian_flowers.jpg|Ukrainian stamp set: "Ukrainian Flowers" (2000), with cornflowers on the right. Painting by Kateryna Shtanko.
See also
Explanatory notes
External links