Centaurea cyanus L., Cornflower

Account Summary

Growth form and preferred habitats

This beautiful, blue-flowered annual knapweed, easily recognised in flower from June to August, was previously and widely regarded as a serious weed of cereal 'cornfields' (ie wheat, barley, oats and rye). It was particularly prevalent and locally abundant on light-textured, sandy, moderately acidic soils under arable cultivation, although it can grow in almost any moist, well-drained, near-neutral soil in lowland sites. Formerly, it was particularly associated, in England at least, with fields of Rye (Secale cereale) grown on poorer soils, although it also occurred in other spring and winter planted crops, including turnips and Oilseed Rape cultivated in rotation with cereals as a means of weed control (Stace & Crawley 2015). It can also behave as a casual ruderal in other forms of open, often roadside or waste-ground grassland, provided competition from associated species is sufficiently slight (Garrard & Streeter 1983; A. Smith, in: Stewart et al. 1994). Nowadays, rather than in agricultural fields, C. cyanus is more often found as more or less isolated, short-lived plants on roadsides, waste ground, pavements, marginal areas of gardens, parks, allotments and on refuse tips and pits (P.J. Wilson, in: Wigginton 1999; Stroh et al. 2023)

C. cyanus can germinate in the autumn (September and October) and overwinter as a small plantlet, although it does not recommence growth and produce its tough, wiry, grooved, slender, cottony flowering stems until the return of longer days in the spring. A second peak of germination takes place in May and June of the first spring after seed production (Melderis & Bangerter 1955; Salisbury 1964). Thus Cornflower can be either a winter- or a summer-annual.

A proportion of the seed that does not germinate in the first year after production can survive for several years in the soil seed-bank; the duration of such longevity is determined by the depth of burial, as witnessed by the reappearance of the species after roadworks or other forms of deep digging bring seeds back into the light environment.

Flowering reproduction

C. cyanus grows up to 20-90 cm tall, often developing numerous, slender lower branches. In common with other knapweeds, it produces showy, colourful, solitary flower heads or capitula, 15-30 mm in diameter at the ends of main stems and some branches. Each flower-head consists of tubular florets of two kinds: a central disc of numerous small, deep violet or reddish-purple, bisexual ones, and a small number of much larger, bright-blue, deeply divided, zygomorphic, spreading, sterile ray-florets on the margin of the central disc (Salisbury 1964; Proctor & Yeo 1973). Nowadays, there are also numerous garden cultivars of differing flower colour available, from white to pink or mauve, plus double-flowered varieties (Sell & Murrell 2006).

The fertile central flowers, 12-15 mm long, are self-incompatible, and as is the norm with bisexual flowers in the family, they are protandrous. The corolla tubes of the fertile flowers are deeply cut into five erect lobes and they contain conspicuous anther tubes that are sensitive (sometimes described as 'irritable') and which react when they are touched, their filaments retracting several millimetres. When this happens, the pollen within the anther cylinder gets swept up and out by a ring of hairs just below the short branches of the style, dusting the visiting insect with a certain amount (a puff!) of pollen. This pollen pump mechanism resets after an interval, the anther filaments regaining their former length, so that later insect visitors are also squirted with pollen in this manner.

Subsequently, the flower goes into its female phase, the style elongating and pushing up through the now pollen depleted anther cylinder to present the bi-lobed stigma at the entrance of the flower. There is no perfume, but the plentiful nectar and pollen foods attract many kinds of insects, including bees (especially bumble bees) and flies. (Melderis & Bangerter 1955; Hutchinson 1972; Proctor & Yeo 1973).

Seed and its dispersal

The achenes (single-seeded, dry fruits) are 6 × 3 mm, ovoid or narrowly ellipsoid, polished or shortly hairy, silvery-grey or greyish-yellow in colour. Autumn germinating seed can produce fruits by the following July (P.J. Wilson, in: Wigginton 1999). Each fruit is surmounted with a pappus composed of many rows of straight, bristly hairs, 2-3 mm long, variously described as reddish-brown, greyish, dirty white or tawny in colour that acts as a parachute to assist wind dispersal of the seed (Melderis & Bangerter 1955; Butcher 1961; Salisbury 1964; Sell & Murrell 2006).

The current author (RSF) has not been able to locate any of the basic quantitative statistics regarding the mean number of flowers/capitula, capitula/plant, or the average number of achenes per plant, although an unreferenced source seen has suggested a figure of 800 achenes per plant. Salisbury (1964), who regularly provides the statistics of seed and fruit production, does mention an average achene weight of 0.0045 gm, and he also notes, "It can withstand appreciable frost.", although it is not clear from the context of the statement if he is writing of the growing plant or of its fruits (but see below in 'Fossil record' section).

As is usually the case with members of the Asteraceae, the achenes are quite heavy and the distance travelled by achenes/seed is probably seldom more than a few metres from the parent plant. However, since they can persist in soil, secondary transport in mud or soil may well take place quite frequently on passing animals, vehicles and on other machinery. However, the main method of seed spread in past centuries has undoubtedly been in contaminated crop seed which has been very widely transported around the globe. Salisbury (1964, p. 40) remarked that, "The fruits were probably repeatedly reintroduced [to B & I] with grain and leguminous seeds from abroad, which together with the low standard of weeding before the common fields were enclosed and drilling became prevalent, enabled the high population [of the species] to be built up.".

In addition, there is some evidence that Goldfinches, and perhaps other seed eating birds, may help disperse Centaurea seed, including C. cyanus. Ridley (1930, p. 461) reports work by Thompson in New Zealand (although he provides no reference details), where some seed survives passage through the gizzard and alimentary canal and might possibly be transported internally and voided in viable condition along with faeces. Ridley (1930, p. 521) also asserts that Centaurea, and he includes C. cyanus in a list of species he is referring to, "have a much reduced pappus and basal oil-bodies". These oil-bodies or elaiosomes represent food packets that are attached to the achene and attract ants that may transport seeds to some probably minor distance from the parent plant. The current author has not found any other mention of elaiosomes on C. cyanus achenes, although Meikle (1985, p. 975), who provides the most detailed species' descriptions known to the current author, writing of the related species C. cyanoides, describes its achene as, "….apex yellowish with a marginal excavation surrounding a narrow crest (or ? elaiosome), base dome-shaped; hilum orbicular; pappus absent." It is also, incidentally, rather interesting that The flora of Cyprus does not include C. cyanus (Meikle 1985).

Of course, both birds and ants may also act as seed predators to some extent, but the degree to which this happens is also unknown to the current author.

Irish occurrence

Cornflower is regarded as an introduced species in Ireland. C. cyanus was reported to have occurred in 28 of the 40 Irish VCs, including Co Leitrim (H29) and all eleven VCs lying to the N and E of it. The Cen Cat Fl Ir 2, which provides this information, indicated that from 1950 onwards C. cyanus had only been seen in eight Irish VCs. This was later shown to be incorrect, six of these records being pre-1950 and the remaining two, from Cos Armagh (H37) and Carlow (H13), were never published (Irish Red Data Book).

The Irish Red Data Book declared five once widespread arable weed species, including Cornflower, extinct throughout Ireland as they were long known to be in decline and had not been reported since at least 1970. The authors were obliged to amend this view with a note added to their book in press since, in July 1987, they found it surviving along with one other extinct and two declining weeds on the Aran Islands, Co Galway (H16) (Curtis et al. 1988).

Curtis et al. (1988) reviewed the ecology, past behaviour and occurrence of Cornflower in Ireland and drew out the fact that the species was more prevalent and established in the north of Ireland, being associated there with the widespread cultivation of flax for linen manufacture. With the arrival of cheaper flax imports, labour intensive flax cultivation began to decline in Ireland during the second decade of the 20th century and, with it, the populations of Cornflower also melted away.

The decline of C. cyanus across B & I and elsewhere was accelerated by the advent of improved seed cleaning from the 1920s onwards, plus the use of herbicides and the gradual reduction of tillage in favour of grazing animals – the latter trend in agriculture continuing to near extinction in Fermanagh and in much of Ireland.

Fermanagh occurrence

C. cyanus had not been recorded anywhere in Fermanagh since 1902 until RHN noticed it in a newly laid lawn in Enniskillen in August 1996. Buried seed is long-persistent and it might possibly have survived dormant for decades in the topsoil until disturbed. However, much more likely the seed arrived as a contaminant of the just sown lawn mixture.

In recent years, Cornflower has been very commonly included as a colourful component of seed mixtures for the creation of 'wildflower meadows'. The flower again appeared for a second time in the VC when disturbed ground in Enniskillen was resown in this manner. C. cyanus was found at the junction of the Sligo and Ballinaleck roads in the town on 14 September 2003 by RHN & HJN.

Another possible source of C. cyanus is among wild bird seed mixtures very commonly and widely sold to the public for feeding garden birds, mainly in winter (Sell & Murrell 2006; Stace 2010). However, this is probably a very minor source of C. cyanus occurrence in B & I, since a comprehensive and dedicated study of bird seed aliens in Britain concluded that Cornflower was only sporadically present in commercial bird seed, ie single plants appearing only irregularly (Hanson & Mason 1985).

Fossil record

Godwin (1975, pp. 349-50) reported that post-glacial fossil pollen of C. cyanus in Britain only appeared rather late on in the current warm, Flandrian interglacial period, and that the species was essentially a weed associated with agriculture, in particular with cereal cultivation especially production of rye. Godwin went on to comment that, "It remains uncertain how far the cornflower persisted in this country [ie Britain] from the late Weichselian [glacial phase, the last Ice Age] or how far it was reintroduced with the crops [ie cereal including rye], although the zone VIIa record from Moss Lake precedes Neolithic clearances and indicates persistence in the middle Flandrian.".

Godwin (1975, pp. 442-3) also mentions and discusses ruderals and weed species, including C. cyanus, occurring, "at several sites in Late Weichselian [glacial] deposits in western Europe.". He then goes on to quote Salisbury (1932) in saying, in view of, "the considerable area of [glacial] morainic deposits that must have fringed the European ice front throughout the Pleistocene glaciations", that, "it is not improbable that this was the primary home of species [ie weeds including Centaurea cyanus], which today are mainly, if not exclusively, associated with the artificial conditions of cultivated and disturbed soil.".

Thus in Late Weichselian times, the lowlands of B & I must have supported open vegetation, resembling to some extent today's communities of sub-alpine meadows, scree slopes and mountain ledges, where competition is slight or nearly absent (Godwin 1975, p. 443). This helps to resolve the current author's uncertainty regarding the frost-hardiness mentioned above in connection to the seed and the plant (see quote from Salisbury 1964), in favour of the plant hardiness!

Species British status

C. cyanus had always been regarded as, or was simply assumed to be, indigenous in Britain until Webb (1985) suggested a list of 41 species which he believed were probably introduced by man. Webb's list included Centaurea cyanus, a species which is common throughout the European mainland, but probably is native only in the SE of Europe and the Near East. Despite Webb's scepticism regarding his list of assumed native species, recent Floras of Britain continued to refer to C. cyanus as either a native species (eg Stace 1997), or as a qualified native (eg Clapham et al. 1987), based on the fossil evidence published by Godwin (1975). Cornflower also featured as a scarce or rare native plant in need of conservation protection efforts in both the 1994 Scarce plants in Britain survey (A. Smith, in: Stewart et al. 1994), and in the 3rd edition of the British red data book on vascular plants (P.J. Wilson, in: Wigginton 1999).

In previous centuries, indeed possibly from the Iron Age onwards (Preston et al. 2004), and certainly from Anglo-Saxon times, C. cyanus had been a very common and widespread noxious weed in arable agriculture. However, from the 1920s onwards its presence in Britain had rapidly and greatly diminished due to the introduction of much more efficient crop seed cleaning, the decline of rye as a crop and, later on, in the 1940s and 50s, the widespread use of inorganic fertilisers and herbicides in arable agriculture.

As a consequence of this intensification of agriculture, by about the 1960s, C. cyanus had become reduced to the status of a rare casual, or near extinct, species across B & I. Although isolated Cornflower plants still occur over a wide area of Britain, when in 1994 the BSBI helped publish the book, Scarce Plants in Britain, apart from obvious garden cultivars and those populations derived from 'wild flower seed' sowings, it was known from only one persistent, self-sustaining, arable field site in mid-Suffolk and was, therefore, considered critically endangered (A. Smith, in: Stewart et al. 1994; P.J. Wilson, in: Wigginton 1999).

Reassessment of British status and revival of the species

Following Webb's 1985 challenge to BSBI botanists regarding a list of species considered native in Britain that he argued might well prove to be introductions, a major review of the status of every species in the flora was made prior to the publication of the BSBI's New atlas of the British and Irish flora (Preston et al. 2002). The actual review paper appearing subsequently as Preston et al. (2004). This review recognised and defined three species status categories, definite natives, probable archaeophytes (ie ancient, pre-1500 AD introductions) and neophytes (ie post-1500 AD introductions). It was stated that, "The indirect nature of the evidence used to identify archaeophytes means it is usually impossible to be certain about the history of a species; in particular, archaeophytes which have successfully invaded semi-natural habitats are likely to be overlooked as natives. The suggestion that a species is an archaeophyte is best regarded as a hypothesis to be tested by further studies." (Preston et al. 2004).

Following the rapid decline of C. cyanus as a self-sustaining agricultural field weed, from the 1980s to the present day, Cornflower has frequently been included in so-called 'wild flower seed mixtures', widely sown in private gardens and popular in Local Council amenity landscape and parks settings (Stace & Crawley 2015). Some of the sporadic occurrences reported in the 1990s and to today (Stroh et al. 2023) arise when deep excavations are made for road construction or pipe-laying, indicating that, like poppies (Papaver spp.), Cornflower seed does have considerable powers of longevity given suitable soil conditions, namely deep burial. It is thus possible to argue that the individual plants of the Fermanagh Enniskillen cornflower records, might be the sole survivors of much larger populations buried for many years. However, a visit to most garden centres around B & I would quickly demonstrate that there is a strong probability that the origin of these records, and the vast majority of other Cornflower records around at present, arrived recently in seed packets!

It is clear that while horticulturalists may enjoy cultivating small patches of Cornflower, and raise 'sports' of variant colour and size in garden borders as decoration (as mentioned by Gerard in 1633), and the currently very fashionable 'wildflower meadows' may be sown as part of 'landscape design', this very decorative ruderal will no longer raise its bright-blue flower heads among corn of any species. In most cases, C. cyanus nowadays does not persist for long in the types of plantings mentioned, it having become a much reduced, mere casual in these instances.

Farmers rejoice, of course, since Cornflower no longer competes with their crops thus reducing their yields. In the days when cereals were reaped by hand, C. cyanus earned the English common name 'Hurt sickle', since its tough, wiry stems quickly blunted the blade of the scythe (Grieve 1931).

European and world occurrence

As Hultén & Fries (1986) summarise the situation, "C. cyanus is believed to originate in SE Europe, from where it has spread as an agricultural weed to numerous parts of the world including S Africa, C, S & E Asia, N & S America, Australia (Queensland) and New Zealand. The species has today a discontinuous circum-polar distribution.". In S Europe, it is thinly scattered in the northern part of the Iberian Peninsula, and in the Mediterranean Basin it is absent from the Macronesian Islands, Crete and Cyprus, most of Turkey, the Middle East and N Africa. On the other hand, in N Scandinavia it extends well within the Arctic Circle and has been recorded in both Greenland and Iceland (Hultén & Fries 1986, Map 1870).

Uses

Apart from its long-term use as a decorative subject in a range of colours for the cultivated flower garden, the flowers are or were in the past used in modern herbal medicine, as a tonic or stimulant, and as an emmenagogue (ie to induce or increase menstrual flow). Water distilled from Cornflower petals was formerly used as an eyewash for weak eyes. Culpepper (1653) recommended the dried leaves for treating bruises. He also recommended the plant, seeds or leaves, as a remedy for poisons, including scorpion bites and other venoms and poisonous substances and, for good measure, the treatment of plague, infectious diseases and pestilential fevers!

If you had the patience, the expressed juice of the petals apparently also made a good blue ink, and mixed with alum-water it has been used by water-colour artists to achieve a delicate blue paint, although unfortunately the colour is not permanent. Apparently, according to Grieve (1931) dried petals are or were also used by perfumers for adding colour to pot-pourri.

Names

The genus name 'Centaurea' appeared in the writings of the Ancient Roman, Pliny, as 'centaureum' and 'centauria' and from the Greek 'kentauros'. It was named after the mythical centaur (half-man, half-bull), called Chiron, who was supposed to have had knowledge of herbal medicine (Gilbert-Carter 1964; Gledhill 1985; Stearn 1992). The Latin specific epithet 'cyanus', or 'cyaneus', of course simply means 'blue' (Stearn 1992).

Grigson (1955, 1987) gives a list of 30 English common names, twelve of which refer to the bright blue flower colour, eg 'Bluebottle', 'Blawort' from the French names 'Bleuet' and 'Blavet' and the Danish 'Blaurter'. Apart from the most frequent name 'Cornflower', another name, 'Blue Poppy', recognises the cornfield weed connection, and other names refer to the Centaurea 'Hard-head' and 'Knapweed' relationship, including 'Knotweed', 'Knobweed' and 'Logger-heads'.

The name 'Cornflower' dates back to Lyte (1578) as a translation of the apothecaries Latin 'flos frumenti'. Earlier English names were 'Bluebottle' or 'Blue Bothm', going back as far as the 15th century (Grigson 1974).

As Grigson also points out, before the advent of exotic, colourful plants from abroad began to flood into gardens in B & I in the latter half of the 17th century, the 'Cornflower' or 'Bluebottle' was much cultivated for its bright-coloured decorative effect in the flower garden (Grigson 1955, 1987).

Threats

None applicable any longer, since it is now widely recognised as being an ancient introduction.

References

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