Centaurea nigra L., Common Knapweed
Account Summary
Native, common and widespread. Suboceanic temperate, naturalised in N America and New Zealand.
1881; Stewart, S.A.; Co Fermanagh.
Throughout, with a peak in August.
Growth form and preferred habitats
C. nigra is a semi-rosette, polycarpic perennial with a stout rootstock and taproot. Each spring it develops a tough, wiry stem up to 90 cm tall, freely branching above and bearing numerous, dark purple-flowered, solitary, terminal knapweed heads (or capitula), each containing around 100 tubular, bisexual flowers, 15-30 mm long, surrounded by an involucre composed of several rows of overlapping, hard, dark-brown or blackish bracts (or phyllaries), each bract with a fringed, comb-like margin (Salisbury 1964). It is the latter structure of the flower-heads that gives C. nigra its second most frequent English common name, 'Hardheads'. The flowers are usually all similar and tubular, but sometimes the outer ones are much larger than the inner and are deeply divided at their apex into five linear segments, this making the flower-head much more conspicuous (Sell & Murrell 2006).
Well grown plants bear numerous branches, their basal leaves long-stalked and broadly elliptical; the leaf blades are rather variable, usually simple and unlobed, but may be quite deeply pinnately cut. Leaf margins are also variable, usually entire, but less often toothed, both leaf-surfaces being clothed with very short, bristly hairs (Salisbury 1964). Stem leaves are more linear-lanceolate and decrease in size upwards (Sell & Murrell 2006). The aerial shoots die down in winter, but in very mild, lowland areas and near the coast, they may remain semi-wintergreen and produce a small basal leaf rosette with the dead, brown flower stem (Grime et al. 1988, 2007).
C. nigra is a very successful perennial in well illuminated, rough, unfertilized, low-grade grassland across a very wide range of soil types, moisture levels and habitats. It is a constant species in the NVC MG5 grassland community named after it, the Cynosurus cristatus-Centaurea nigra grassland, Centaureo-Cynosuretum cristati Br.-Bl. & Tx 1952, a rather variable grassland composed of numerous very common grasses including Agrostis capillaris (Common Bent), Cynosurus cristatus (Crested Dog's-tail) and Festuca rubra (Red Fescue), and legumes such as Lathyrus pratensis (Meadow Vetchling), Lotus corniculatus (Common Bird's-foot-trefoil), Trifolium pratense (Red Clover) and T. repens (White Clover). This grassland is becoming drastically reduced and increasingly rare across lowland B & I as a result of agricultural improvement involving re-seeding and fertiliser spraying (Rodwell et al. 1992). The species occurs across a wide variety of infertile soil types and moisture regimes, but on account of its deeply penetrating root system, it avoids cold, waterlogged ground and very acid, peaty soils below about pH 4.0.
C. nigra mostly appears in old meadows and pastures, where it can become dominant if the ground is dry or well-drained, infertile and ill-managed or neglected for a time. Common Knapweed grows vigorously and can tolerate moderate sheep grazing (especially in autumn and winter) and mowing for either silage or hay (Rodwell et al. 1992). It is less likely to occur in fields grazed by cattle, probably on account of the heavier trampling damage it would sustain. It can persist on occasionally trampled sides of tracks, but it cannot survive in very disturbed situations, or where there is severe competition and shading from tall, robust, clonally-spreading, more aggressive species (Grime et al. 1988, 2007).
The established strategy of C. nigra is categorised as "variable", due to the wide ecological range of the species, but the majority of populations are allocated to the C-S-R strategy, meaning a general balance of all three main strategies, Competitor, Stress-tolerator and Ruderal (Grime et al. 1988, 2007).
Vegetative reproduction
Grazing or cutting stimulates the production of additional side-shoots around the crown and this can lead to shoot detachment and the formation of small, vegetative clones. While this qualifies as a minor form of vegetative reproduction, the principal means of species increase and dispersion is very definitely achieved by seed production (Grime et al. 1988, 2007).
Flowering reproduction
The flowers, which are said to be typically self-incompatible (Marsden-Jones & Turrill 1954), are produced from June to September. The flowers are amongst the most productive in terms of nectar and pollen in the flora of B & I and are particularly attractive to bees, although flies, hoverflies and butterflies also visit them to feed (Proctor & Yeo 1973).
There is a rudimentary pappus to assist dispersal and achenes are produced from July onwards. Being rather poorly equipped for dispersal, however, a proportion of the dry fruits often remain in the dead or dying inflorescence which becomes attacked by insects and, eventually, by mice and other small mammals. The seed weight is quite heavy at 2.55 mg, but not when compared with the achene of Greater Knapweed (C. scabiosa), which weighs an average of 7.46 mg.
Germination is probably mainly in the spring as only 36% of freshly collected seed were capable of immediate germination (Grime et al. 1981). A few dormant seed may survive in the soil for a few years, but no substantial seed bank exists (Roberts 1986).
The current author (RSF) has not been able to locate any estimates of average flowering capability or statistics of seed production and dispersal for this species.
Variation
The species is an out-breeder and since it contains considerable genetic variation, attempts have been made to recognise two subspecies in B & I (Sell & Murrell 2006). Along with the majority of B & I recorders, however, the Fermanagh recorders have not sought to distinguish them.
Toxicity
Some species of the genus Centaurea, including C. nigra, contain pyrrolizidine alkaloids that can degrade and become poisonous. They can cause liver damage and induce cancer in animals that graze the plant. Plants should therefore not be handled without wearing gloves (https://glenlivet-wildlife.co.uk/plants/knapweed/, accessed 13 February 2024). Removed hyperlink
Having said this, C. nigra cannot be particularly dangerous as it is not mentioned at all by Cooper & Johnson (1998) and grazing stock like cattle and horses always seem to know to avoid potentially poisonous plants, unless grass and other fodder is in very poor supply.
Fermanagh occurrence
In Fermanagh, as elsewhere in B & I, C. nigra is very common and widespread and it has been recorded in 449 tetrads, 85% of the VC total.
British and Irish occurrence
C. nigra has a wide ecological range and, consequently, it is very common, widespread and locally abundant throughout the whole of B & I, including the Channel Isles, Orkney and Shetland. In Britain, it is most frequent in southern and central regions, a fact not conveyed in hectad dot-maps. However, the trend analysis provided in the BSBI Atlas 2020, shows the species has been in steady decline for some years. Also, trending in the opposite direction, since the 1980s the species s.l. has been very widely sown in the majority of so-called 'wild flower seed mixtures', which in most cases will have contained non-native (foreign, alien) seed of the species from Europe or elsewhere (Stroh et al. 2023).
European and world occurrence
C. nigra belongs to the Suboceanic European phytogeograpical element and is native and endemic to Europe, where it is most prevalent in the west. It is considered an adventive introduction in more outlying localities by Hultén & Fries (1986), at least towards the north of its range in Norway, Sweden and Finland. It does not penetrate southwards beyond mid-way on the Iberian Peninsula and in Italy and is absent from most of the Mediterranean except SE France and NE Spain. It is also absent from Iceland, the Faeroes and Macronesia. It is an introduction in eastern and western N America and also in New Zealand (Hultén & Fries 1986, Map 1871)
Names
The name 'Centaurea' is derived from the Greek in Hippocrates and is called after Chiron the Centaur, whom legend reputed had a wide knowledge of herbs; he used it to heal his foot (Johnson & Smith 1946; Gilbert-Carter 1964).
Grigson (1955, 1987) listed as many as 53 common names for C. nigra, most of which refer to the hard, knobby or hairy heads, the toughness of the stalk or the bottle-shaped involucre of the inflorescence. The current author's (RSF) favourite is 'Churl's Head', derived from its rough, hairy, brown involucre (Prior 1879). It is very possible that some of these names were also applied to C. scabiosa (Greater Knapweed), which has larger flower-heads than C. nigra, does not occur so widely, and has not been recorded in N Ireland.
Uses
By whatever name we call it, this plant, together with C. scabiosa, has been used by herbalists for all sorts of wounds, ruptures, sores and scabs (Culpeper 1653; Grieve 1931).
Threats
None.
References
Marsden-Jones, E.M.and Turrill, W.B. (1954); Proctor, M.and Yeo, P. (1973); Roberts, H.A. (1986); Salisbury 1964; Grigson, G. (1955, 1987), Grieve, M. (1931); Culpeper, N. (1653); Gilbert-Carter, H. (1964); Johnson, A.T. and Smith, H.A. (1946); Grime, J.P., Hodgson, J.G. and Hunt, R. (1988, 2007); Grime, J.P., Mason, G., Curtis, A.V., Rodman, J., Band, S.R., Mowforth, M.A.G., Neal, A.M. and Shaw, S. (1981); Prior, R.C.A. (1879); Hultén & Fries 1986; Stroh et al. 2023; Cooper & Johnson 1998; Sell & Murrell 2006; Rodwell (ed.) et al. 1992;