Knautia arvensis (L.) Coult., Field Scabious

Account Summary

Growth form and preferred habitats

Knautia arvensis is quite common and striking, but is also a very variable plant across both B & I. It is particularly variable in height, growing anywhere between 25 and 90 cm tall. The rootstock is perennial, dark in colour and somewhat woody, and when it becomes established it takes such a firm hold in the ground it becomes difficult to eradicate, should one want to do so (Grieve 1931). The branched rootstock or rhizome frequently gives rise to clumps of stoloniferous, leafy, frost-hardy, over-wintering rosettes (Sell & Murrell 2006). Plants also possess a ± deep taproot that can penetrate to depths of around 100 cm (Varga et al. 2022).

Another notable feature is that leaves are very variable in this species. The basal rosettes are composed of short-stalked, opposite leaves, lanceolate in shape with their margins toothed, but the blade or lamina usually undivided, although occasionally some plants produce lyrate or pinnately-cut leaves. The aerial stem is pale green, but is often tinted or spotted purplish and it may branch above. The stem is terete (round in section), feels rough to the touch and at least the lower part is clothed with downward directed stiff hairs of varying length, bulbous at their bases. The stem leaves, that are again opposite, are sessile and often are crowded towards the lower half of the stem. The stem leaves are large, deeply pinnately cut or lobed and are very variable in general outline. The terminal segment of the lobed stem leaves is generally elliptic. Some of the rather few upper stem leaves are entire and they get smaller and more bract-like higher up the stem. All leaves are ± hairy, deep green in colour and arranged in successive pairs at right-angles (Melderis & Bangerter 1955; Hutchinson 1972; Clapham et al. 1987).

Field Scabious is a plant of dry or well-drained grassy places on light, moderately acidic to near neutral, often sandy or calcareous soils. It is most frequent in B & I in soils with pH between 7 and 9 (Hill et al. 2004). It is a plant of full-sun or light shade conditions and is characteristic of open, disturbed habitats that include calcareous or chalky meadows, open areas in woods, hedge-banks and rough, grassy roadside verges or on waste ground in old quarries and sand-pits, churchyards and the like. It is also occasional in some parts of England as a weed of cereal fields, or on field margins (G.T.D. Wilmore, in: Preston et al. 2002).

K. arvensis is often said to be intolerant of grazing, yet it can generally withstand the regime of occasional mowing applied to roadside verges one or two times per year (Sinker et al. 1985). Grazing, mowing, trampling or other forms of disturbance reduce the ability of the plant to flowering and fruit but, on the other hand, the disruption opens the surface of soil or vegetation somewhat, enabling germination and better seedling establishment. Experiments have shown that without some degree of vegetation disturbance, K. arvensis is a poor competitor whenever it is a new invader of grassland plots (Varga et al. 2022).

The established strategy of K. arvensis is categorised as a CSR species, ie it combines traits of all three strategies, Competitor, Stress-tolerator and Ruderal (Grime et al. 1988, 2007).

Although Sinker et al. (1985) considered it, "not very persistent" in the Shropshire region, a long-term, permanent quadrat study of species behaviour and persistence in a hay-meadow in C Sweden found that K. arvensis maintained or increased its local population presence over the 13 year period of the study (Tamm 1956). "In moist, well-drained soil of low to moderate fertility, provided the species can at least occasionally flower, set seed and disperse, it will be able to persist in the habitat." (Varga et al. 2002).

Fermanagh occurrence

K. arvensis can form quite large stands on roadsides, which is the habitat it most frequents in Fermanagh. Apart from the non-specific county reference made by Praeger (Irish Topographical Botany), there are 13 sites in Fermanagh with records dating from 1950 onwards. Eight of these sites have post-1975 records. As the tetrad map indicates, K. arvensis appears to be confined to the lowlands lying E of Lough Erne where soils are more fertile and better drained. The local habitats include lakeshores, gravel pits and old quarries.

At one local site in the VC (Clonmaulin, near Clontivirin), this species is known to have persisted since 1951.

Flowering reproduction

Flowering takes place from July to September and the plants are gymnodioecious, ie the inflorescences are compound flower-heads that are either bisexual or, less commonly, purely female. The flower-head in bisexual plants is relatively large, ± convex, 30-40 mm diameter, while in female plants it is considerably smaller, around only 15-30 mm in diameter. Both forms of inflorescence are a delicate bluish-mauve, occasionally lilac or purple, and they are composed of up to 100 small flowers or florets, the outer blossoms with larger, mostly irregular petal lobes, helping to make the heads more conspicuous and most attractive in appearance. The flower-heads are slightly domed and are often held three together, each one on a long stalk, the middle one the tallest and its flowers opening first. The petals of both the bisexual and the female florets have a short corolla tube that varies from 5-7 mm in length, the shortest flowers being the central ones in each head (Hutchinson 1972; Sell & Murrell 2006).

Pollination

The flowers are scented and nectar is secreted on top of the upper part of the inferior, single-ovuled ovary. The nectar food reward is sheltered from the diluting effect of rain by hairs inside the corolla tube. Since the floral tube is short, the nectar is readily available to both short- and long-tongued insects, and when pollen is produced, it is equally easily accessed. The inflorescences are visited by a range of insects including bees, butterflies, beetles and flies, although butterflies are undoubtedly the most frequent callers. Bisexual flowers are self-compatible, although the level of seed production remains very dependent on cross-fertilisation (Varga et al. 2022).

The bisexual version of the flowers is very strongly protandrous, ie the anthers mature first, release their pollen and wither before the stigmas mature and become capable of being pollinated. Thus the bisexual flower-heads are entirely male for a time, and then become equally completely female, ensuring cross-pollination takes place (Hutchinson 1972; Hickey & King 1981; Richards 1997).

Seed and fruit

Seed set is from August onwards. The fruit of each floret is a solitary, rather large, 5-6 × 2 mm, ± hairy achene (ie a single-seeded dry fruit) or nutlet, like that found in the Asteraceae. The similarity between these two flower families extends to the achene being surmounted by a pappus-like, bristly, hairy calyx, but in Knautia arvensis this is deciduous ‒ unlike in the closely related genus Succisa which likewise contains only one species in B & I (S. pratensis Moench, Devil's-bit Scabious), where the pappus is persistent and presumably (or obviously!) assists in wind-dispersal. Salisbury (1978) reported from his measurements a mean seed production of 520 seeds per plant, but the figure can vary up to 2,860 seeds per plant (Varga et al. 2022).

In all Knautia species the surface of the cylindrical fruit achene or nutlet has a small, attached, basal cylindrical eliaosome food appendage that attracts ants that may help to disperse it, although probably they only transport the rather large propagule a short distance from the parent plant (F. Ehrendorfer, in: Tutin et al. 1976; Clapham et al. 1987; Varga et al. 2022). Seed predation pre-dispersal by birds (especially Goldfinches), and post-dispersal by slugs, beetles and ants, can be a major factor limiting seed survival (Varga et al. 2022).

Buried K. arvensis seed is known to be long persistent in the soil seed bank, surviving for ten or more years (Thompson et al. 1997).

Vegetative reproduction

Plants of K. arvensis tend to occur as solitary individuals, although due to rhizome growth, older plants can form clumps and there is a very limited degree of clonal spread, provided by a combination of rhizome and stolon growth (Varga et al. 2022).

Fossil record

There is very little in the way of a fossil record for K. arvensis. Godwin (1975) reported pollen at the supposed Anglo-Saxon level in lake deposits from Norfolk, and Ingrouille (1995) found pollen in Britain from the Early Holocene (11,000 to 8,000 BP), probably present as the result of vegetation disturbance by Megalithic humans making use of natural communities, if not actually farming, in any event creating openings for colonisation by weedy species.

British and Irish occurrence

Much more frequent in the S and E of both B & I (BSBI Atlas 2; New Atlas), K. arvensis is rare, scarce or absent in much of N & W Scotland and, likewise, in W & N Ireland, presumably due to a combination of unsuitable climate and the predominant strongly acidic soils in these regions. In common with Dipsacus fullonum (Wild Teasel), in the north of its overall B & I distribution K. arvensis becomes increasingly confined to coastal sites (FNEI 3).

Although recorded at least once in every Irish VC (Cen Cat Fl Ir 2) and in 56% of Irish hectads (New Atlas), in Ireland Field Scabious is very much more frequent and widespread in land lying to the east of the River Shannon.

The New Atlas map indicates there have been considerable losses in K. arvensis populations across its range in B & I and analysis of the database indicates this has occurred gradually since 1950 (G.T.D. Wilmore, in: Preston et al. 2002). The intensification of agriculture and associated modern practices such as widespread spraying of herbicides and nitrogen-rich slurry and use of artificial NPK fertilisers explains this, since K. arvensis does not tolerate the soil enrichment and subsequent increased competition from vigorous species this practice produces (Crawley 2005).

In recent years, K. arvensis has become a regular component of so-called 'wild flower seed mixtures' that are frequently sown by private gardeners and local district councils and landscape architects in urban gardens, roadsides and amenity areas; some of the New Atlas recorded distribution may be accounted for by spread from such sowings (G.T.D. Wilmore, in: Preston et al. 2002).

European and world occurrence

A very variable and polymorphic species, K. arvensis belongs to the Eurosiberian temperate phytogeographical element and is widespread across Europe northwards to 68^oN in Norway and in adjacent parts of N Africa and Asia. It also spreads eastwards to reach W Siberia. Seed has been spread widely along with farming and has colonised Japan, N America and New Zealand (Hultén & Fries 1986, Map 1746; Clapham et al. 1987).

Names

The Latinised genus name 'Knautia' is named in honour of Christoph Knaut, a 17th century German doctor and botanist (Stearn 1992). The Latin specific epithet 'arvensis' means 'of cultivated fields' as the plant has a long history as a crop weed (Melderis & Bangerter 1955). Thirty-five English common names are listed by Grigson (1955, 1987), many of them rather aptly referring to the inflorescence head resemblance to a pincushion. The name 'Scabious' is,"an abbreviation of 'scabiosa herba', 'the herb for scabies' [a form of leprosy according to Grieve (1931)], otherwise known as 'scab' [a scaly sore], 'the mange' and 'the itch'" (Grigson 1955, 1987). The commonly applied 'Doctrine of Signatures' was probably invoked by herbalists noticing the rough feel of the stems with their stiff hairs.

Threats

None.

References

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