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Pilosella officinarum F.W. Schultz & Sch. Bip., Mouse-ear-hawkweed

Account Summary

Native, common. European temperate, naturalised in N America and New Zealand.

1881; Stewart, S.A.; Co Fermanagh.

Throughout the year.

Growth form

Pilosella officinarum is a very variable, winter-green, rosette-forming, polycarpic perennial herb with a ± slender, vertical rootstock and far-spreading, white, prostrate leafy runners that efficiently vegetatively propagate the plant.

Identification and variation

The rosettes of P. officinarum with their small entire leaves firmly appressed to the ground, rimmed by some of the dense white, star-like (stellate), branched hairs that constitute the woolly covering (tomentum) on the underside straying across the margin, the single, distinctive solitary lemon-yellow heads on usually leafless stems or scapes up to 30 cm tall, plus the small, daughter rosettes attached by long, slender, prostrate, creeping hairy stolons, make this the most readily recognised Hawkweed of them all. Possession of stolons is an instantly diagnostic characteristic of the genus Pilosella (Silverside 1990). Having said that, the species is very variable both in terms of chromosome number and phenotypically plastic in its form in response to the environment, especially with respect to rosette size, the pattern of clonal growth and its flowering behaviour (Bishop & Davy 1994).

Forms of the plant exist in a more or less geographically localised manner in B & I, with chromosome numbers forming a complete series from diploid (2x=18) to hexaploid (6x=54), with tetraploid (4x), pentaploid (5x) and hexaploid forms all having been recorded in Ireland.

The genomes or cytotypes of P. officinarum differ in their reproductive mode, some forms being sexual and others apomictic, their achenes merely contributing to the clonal growth produced by the stolons. A survey found that sexual reproduction is confined to plants with an even ploidy level (ie 2x, 4x and 6x). Most 4x and most 6x plants from mountainous habitats reproduced sexually, while 5x, 7x and lowland 6x plants were apomictic (Gadella 1991). Triploid plants are not mentioned, but they are capable of reproducing apomictically. Some 5x and 7x plants studied by Gadella (1991) were completely sterile, and since sexual plants are strictly self-incompatible, this suggests that plants colonising remote or new areas are more likely to be apomicts.

As many as seven subspecies of the plant have been described and are distinguished by differences in the hairs on the involucral bracts around the flowerhead (P. Sell, in: Rich & Jermy 1998). All forms of the plant in B & I are more or less connected by intermediates, possibly of hybrid origin. Two of these divisions, subsp. micradenia (Nägeli & Peter) P.D. Sell & C. West and subsp. trichosoma (Peter) P.D. Sell & C. West, have been listed for Fermanagh.

Stace (1991, 1997) is sceptical of these divisions, taking the view that the subspecies of P. officinarum are not, or are only partially, geographically separated and, therefore, are no more than varieties. Indeed, he goes much further, suggesting the genus Pilosella itself is doubtfully distinct from Hieracium. The term used to describe this doubt is 'equivocal' (Stace 2019). Sell & West (1974), however, consider Pilosella sufficiently different from Hieracium in its achene structure, stoloniferous habit and capacity for sexual reproduction to merit elevation to generic status.

Fermanagh occurrence

The 414 records in the Fermanagh Database are lumped as P. officinarum (= Hieracium pilosella L.). P. officinarum has been recorded in 218 Fermanagh tetrads, 41.3% of those in the VC. As the distribution map shows, it is widespread throughout the county, but is more prevalent in the western half.

Habitat preferences

Despite being shallow-rooted, roots seldom reaching depths beyond 15 cm, P. officinarum grows on a wide variety of soils that typically are dry, often of calcareous or low nutrient status, and generally are disturbed to some extent. It is most typically found on banks, pastures, lakeshores, waste ground and roadside verges. It also frequently grows on skeletal soils, such as those on screes, gravel, in cliff rock crevices, or in quarries (Grime et al. 1988, 2007; Bishop & Davy 1994). When it occurs on better developed soils of sand dunes, grassland or heaths, it grows where low levels of nitrogen and phosphorus typically limit the growth of more vigorous competitors. In these situations, low productivity is frequently exacerbated by soil impoverishment resulting from overgrazing (Ellenberg 1988).

Mouse-ear-hawkweed is an important constituent or constant species (ie with a constancy value greater than 60%) of short or sparse calcareous grasslands (Bishop & Davy 1994). However, it is also found to a lesser extent in calcifuge, acidic grasslands and has been reported occurring at high frequencies in acidic degraded pastures on gravelly soil in N America at a pH of 3.7-4.8 (Vander Kloet 1978).

The low rosette growth form of P. officinarum restricts it to habitats where taller, vigorous and potentially more competitive species are limited by local environmental factors, eg by exposure, soil impoverishment, lack of moisture or grazing pressure. On the other hand, as recent experience in N America and New Zealand has proven, P. officinarum has the ability to colonise nutrient-poor permanent pastures. In its natural habitats in B & I, where it contends with a long list of associated herbivores and a rust fungus disease, it appears to be a good, often patch-forming competitor, eg with Festuca ovina (Sheep's Fescue) (Watt 1962).

The established strategy of P. officinarum is categorised as S/CSR, meaning it is intermediate between a Stress-tolerator and a more balanced mix of all three strategies, Competitor, Stress-tolerator and Ruderal (Grime et al. 1988, 2007).

Toxicity and herbivory

Leaves and roots of P. officinarum contain a number of phenolic compounds which are toxic to some other plant species and they probably confer a competitive advantage through allelopathy. These substances do not deter herbivores however and the species is palatable to rabbits, sheep and a long list of invertebrates (Bishop & Davy 1994). While it is intolerant of taller, shading species, P. officinarum is very able to withstand frost and, despite being very shallow rooted, it is an exceptionally able xerophyte, evading wilting to a lower soil water content than any other chalk plant tested (Anderson 1927).

Flowering reproduction

Flowering begins in late May, peaks in June, and may be followed by a second flowering period in a mild autumn. The individual leaf rosettes flower once and then die (ie they are monocarpic). The solitary heads of strap-shaped (ligulate), lemon yellow florets, the outermost striped red on their exposed face, are visited by a wide range of pollinating insects. The number of achenes in a head varies considerably within the range 60-130, depending on the population and growing conditions. Achene production was estimated to be around 500 achenes/m2 in a pasture moderately grazed by rabbits, and 13,000 achenes/ m2 when ungrazed (Bishop & Davy 1994).

Fruit dispersal is both by wind, the achenes being furnished with a short, even pappus, and by birds, the fruit surviving passage through the animal's gut (Salisbury 1964). Despite seed production and dispersal on this scale, field sightings of seedlings were infrequent during a four year study by Bishop & Davy (1994). It is very likely that the vast majority of rosettes (probably around 99%) are produced vegetatively by the obviously very vigorous stoloniferous growth of the plant.

British and Irish occurrence

P. officinarum is frequent and widespread throughout the whole of B & I, being absent only from a few hectads in the English Fens, plus parts of N Scotland, including Shetland (VC112) (Atlas 2020). The distribution is largely unchanged since the 1960s although present recording coverage is definitely better than before. It occurs on both acid and base-rich soils and from coastal sand-dunes to an altitude of 915 m on Ben Macdui, South Aberdeenshire (VC 92) (Bishop & Davy 1994; Stroh et al. 2023). There is insufficient information to map the subspecies or varieties.

European and world occurrence

P. officinarum occurs throughout temperate and sub-Arctic Europe. As reported by Bishop & Davy (1994), "tetraploid plants are widespread in the lowlands of S Europe, but scarce in the northern parts". The pentaploid form occurs in N Europe including Scotland and the mountainous parts of S Europe (Gadella 1972). P. officinarum extends eastwards to W Asia, but it is absent from the Azores, the Faeroes, Iceland and Spitsbergen. According to Hultén & Fries (1986), in the Mediterranean region a closely related form (Hieracium pseudopilosella) has been recorded in Mallorca, Corsica and N Africa, but it is absent from Sardinia, Sicily, Crete and S Greece including the islands.

P. officinarum has been introduced to eastern N America and to New Zealand (Hultén & Fries 1986, Map 1891). An indication of the colonising success of the plant is the 'Red alert' listing of it among exotic pest plants of greatest ecological concern in Oregon and Washington on a 2001 Website. https://www.cabidigitallibrary.org/doi/10.1079/cabicompendium.27162

Uses

None of the Hawkweeds are now used much in herbal treatment except Mouse-ear-hawkweed. It is regarded as having sudorific (ie sweat inducing), tonic and expectorant properties. It is also regarded as a specific cure for whooping cough and useful for all disorders of the lungs. Vickery (1995) quotes living correspondents who took 'mouse-ear' for coughs and colds. It is also recommended as an astringent for bleeding, both internal and external (Grieve 1931). Culpeper (1653) also provides a long list of medicinal uses, some of which may have a real basis.

Names

The genus name 'Pilosella' is the feminine diminutive of 'pilosus', meaning hairy, and in botanical Latin usually implies soft, distinct hairs; it made its first appearance in the Herbal of Rufinus, as Hieracium Pilosella (Gilbert-Carter 1964).

The English common name 'Mouse-ear-hawkweed' refers very aptly to the shape and the long, soft hairiness of the leaf (Prior 1879). Grigson (1974) suggests 'Mouse-ear' is a 13th century name, derived from 'musere' (the hairiness of the small leaves), translating medieval Latin 'auricula muris', in turn translating the Greek 'muos ota' (from Dioscorides herbal). Alternative English common names include 'Erswort', 'Fellon-herb' (from E Cornwall) ['fellon' possibly referring to inflammation], 'Ling Gowlans' (Yorkshire) [a very non-specific name] and 'Common Hawkweed' (Britten & Holland 1886).

Threats

None.

References

Stace, C. (1997); Stace, C. (1991); Stace 2019; Salisbury, Sir E. (1964); Grime, J.P., Hodgson, J.G. and Hunt, R. (1988, 2007); Silverside, A.J. (1990); Rich, T.C.G. and Jermy, A.C.(eds.) (1998); Ellenberg, H. (1988); Bishop, G.F. and Davy, A.J. (1994); Sell, P.D. and West, C. (1974); Gadella, T.W.J. (1991); Vander Kloet, S.P. (1978); Watt, A.S. (1962); Anderson, V.L. (1927); Grieve, M. (1931); Culpeper, N. (1653); Prior, R.C.A. (1879); Vickery, R. (1995); Britten, J. and Holland, R. (1886); Stroh et al. 2023; Hultén & Fries 1986; Gadella 1972; Gilbert-Carter 1964; Grigson 1974