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Petasites hybridus (L.) P. Gaertn., B. Mey. & Scherb., Butterbur

Account Summary

Native, frequent. European temperate, introduced in N America and New Zealand.

1882; Barrington, R.M.; Castle Hume estate, Lower Lough Erne.

January to November.

Growth form

Like Tussilago farfara (Colt's-foot), Petasites hybridus flowers before its leaves appear. Again, like Colt's-foot, it is a fairly low-growing, herbaceous perennial possessing a thick rootstock and an extensive, spreading underground rhizome. In this case, the green aerial parts of the plant become 'tatty' in September, dying down in October and more-or-less disappearing completely in winter. This mode of overwintering means the plant is described as a 'geophyte', characterised by having dormant buds that are protected by being buried in the soil or in leaf litter. The leaves are all basal, margins shallowly lobed, remotely toothed and cordate at the base – ie deeply indented, heart-shaped, their blades being ± broadly ovate to reniform (ie kidney-shaped).

The leaves are also large, often more than 30 cm in diameter, but occasionally they grow to be massive (up to 90 cm across on leaf-stalks up to 200 cm tall), so that the plant could almost be mistaken for Rhubarb (Rheum × hybridum) (Melderis & Bangerter 1955). Indeed, one of the alternative English common names applied to the species is 'Wild Rhubarb' (Allen & Hatfield 2004).

Like T. farfara, the leaves are covered with downy hairs on both surfaces at first, but the upper surface loses much of its cottony coating, while the lower surface remains floccose, being covered with a white or grey, cobweb of long 'arachnoid' hairs which also clothe the stout, hollow, leaf stalk (Butcher 1961; Sell & Murrell 2006). In the past, in Britain and Europe the large leaves of P. hybridus were used to pack butter and it was planted to provide this resource (Mabberley 2017).

Fermanagh occurrence and preferred habitats

Damp or wet ground on shifting, sandy riverbanks, ditches and lakeshores, plus damp copses and wet woodland at the base of cliffs provide the most typical natural habitat of this species in Fermanagh. The species also commonly forms large stands in occasionally disturbed ground in lowland wet meadows, grassy roadsides and in waste ground near buildings, including old churches, disused railways and quarries. It is widely scattered throughout the lowlands of the survey area, having been recorded in a total of 89 tetrads, 16.9% of those in the VC.

P. hybridus can colonise, persist and survive on a range of usually moist but not waterlogged soils, although it is associated with waterside habitats that may flood briefly in winter. The substrate can be stony, but more usually is of moderately fertile clay or silt. Generally there is little humus present, partly as a result of scouring and silt deposition from flooding. P. hybridus cannot tolerate strongly acidic conditions, heavy shade or competition (Grime et al. 1988, 2007).

Mobilisation of nutrients stored in the Butterbur rhizome enables a rapid expansion of the dense leafy canopy in April, quickly shading out all competing plant species, so that it tends to occur as monocultures on otherwise bare soil. The huge leaves borne in dense stands draw attention to the plant, which as a result frequently suffer damage through cutting, trampling and grazing (Grime et al. 1988, 2007).

Flowering reproduction

In late February or early March through into April or May, beginning shortly before the leaves appear, the stout fleshy rhizome sends up many erect, thick, scale-bearing, hairy flower stalks up to 40 cm tall, each bearing from 50 to over 100 flowerheads in an inflorescence up to 30 cm long. The colour of the flowerheads is variable and is variously described, ranging from dull white or cream, both tinged with purple, or violet, or pinkish-mauve, or pinkish-lilac! Hutchinson (1945, 1972) describes them more simply as, "pink to purple". The arrangement of the flowerheads is also variously described, from a dense, ± cylindrical or conical spike to a raceme inflorescence (Perring & Walters 1989; Parnell & Curtis 2012).

Dioecy

The flowers of all 18 species of the genus Petasites are dioecious, having quite different looking, male and female flowers borne on separate plants (Stace 1989). Dioecy is quite rare in the flora of B & I, only occurring in about 2% of the 3,500 or so recognised species (Berry 1977). As is quite often the case in dioecious species, the sexual separation is rarely completely and perfectly realised. Thus in P. hybridus a few female florets are usually present around the edge of male flowerheads and a few (one to three) males occur in the centre of the female heads, each of which contains around 150 female flowers. In both cases, the 'misplaced florets' in the flowerheads are sterile (Valentine 1947; Parnell & Curtis 2012). On account of the imperfect separation of the sexes, it is perhaps better to refer to the P. hybridus clones as either 'functionally male' or 'functionally female' (Stevens 1990; Yeo 1990).

Referring back to the appearance of the inflorescences mentioned above, the female clones produce looser, taller inflorescences than the males (Valentine 1946, 1947). In both male and female flowerheads, all the florets are tubular, there being no ligulate ray-florets present in the genus. The involucral bracts around the flowers are mostly in one row. The male flowers contain fertile anthers and a non-receptive female style and stigma (ie the stigma is bi-lobed, but it never opens) and a sterile ovary, surmounted by a short, non-functional, completely redundant pappus (Hutchinson 1945, 1972, p. 603, Fig. C). Thus the flowers on 'male' flowerheads are described as hermaphrodite (bisexual), but functionally male, and similarly 'female' flowerheads contain many fertile female florets surrounding a few hermaphrodite but sterile central 'male' tubular florets (Clapham et al. 1987).

The style is present in the functionally male florets, being an essential organ, required to push the pollen from the anthers out of the floral tube entrance (ie the 'stylar brush' mechanism typical of the Asteraceae family). The extending style makes the pollen available to insect visitors, chiefly bees, which also collect nectar from flowers of this sex only (Clapham et al. 1987).

Female flowerheads are more widely bell-shaped than the males and the stalk of the female inflorescence also becomes longer in fruit than those of male clones. Female flowers are very narrowly tubular, four-toothed at the apex, the style shortly two-lobed at the apex (Hutchinson 1945, 1972).

British and Irish clonal occurrence

Study in the 1930s of the distribution of the sexes of P. hybridus first showed that while male plants occurred throughout much of lowland B & I, as well as most of N Europe, female clones, which are much taller and more conspicuous than male ones, were very much more restricted in distribution. In Britain, the female plants were shown to be common only in parts of N England and the N Midlands in a zone lying north of Leicestershire (VC 55) reaching Lancashire and Yorkshire with outliers beyond to the southern shore of the Firth of Forth (Valentine 1947; Perring & Sell 1968, Map 509/1; Clapham et al. 1987). It later transpired that in the English Midlands, female plants were generally, "much more abundant than 'males', and frequently occur on their own without it" (D.H. Valentine MS, quoted in Perring & Sell 1968, text with Map 509/1). It has been suggested that the restricted area of Britain to which the functionally female Butterbur plants occur, may well represent the true native distribution of the species, ie from the central Midlands of England to the Scottish borders (Stace & Crawley 2015).

The BSBI Critical Atlas map of P. hybridus 'female' distribution indicates an occurrence in just one Irish hectad square in Co Louth (H31) (Perring & Sell 1968, Map 509/1). Thus all the other P. hybridus clones growing everywhere else in Ireland are male and might possibly represent introductions deliberately planted to provide food for honey bees early in the year (see below).

Vegetative reproduction

It is apparent, therefore, that male clones reproduce perfectly efficiently in sexual isolation, relying entirely on vegetative lateral spread of their rhizome, plus occasional fragmentation of this fleshy, brittle underground organ. Vegetative dispersal and reproduction of the male plants often occurs along waterside banks – probably rather too frequently assisted by various forms of human disturbance.

Successful sexual reproduction

Where some female plants or clones occur intermixed with male clones, flowers regularly achieve pollination and subsequently produce slightly angular, smooth, 2.5 mm long brown achenes surmounted by a white, almost smooth pappus plume (nearly twice as long as the corolla), which wind disperses the 'seed' (Hutchinson 1945, 1972; Melderis & Bangerter 1955). However, since the seed is short-lived and does not form a persistent buried seed bank in the soil, establishment of new colonies by seedlings is likely to be a rare event (Grime et al. 1988, 2007).

Flowers as bee food providers

The fact that male flowers produce a copious supply of energy-rich nectar, plus protein and mineral-rich pollen means they provide a very significant early season balanced food source for honeybees. This has led to the suggestion that beekeepers have deliberately introduced male clones to areas beyond the natural distribution of the species (Tutin et al. 1976; Stace 1989). Evidence to support this preferential planting hypothesis has not yet been uncovered, so it merely remains a possible explanation of this puzzling sexual distribution. However, other dioecious species also show differing distributions of the sexes (eg Stratiotes aloides (Water-soldier)), and while these may also have a history of cultivation, alternative explanations involving differential survival governed by natural factors are perfectly feasible (Forbes 2000).

European and world occurrence

P. hybridus belongs to the European temperate phytogeographical element and occurs in most of Europe from Spain and Portugal northwards to S Sweden and Norway to 63oN, and eastwards into adjacent parts of N, W & C Asia. It is less prevalent in S Europe and, although it penetrates into S Italy, it is absent from all of the Mediterranean islands. However, it has been widely introduced either as a medicinal plant and/or to feed honey bees, for instance in parts of Scandinavia, N America and New Zealand. It has been said that elsewhere in Europe the "male clones exist alone only where the species is introduced" (Mabberley 2017). It is therefore, in many situations, difficult or impossible to distinguish native from deliberately planted alien occurrences (Hultén & Fries 1986, Map 1831; Clapham et al. 1987).

Names

The name 'Petasites' appears in Dioscorides herbal writings and it is derived from the Greek name for a wide-brimmed traveller's hat, a 'petasos', used to protect the wearer from both rain and sun (Grigson 1955, 1987). The plant has at least 29 local English common names, most referring either to the use of the leaf as a shelter, or as a wrapper for butter (eg, 'Butterbur' or 'Butter-dock') (Grigson 1955, 1987; Garrard & Streeter 1983). Interestingly, Melderis & Bangerter (1955) point out that, since the species frequently grows near water, the large leaves provide shelter, cover and shade for water-fowl and other creatures.

Medicinal uses

The rhizome of P. hybridus was used in herbal medicine mainly as a diuretic and a cardiac tonic, although it also had a reputation both as a plague remedy and in "love divination" (Grieve 1931). In placebo-controlled medical trials, Butterbur root reduced the incidence of migraine attacks by 60% in 128 sufferers. Butterbur is thought to act by affecting muscle tension: a dose of 50 mg was taken twice daily by the subjects, but it needed to be taken for a couple of months in order to be effective (Olivier 2001).

A Swiss and German medical study found that a Butterbur extract called 'petasine', a terpene believed by researchers to have antispasmodic and anti-inflammatory action, proved as effective as the antihistamine 'cetirizine' (marketed as 'Zirtek') in treating hay-fever. Moreover, it did not induce as much drowsiness and fatigue as did the previously used supposedly non-sedating antihistamine. Another advantage of the Butterbur extract was that it proved effective in completely eliminating hay-fever symptoms within 20 to 30 minutes of oral administration, so that the remedy need only be taken as required, rather than pre-emptively. It also reduced nasal congestion, which usually cannot be achieved by antihistamines (Schapowal 2002; Woodham 2002; Mabberley 2017).

Toxicity

Pyrrolizadine alkaloids toxic to the liver are also present in Butterbur, but these can be completely removed by a patented Swiss procedure during extraction of petasine (Woodham 2002).

Threats

None.

References

Perring, F.H. and Sell, P.D.(Eds.) (1968); Grime, J.P., Hodgson, J.G. and Hunt, R. (1988, 2007); Stevens, D.P. (1990); Yeo, P. (1990); Berry, R.J. (1977); Valentine, D.H. (1946, 1947); Forbes, R.S. (2000); Tutin, T.G. et al.(Eds.) (1976); Stace, C. (1989); Grigson, G. (1955, 1987); Grieve, M. (1931); Olivier, S. (2001); Stace & Crawley 2015; Garrard & Streeter 1983; Melderis & Bangerter (1955); Hultén & Fries 1986; Clapham et al. 1987; Mabberley 2017; Hutchinson 1945, 1972; Parnell & Curtis 2012; Perring & Walters 1989; Allen & Hatfield 2004; Butcher 1961; Sell & Murrell 2006; Schapowal 2002; Woodham 2002; Mabberley 2017.