Arctium minus subsp. nemorosum (Lej.) Syme, Wood Burdock

Account Summary

Growth form and preferred habitats

The genus Arctium, as we shall see below, is fraught with nomenclatural difficulties. But Wood Burdock, however we manage to consider or confuse it, is a very large-leaved, robust, conspicuous, rosette-forming monocarpic biennial or short-lived perennial possessing a deep taproot that occurs both as isolated individuals and in patches. The stout, hollow, branched, flowering stem produced in the second year or later, can be 200 cm or more in height. The broad, long-petioled, rhubarb-sized, rosette leaves readily overtop and cast such heavy shade that few species can compete with and co-exist under them in herbaceous vegetation (Gross et al. 1980).

A. minus is typically found in open, occasionally disturbed, lowland areas of pastures, rough grassland, in clearings and alongside paths in woods, roadsides, waste ground, hedgerows and riverbanks. It characteristically occurs on moderately fertile soils of near neutral pH, but can grow on all but the most acidic conditions (Grime et al. 1988, 2007).

The established strategy of A. minus agg. was categorised as intermediate between a Competitor and a Competitive-Ruderal, being rather confined to soils that are only occasionally disturbed or to sizeable vegetation gaps, although it can more rarely be found in closed turf vegetation (Gross et al. 1980; Grime et al. 1988, 2007). Depression of surrounding vegetation by shade from the basal leaf rosette is important in creating openings for seedling production and establishment and local maintenance of the species' monocarpic population. Clearings beneath patches of adult plants have been observed enabling large seedling populations to develop, and in one case in Kalamazoo, Co., Michigan, well over 100 seedlings/m² were present in bare ground within such clearings on 21 Oct. 1978 (Gross et al. 1980). Seedling emergence and survival were reduced by the presence of litter and vegetation in an old-field population study in SW Michigan, USA (Gross & Werner 1983).

Burdock, of whatever persuasion, is not a very important or serious crop weed since it is easily controlled by cultivation. In pastures it can be a nuisance as its burrs, attaching to stock animals and to pets, particularly to long-haired dogs and cats, may cause skin irritation and discomfort. It is also considered unsightly and would certainly reduce the value of wool (Gross et al. 1980). It is claimed that cows are fond of eating A. minus foliage and that when they do so, it gives their milk a bitter flavour if they consume it in large quantities (Georgia 1931). This strikes the current author (RSF) as being an extremely unlikely event!

The main period of germination is in spring and growth of seedlings is slow, such that some may even retain their cotyledons through the whole growing season and produce only one or two small leaves by the autumn. Subsequent growth often continues slowly, so that many plants with rosettes of six or seven larger leaves, up to 40 cm long, must be older than two years. Regardless of the eventual size of the autumn rosette, all leaves die back in winter and fresh ones develop in the spring (Gross et al. 1980).

Taxonomic difficulties and decisions taken

Faced with this very variable and taxonomically ill-worked genus, the current author (RSF) decided to follow the example of An Irish Flora (1977) and has taken a 'lumpers' approach. Dickie (1864) listed A. lappa as being distributed, "from Armagh to Enniskillen along with Conium", but he was in error as this species has never occurred anywhere in Fermanagh.

It has been assumed that all Arctium plants found in Fermanagh are the most widespread Irish form (and the only subspecies so far recorded anywhere in the north of the country), A. minus subsp. nemorosum (or A. nemorosum). The reasons for taking this decision lie in the fact that Webb et al. (An Irish Flora 1996) suggest that some Irish forms of the genus do not fit into the taxonomic segregates suggested by F.H. Perring in the BSBI Plant Crib (1988), since they display a mixture of his defining characters. In addition, the monograph revision proposed by Duistermaat (1996), accepted by New Flora of the BI (1997), and treated in the revised BSBI Plant Crib (1998), Sell & Murrell (2006) and Parnell & Curtis (2012) happens to have post-dated a large proportion of the field recording for the now published County Fermanagh Flora (Forbes & Northridge 2012).

Identification and taxonomic difficulties arise in Arctium because occasional hybrids and outcrossing occur in what is possibly, or probably chiefly, an autogamous genus, and this has led to the production of numerous intermediate forms (Gross et al. 1980). A. nemorosum can hybridise with other forms of A. minus and with A. lappa (Greater Burdock), but Stace et al. (2015) recognised that this is really just more evidence of the difficulty met when trying to distinguish the taxa of Arctium (Stace et al. 2015).

Duistermaat's revision adds further to the confusion of Irish material, since his A. minus (Hill) Bernh. is described in the New Flora of the BI (1997) as being, "apparently common throughout the British Isles", while his A. nemorosum Lej. is considered a plant, "mostly of open woods and semi-shaded disturbed ground, especially on calcareous soils", and it is considered, "possibly rare or absent from SW Britain, the Channel Isles and Ireland". Obviously resolving the questions raised is an all-Ireland project, awaiting an interested party with the time and the funds to pursue it.

In the latest version of Webb's An Irish Flora, Parnell & Curtis (2012) still recognise A. lappa (= A. major) (Great Burdock) as being distinct from A. minus and they mention three subspecies of the latter, subsp. nemorosum, subsp. minus and subsp. pubens. These authors also state that of the three subspecies, A. minus subsp. nemorosum is the common form of the species everywhere [in Ireland], and remains the only subspecies recorded in the north [of the island].

In comparison, Sell & Murrell (2006) also recognise two species, A. lappa and A. tomentosum (Woolly Burdock), the latter an introduced grain casual that can hybridise with A. lappa. Their A. lappa is subdivided into four subspecies, subsp. lappa (Greater Burdock), subsp. minus (Hill) Hook. fil. (Lesser Burdock), subsp. nemorosum (Lej.) P.D. Sell (Northern Burdouck) and subsp. pubens (Bab.) P.D. Sell (Hairy Burdock).

In view of the uncertainties now and in the past, RSF and RHN in the County Fermanagh Flora did not attempt to distinguish species or subspecies other than A. minus subsp. nemorosum. Together they also decided to follow the example set in FNEI 3, in that plants referred to as A. minus in older literature were amalgamated into this taxon.

Flowering reproduction

Reproduction is entirely by seed, flowering taking place between July and September whenever the plant has sufficient accumulated photosynthetic reserves to enable it to bolt in vertical growth and flower. Under normal field conditions with moderate to high densities of grasses and herbaceous dicots, this can take anything between two to four or more years to achieve, depending upon local growing conditions (Gross et al. 1980; Gross & Werner 1983). The inflorescence heads or capitula are numerous and are arranged in a raceme that sometimes is somewhat corymbose, ie organised in a ± flat-topped manner. The flower heads are 1-2.5 cm in diameter, ovoid, sessile or sub-sessile, terminal or in the axils of upper leaves. The outer bracts (or phyllaries) are lanceolate, green, variably hairy, appressed upwards and strongly hooked at the tip (Gross et al. 1980).

The flowers are all perfect (bisexual), with tubular corollas 8-9 mm long, usually purple in colour. Although the flowers are self-compatible and can readily self-pollinate, an unquantified degree of out-breeding is known to occur and, indeed, some believe allogamy may be more common in A. minus. The range of insect visitors includes species of Lepidoptera, Hymenoptera and Coleoptera. Honey bees are often, but not always important amongst the visiting insects (Gross et al. 1980). The number of flower-heads per plant and seeds per head are very variable, but seed production figures are large and, in Canadian studies, averaged between 11,700 and 13,400 per plant (Gross et al. 1980).

Whatever the balance is between in- and out-breeding, the observed breeding behaviour produces a confusing array of intermediate forms which are fully fertile, breed true and persist (Rich et al. 1988). There is a resultant high degree of variation in hairiness, peduncle length and floret colour (typically purple or lavender).

The many involucral bracts or phyllaries surrounding the flower-head have long, stiff, spreading hooked tips and thus the whole head and its contained fruiting achenes are extremely efficiently dispersed as tightly clinging burrs on the coats or tails of passing animals (including ourselves). RSF regularly risks serious puncture injury in the autumn months when extricating these propagules from the hairy oxters of a displeased and very uncomfortable Scottish terrier who has amazing teeth and jaws. The burrs are light and possess many tightly packed, radiating spines, so they are also very well adapted to float in water. It is no surprise that Burdock spreads efficiently on river- and stream-banks as well as along other obvious linear habitats.

Seeds are mature by September and are shed continuously until the following spring (Gross et al. 1980). The achenes are relatively large, 5-7 mm long, brownish-grey with dark blotches and it is crowned with a pappus of deciduous bristles 1-3.5 mm in height (Clapham et al. 1987). They are also relatively heavy, weighing on average 0.0075 g, which makes them more than three times heavier than the achenes of Geum urbanum (Wood Avens) and over twice the average fruit weight of 15 other herbaceous plants of scrub and woodland margins compared by Salisbury (1942). The pappus attached to the achene has become modified into irritating, stiff, deciduous bristles and it has been suggested that the bristles may help minimise seed predation by mammals and birds (Gross et al. 1980).

Seed longevity estimates in NW Europe numbered four, one transient (less than one year), two short-term persistent (one-five years) and one long-term persistent (at least five years) (Thompson et al. 1997).

Fermanagh occurrence

A. minus subsp. nemorosum is frequent and very widespread in lowland Fermanagh, having been recorded in 138 tetrads, 26.1% of those in the VC. As the accompanying tetrad map indicates, the plant has been recorded particularly often around the shores of both Upper and Lower Lough Erne, but has also been noted in very many boundary areas of the VC.

British and Irish occurrence

A. minus s.l. (including subsp. nemorosum) is widespread, if often only locally frequent in most of lowland B & I. It becomes more scattered and occasional in Scotland and in W Ireland. It reaches its highest point at around 390 m in Cumberland (VC 70) (F.H. Perring, in: Preston et al. 2002). The distribution was considered stable by Perring in the New Atlas account. Stace (2019) remains aware that there is identification confusion between A. minus (Hill) Bernh. and A. nemorosum Lej., affecting their perceived distributions, but while he regards the position uncertain, he considers A. nemorosum much less common than A. minus in S Britain and S Ireland and absent from SW Britain and the Channel Isles. He also considers A. nemorosum the more common of the two in N Britain, the Isle of Man and N Ireland (Stace 2019).

European and world occurrence

The native distribution of A. minus takes in most of Europe to 66^oN in Scandinavia and Finland, together with adjacent parts of N Africa and W Asia, to the Caucasus and beyond (Clapham et al. 1987). It is widely introduced in N America, especially in E & C areas of the USA (Hultén & Fries 1986, Map 1853). It is Eurasian temperate in phytogeographical terms, although due to introductions, it is now disjunctly circumpolar. The uncertainty regarding the distribution of the main taxa, A. minus subsp. nemorosum (= A. nemorosum) and A. minus that occurs in its European range are closely mirrored in N America, where Burdock, undoubtedly of both forms under discussion here, and perhaps other taxa, occurred there back in the early days of English and French colonialism in the 1630s (Gross et al. 1980).

Toxicity

Burrs of A. minus readily attach to the coats of animals and human clothing and can cause irritation. When animals attempt to remove the burrs using their teeth and lips, the sharp hooked spines can cut the mucus membranes of mouth and tongue. This can lead to a condition known as 'granular stomatitus' that involves excess salivation, ulceration around the mouth and pain when attempting to eat. It is said to be of quite common occurrence, but is not regularly reported (Cooper & Johnson 1998). The hooked spines or barbs on the fruit are also said to contain a fluid which can induce localised allergic reactions, and the leaves when eaten are reputed to cause diuretic effects that led to A. minus being listed as a poisonous plant in Michigan, USA (Woodcock 1925).

Uses

Old English herbals attributed Burdock both culinary and medicinal properties, eg infusions of leaves, flowers or seed were used to prevent or cure rheumatism, coughs, asthma, blood and skin diseases, and to serve as a diuretic and laxative (Grigson 1955, 1987; Gross et al. 1980; Vickery 1995). A decoction of the roots was widely used in Ireland in particular as, "a forceful cleanser of the system and eliminator of boils and skin complaints" (Allen & Hatfield 2004). It was also used in herbal treatment of scrofula, arthritis, eczema, epilepsy, convulsions, hysteria, nervousness and anorexia nervosa (Mabberley 1997). In other parts of B & I, the leaves were used to poultice burns, cuts and boils, and the plant provided treatment for dropsy, kidney and urinary problems (Allen & Hatfield 2004).

Folklore

Like Galium aparine (Goosegrass), sticking Burdock fruits on the backs of people is a longstanding childish game, the sticking burr indicating the sweetheart, the cuckold and the clot (Grigson 1955, 1987).

Names

The genus name is derived from the Greek 'arction' or the Latin 'arctium' or 'arcturus' (both in Pliny), meaning 'a bear', referring to the woolly covering of the plant, and perhaps also to the clinging of the burrs to the same animal (Gilbert-Carter 1964; Stearn 1992). The Latin specific epithet 'minus' means 'small' or 'smaller' and 'nemorosum' means 'full of woods', 'shady' or 'growing in woods' from 'nemus', 'oris', meaning 'wood', 'grove' or 'glade' (Johnson & Smith 1946).

Prior (1879) very clearly explains that, "'Burdock' is a name that, properly speaking, is a pleonasm; for 'bur' and 'dock' meant originally, the one in French, and the other in the Germanic languages, a flock or lump of wool, flax or hemp combed out in carding: but upon 'dock' being extended from the 'bur-dock' to other broad-leaved plants, the first syllable was added, to distinguish this species (which pre-eminently deserved the name by the trouble it gave housewives) from the plants of the sorrel and other tribes.".

In common usage, the several species of Burdock are not distinguished but lumped and given a list of 53 local English common names and associated customs, the oddest of which must be the remarkable 'Burry Man' at South Queensferry in Lothian (see Vickery 1995, p.53; Grigson 1955, 1987).

Interestingly, one of the older names, 'hegerife', is Anglo-Saxon, a combination of 'hege' = 'hedge' and 'reafa', which means both 'a tax-gatherer' and 'a robber' (Prior 1879)! The 'Hedge robber' name possibly refers to the way in which the plant burrs plucked wool from passing sheep, or maybe that wool contaminated with burrs was useless to the shepherd. The name was originally applied to Arctium species, but it has been transferred at some unknown stage to Galium aparine.

Uses

Old English herbals attributed Burdock both culinary and medicinal properties, eg, infusions from leaves, flowers or seed were used to prevent or cure rheumatism (Grigson 1955, 1987; Vickery 1995). It was also used in herbal treatment of arthritis, eczema and Anorexia nervosa (Mabberley 1997).

Threats

None.

References

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