Galeopsis tetrahit L., Common Hemp-nettle
Account Summary
Native, occasional. European boreo-temperate.
1860; Smith, T.O.; Ardunshin.
June to December.
Growth form and preferred habitats
This bristly hairy, annual ruderal weed produces simple or branched stems, 30-75 cm tall, that are usually swollen below the nodes. The opposite, nettle-like, heavily toothed lanceolate to ovate leaves are pubescent on both surfaces, but the bristly hairs are more abundant on the upper side. The bristly hairs that cover the plant are sufficiently stiff to penetrate the skin when handled, but they carry no sting. G. tetrahit is a tetraploid with a chromosome count of 2n=32 (O'Donovan & Sharma 1987).
G. tetrahit is occasional in a wide variety of semi-natural habitats ranging from damp semi-shade of woodland clearings, hedges, riverbanks and lakeshores to the very different situation of open, somewhat disturbed ground on agricultural field margins and damp waste places. It often grows on moisture-retaining peaty soils, although it can grow in almost any damp, moderately fertile, mildly acid soil with a pH between 4.5 and 7.0, such as would be suitable for arable agriculture. Its occurrence in Britain is centred on heavily disturbed, productive vegetation (O'Donovan & Sharma 1987; Grime et al. 1988, 2007).
While this summer annual favours open habitats, it produces only one tall-growing population each season and thus is not able to compete and survive to reproduce in more heavily disturbed ground that is regularly grazed, mown, or trampled. The phenology involves germination stimulated after low winter temperatures in early April, followed by a first large flush of growth in early spring and later smaller flushes at regular intervals throughout the growing season (O'Donovan & Sharma 1987). Also, despite individuals reaching a metre or so in height, G. tetratit does not form large, conspicuous patches or stands. The established strategy of G. tetrahit is categorised as C/R, Competitive-Ruderal, ie intermediate between a competitor and a purely ruderal species (Grime et al. 1988, 2007).
Polyploid origin
The distinctly odd variety of habitats occupied by Common Hemp-nettle may be a reflection of the polyploid origin of this species, an amphidiploid formed by a complicated cross between two diploids, G. pubescens and G. speciosa, to produce a new tetraploid species (Muntzing 1930). The same two parent species at some stage also crossed to give rise to the rather similar G. bifida. The two hybrid products are inter-sterile with their parents (though not with one another) and, being morphologically separable, they are both given specific rank.
Since many field botanists struggle to distinguish these two species, it is not surprising that there are only a very few, scattered records of the hybrid between them since it was first discovered in Britain in 1955. The hybrid is markedly, but not completely sterile and usually occurs as single plants without any back-crossing (Stace et al. 2015). Through the isolation created by occupying fundamentally different open and semi-closed vegetation habitats, populations of G. tetrahit may well be in the process of establishing genetic barriers to gene flow, which will eventually lead to divergence and further sympatric speciation (Stace 1975).
Flowering reproduction
Flowering takes place from July to September, the stalkless, bisexual flowers being borne in dense clusters (contracted cymes called verticillasters), in the axils of the upper leaves. The calyx is 12-13 mm long and elongates further when in fruit. It is tubular and has five unequal lobes, each one linear-lanceolate and with a spine-like apex. The calyx is also densely covered with stiff eglandular hairs and minute glands. The corolla, which is 15-23 mm long, is rather variable in colour, ranging from purple to mauve-pink, white or variegated (Sell & Murrell 2009). The flowers are self-fertile and although they produce nectar and pollen they are mainly autogamous (ie they self-fertilise). Autogamy in G. tetrahit is more pronounced than in related species (O'Donovan & Sharma 1987).
Seed output is modest for such a large plant. Each flower produces the four nutlets typical of the family and, therefore, an average plant might produce a total of around 2,000 nutlets, although this is very variable with environment and will be significantly reduced at lower temperatures (Salisbury 1942; O'Donovan & Sharma 1987). Nutlets are single seeded dry fruits (ie achenes) and are often simply referred to as seeds. The level of G. tetrahit seed production quoted is less than a tenth of the annual seed output of Senecio sylvaticus (Heath Groundsel), for instance, while Epilobium angustifolium (Rosebay Willowherb) produces vastly more – around 76,000 seeds/annum (Salisbury 1942, pp. 188, 191).
Seed are dispersed by wind and by flowing water, being able to float for up to two days (Grime et al. 1988, 2007). They can also survive ingestion by stock animals and birds (Ridley 1930). In cultivated fields, seed is scattered by farm machinery during seeding, tillage and harvesting operations and may also be transported over long distances as a contaminant of crop seed and livestock feed (O'Donovan & Sharma 1987).
Seeds of G. tetrahit display strong dormancy, germination being only possible after cold stratification. Germination is favoured by lower temperatures, the optimal fixed temperature being 13°C, although a fluctuating temperature regime alternating between 5° and 15° was even better (O'Donovan & Sharma 1987). Estimates of buried seed survival in soil vary greatly from transient (less than one year) to long-term persistent (at least five years) (Thompson et al. 1997). At least some seed is almost certainly long-persistent (Roberts 1986)
Fermanagh occurrence
In Fermanagh, where arable fields are now very rare, the G. tetrahit agg. is thinly and rather widely scattered in the lowlands, but it has been recorded in 69 tetrads (13.1% of those in the VC), 57 of them with post-1975 records.
G. tetrahit s.s. has only been discriminated from the species aggregate on six occasions in separate tetrads and thus it requires much further study in the VC. The details of the records are: NE shore of Cornaleck Td, E of track, Upper Lough Erne, 23 July 1986, EHS Habitat Survey Team; ditch at Curragh and Drumguiff Tds shore, 6 August 1986, EHS Habitat Survey Team; waste ground, Killycarnan, near Lough Nacallagh, Upper Lough Erne, 4 September 1988, RHN; E of Rossmore Point, Castel Archdale, Lower Lough Erne, 26 July 1995, RHN & HJN; sand pit at Pubble Bridge, Tempo River, 20 August 1999, RSF & RHN; and halfway between Newtownbutler and Galloon Bridge, Upper Lough Erne, 1 September 2001, RHN & RSF.
British and Irish occurrence
The New Atlas map shows that while G. tetrahit agg. is rather widely recorded in Ireland, at the same time it is much more consistently and frequently recorded in the north in comparison with elsewhere on the island, as is also the case with G. speciosa (Large-flowered Hemp-nettle). The current author (RSF) cannot suggest any obvious reason why this should be the case, since in Britain this taxon is common and widespread throughout lowland regions with acidic soils.
The recent BSBI sampling re-survey effort in Britain (Change in the British Flora 1987-2004) has found that G. tetrahit agg. is markedly declining there (a calculated Change Factor of -23), although again no explanatory mechanistic factor is identified by the editors of the published report (Braithwaite et al. 2006).
European and world occurrence
Nowadays, G. tetrahit is widespread across most of Europe, although absent or rare in the south and on many of the Mediterranean islands (Sell & Murrell 2009). It is not clear where it originated, but Hultén & Fries (1986, Map 1586) show it more definitely present in Denmark and Scandinavia, especially around the Baltic Sea, as if this might be significant. They also question whether G. tetrahit extends eastwards to Siberia, although this is claimed by Sell & Murrell (2009).
Hultén & Fries encountered a difficulty in their mapping as they remark on the problems that exist in distinguishing G. tetrahit from G. bifida, and they map the latter as the much more widespread of the two closely related species (Hultén & Fries (1986, Map 1587).
O'Donovan & Sharma (1987) appear clear that in their view G. tetrahit is native in Europe and temperate regions of Asia, and that it has been found as a naturalised introduction mainly in the northern continental United States, Alaska and Canada, where it occurs to the northern limits of agriculture.
Names
The genus name 'Galeopsis' is an ancient Greek name derived from 'gaľe', 'weasel' and 'ŏpis', 'appearance', thus translating as 'weasel-resembling' (Stearn 1972). The Latin specific epithet 'tetrahit' may be associated with 'teter', meaning 'foetid', a reference to the goat-like smell of the root (Gilbert-Carter 1964). However, it may also refer to the tetraploid nature of the species, meaning 'four times', ie four sets of chromosomes (Gledhill 1985).
G. tetrahit has a total of 17 English common names in Britten & Holland (1886), mostly associated with 'Dead-nettle' (ie not stinging nettle), such as 'Bee-nettle', 'Blind Nettle', 'Day-nettle', 'Deye-nettle', 'Dog-nettle'. A couple of names actually suggest the plant does sting, eg, 'Stinging Nettle' and 'Sting Nettle'. 'Hemp-nettle' is a name first given by Gerard (1597), who also called the plant 'Bastard Hemp' and 'Wild Hemp'. Other interesting names that have no or poor explanation include 'Glidewort' and 'Hoyrope', another Gerard (1597) name more often applied to Eupatorium cannabinum (Hemp-agrimony).
Threats
None.