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Galeopsis speciosa Mill., Large-flowered Hemp-nettle

Account Summary

Introduction, archaeophyte, very rare. Eurosiberian boreo-temperate, but widely naturalised, including in W & N Europe and N America.

1947; MCM & D; peaty potato patch near Derrygonnelly.

August and September.

Growth form and preferred habitats

Previously, this coarsely hairy winter or summer annual with golden-green, ovate-elliptic, nettle-like leaves broadly wedge-shaped at the base was a more frequent weed, especially of potato and turnip fields, but also in other broad-leaved arable crops, particularly those growing on improved, fertile, manured, sandy or peaty soils. It survives best and can be locally common on winter-wet clay and peat soils. It can also occur very rarely on roadsides and on disturbed soil tips (Crawley 2005).

G. speciosa is one of only four cold-winter tolerant archaeophytes in the flora of B & I (Stace & Crawley 2015), the species normally germinating in late April and behaving as a summer annual. G. speciosa is a rather late spring developer, slow to germinate and not reaching the flowering stage until the height of summer.

G. speciosa is similar in appearance to the closely related native annual, G. tetrahit (Common Hemp-nettle), but it is rather larger, with more acutely-pointed leaves and much larger flowers with a corolla 27-35 mm. The hollow, much-branched stems of G. speciosa are rough and bristly hairy, and the nodes are swollen. The stems additionally carry yellow-tipped glandular hairs. The long yellow corolla, usually with a purple mark on the lower lip and the upper lip strongly hooded and covered with hispid hairs, are very distinctive features (Melderis & Bangerter 1955; Blamey & Grey-Wilson 1989).

The decline in arable farming using traditional methods of rotation and manuring and the advent of modern methods of weed control together have contributed to the decline, almost to extinction, of this distinctive and attractively flowered annual weed. However, as it also used to occur on other types of marginal and disturbed ground, these changes in farming and weed control measures do not completely account for the species near total decline and disappearance since the 1960s (BSBI Atlas 2; New Atlas).

Species status

In previous years, G. speciosa was regarded by Irish botanical authorities as a definite or possible introduction (An Irish Flora 1977; Cen Cat Fl Ir 2), whereas in recent years British Flora writers continued to regard the plant as native (Clapham et al. 1987; New Flora of the BI 1997). The New Atlas editors reassessed the status and now recognise this agricultural weed as being an ancient introduction or archaeophyte in both B & I (Preston et al. 2002, 2004).

Variation

G. speciosa is a diploid plant with 2n=16 chromosomes and is one of the parent species of the natural tetraploid hybrid, G. tetrahit, the other parent being G. pubescens (O'Donovan & Sharma 1987). In fertile, damp, arable situations, plants can grow up to 100 cm in height, overtopping the potato or other root-crops with which it frequently co-habits (Salisbury 1964, p. 317). There is great morphological plasticity, however, with respect to local growing conditions, and in the current author's (RSF's) albeit limited experience in Donegal, the plants encountered were small, reaching only around 15-30 cm or so in height and they clearly were not capable of strongly competing with potato plants.

The corolla colour can also vary: usually it is yellow with a purple blotch on the lower lip, but it can be a pale yellow with deeper yellow protuberances and the purple patch on the lower lip, or it can be yellow with the lower lip variegated white, or the whole lower lip can be purplish in colour (Sell & Murrell 2009; Stace 2019).

Flowering reproduction

Flowering takes place from July to September, the flowers being produced in dense whorls in the axils of the upper leaves and bracts. The conspicuous bi-coloured corolla and the nectar it contains attract large bumble-bees with a tongue at least 10 mm long to the plant (Faegri & van der Pijl 1971, p. 226). If insect-pollination fails to occur, the flowers are self-fertile and may self-pollinate, although seed yield is greatly depressed by selfing (Muntzing 1930; Garrard & Streeter 1983). A well-grown plant may bear up to 50 or more flowers and seed production can be prolific.

Each fruit of G. speciosa contains four nutlets or achenes (single-seeded dry fruits). The nutlets measure 3 × 2 mm, are ovoid, trigonous, papillate, tuberculate and dark brown in colour (Butcher 1961). The seeds are originally dispersed by wind and ground water flow. In agricultural settings, they are also scattered by farm machinery operations on the crop. In some situations, eg in grain crops such as oats, the seed may be transported over long distances as a contaminant of crop seed (O'Donovan & Sharma 1987).

There are only three estimates of G. speciosa seed (nutlet) persistence in the recent survey of NW European soil seed banks. Two studies regarded it as transient (ie persisting for less than a year), while the third source could not assign a seed-base type to the species (Thompson et al. 1997). G. speciosa seed does, however, demonstrate strong dormancy (O'Donovan & Sharma 1987) and is long-persistent. There is considerable variation in germination, which generally occurs around late April in open conditions in cultivated or recently disturbed fertile soils. Only a proportion of the buried seed is released from dormancy each winter and manages to germinate in spring, the remainder surviving buried to appear in later seasons (Karlsson et al. 2006).

Fermanagh occurrence

While previously this was a quite frequent weed, G. speciosa is now exceedingly rare and has not been seen in Fermanagh since 1988. There are a total of six records in the Fermanagh Flora Database, four from the 1947-53 period, when Meikle and co-workers were recording, and two from the 1980s. The details of the other five records are as follows: field near the river below Manyburns Bridge, 1951; fields N of Corraslough Point, Cam Td, Upper Lough Erne, 1951, and seen here again on 2 September 1984 by RHN; waste ground near Lisnaskea, 1947-53; and quarry beside Keenaghan Lough, 3 km E of Belleek, 17 September 1988, RHN.

British and Irish occurrence

The decline of G. speciosa to occasional, sporadic or casual status appears to have occurred throughout its range in B & I, as shown by many recent county Floras (eg Trueman et al., 1995; Brewis et al., 1996; Crawley 2005).

While the Cen Cat Fl Ir 2 indicates that Large-flowered Hemp-nettle has previously been recorded at least once from a total of 27 Irish VCs (Scannell & Synnott 1987), the species has always been much more prevalent in the north of the island. The 1962 BSBI Atlas showed that G. speciosa had much greater presence in Cos Donegal (H34, H35) and Londonderry (H40) than elsewhere in Ireland when the survey was carried out in the late 1950s and early 1960s (Perring & Walters 1962, 1976). The New Atlas hectad map confirms this northern pattern has been maintained up to 2000 (Preston et al. 2002). The new map certainly indicates that Co Londonderry (H40) is now the Irish stronghold of this rather attractively-flowered Hemp-nettle.

In Britain, G. speciosa was previously common and widespread throughout, although always regarded as casual in SW England. The New Atlas map shows that the distribution is better recorded than in the 1962 Atlas, but also that the species presence has shrunk rapidly to rarity during the intervening four decades.

The distribution pattern of the surviving G. speciosa populations in B & I is a very curious one, especially when compared with those of related annuals and other weeds of arable cultivation on acidic soils, eg Chrysanthemum segetum (Corn Marigold), Fumaria bastardii (Tall Ramping-fumitory), F. muralis (Common Ramping-fumitory) and Galeopsis bifida (Bifid Hemp-nettle), and G. speciosa has a definite northern and lowland distribution, its remaining southern strongholds being in N Wales, Lancashire and the peaty fenland south of The Wash. It is also prevalent in lowland Scotland, from Dumfries northwards and in the NE from Perthshire to Aberdeenshire (New Atlas).

The most recent BSBI sampling re-survey (unfortunately confined to Britain) indicates that G. speciosa remains a frequent weed of arable crops towards the upper limits of cultivation, often on damp, peaty soils, but that elsewhere it is rare or scarce and declining rapidly. Compared with other arable weeds, it is late in flowering and setting seed and it only really succeeds among root crops and in some game cover and exotic crops such as Phacelia tanacetifolia (Braithwaite et al. 2006).

European and world occurrence

G. speciosa is widespread in Europe N of the Alps and other southern mountain ranges and it extends into temperate W Asia. The species must be cold-tolerant, since it penetrates well beyond the Arctic Circle in N Scandinavia. The map published by Hultén & Fries (1986) shows it also present in S Iceland, although it is not mentioned in the Flora of Iceland (Löve 1983). It is mentioned in the earlier The Flora of Iceland and the Faeroes as having been accidently introduced and only found around the capital Reykjavik (Ostenfeld & Gröntved 1934). It has been introduced to N America, but the published map shows it very rare and scattered there (Hultén & Fries 1986, Map 1585).

Toxicity

In common with other members of the family, G. speciosa contains irritant oils which can prove poisonous – dangerously so to young stock such as pigs and horses, although there are no recent reports of poisoning except in Russia (Cooper & Johnson 1998).

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 'speciosa' translates as 'showy' or 'handsome' (Gilbert-Carter 1964).

Threats

Modern farming methods of cultivation and herbicide weed control have reduced this annual weed species almost to extinction.