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Gunnera tinctoria (Molina) Mirb., Giant-rhubarb

Account Summary

Introduction, neophyte, a very rare garden escape.

1 April 1989; Northridge, R.H.; streamside, Clonelly, NW of Kesh.

April to May.

Growth form, identification and preferred habitats

G. tinctoria is one of the most popular, decorative, architectural-scale garden plants for large gardens and demesnes with appropriately spacious water features to display it. It was introduced from its native Chile and Argentina in the mid-19th century and was promoted in Victorian and Edwardian horticulture for planting around ponds and in damp areas. This huge, herbaceous, slightly tender, coarse perennial with thick, surface-spreading stems that are entirely rhizomatous, has enormous rhubarb-like leaves ± 2.5 m high and 1.5 m across, palmately 5-9-lobed, rounded in outline with sharply pointed, jagged-toothed lobes, cordate at the base, borne on 2 m tall, stout petioles, densely studded with short, conic, prickly spines that give the leaf stalk a reddish-brown appearance (Sell & Murrell 2009). The huge leaves and the up to 100 cm tall inflorescence at fruiting are deciduous, wilting and decaying in the autumn to produce a very heavy leaf litter that protects the surface-exposed horizontal rhizome from frost. Massive, winter-resting, perennating buds, up to 25 cm long are formed on the rhizome (Webb et al. 1988).

Throughout B & I, there has been a history of confusion between G. tinctoria and G. manicata Linden ex André from Brazil, that were introduced to European gardens at much the same time, but the latter does not set viable seed in B & I. Other differences that distinguish G. manicata include its leaves that are even larger (2 m across) than those of G. tinctoria and are pedately lobed (ie five leaflets arising from a single point on the petiole) and although the petiole is covered with numerous small spines, each with a red tip, the overall appearance of the leaf stalk remains green. G. manicata has a large, erect, conical inflorescence, while that of G. tinctoria is somewhat more cylindrical in shape and its numerous individual side branches or catkins are shorter and less open than those of G. manicata. Also, G. manicata is very rarely cultivated in Ireland and there is only one reliable record of its naturalisation in the country, in W Galway (H16) (Clement & Foster 1994; Scannell & Jebb 2000).

In B & I overall, G. tinctoria is a fairly frequent escapee 'over the garden fence' and has become naturalised and persistent in a variety of damp to wet and sometimes already shaded habitats, such as rough grassland on riverbanks, stream-sides, ditches and damp hollows, especially in lowland areas near the coast in Ireland (Webb & Scannell 1983; Jarvis 2011). Remarkably, considering the enormous size of the plant, it sometimes establishes itself on sheltered sea cliffs and on old quarry rock faces. In addition to self-sown seed, or in place of it, some extra-garden populations must originate from material discarded by desperate gardeners unable to accommodate the gigantic scale to which the plant grows when given suitably damp, nutrient-leached, boggy ground (Clement & Foster 1994; T.D. Dines, in: Preston et al. 2002).

The South American native range of G. tinctoria falls within the temperate climatic zone with a predominantly Mediterranean climate and a tropical moist climate subzone where annual rainfall is high (>200 cm) and mean annual temperatures are cool (10°-14°C) (Williams et al. 2005). Beyond its native range, the species grows and thrives in regions where winter temperatures are mild, any frosts are not severe, and both precipitation and humidity are typically high (Gioria & Osborne, 2013).

In garden growing experiments in Dublin, four days without water caused permanent wilting and desiccation of G. tinctoria leaves, with no sign of recovery of the outer edges of the leaves after re-watering (Hennessy 2009). Young seedlings were found to be unable to survive being kept two consecutive days under waterlogged conditions (Gioria 2007). Adults are rarely found in areas where the entire rhizome is permanently under water (Campbell 1994). G. tinctoria grows on a range of soil types, but in Ireland is typically found on mineral soils and relatively acidic, wet soils and is not common on organic/peat substrates (Gioria & Osborne 2013).

G. tinctoria is commonly associated with a range of ruderal or

competitor-ruderal species, including Apium nodiflorum (Fool's-water-cress), Galium aparine (Cleavers), G. palustre (Marsh-bedstraw), Persicaria maculosa (Redshank), Stachys sylvatica (Hedge Woundwort) and Urtica dioica (Common or Stinging Nettle).

In coastal areas, it is associated with cliff faces dominated by Armeria

maritima (Sea Pink or Thrift), Festuca rubra (Red Fescue) and Plantago species (Plantains) (Gioria & Osborne 2010, 2013). Its competitive ability, said to be only realised in wet and/or humid habitats (Campbell, 1994), particularly along water courses, coastal cliffs and in wet meadows, derives from a range of traits including its very large stature, perennial nature, capacity for fixing nitrogen through a unique intracellular symbiosis involving cyanobacteria (Nostoc), high relative growth rate, early season growth, dense leaf canopy, abundant litter and the persistence of its seeds and rhizomes (Gioria & Osborne 2013).

Gunnera tinctora is considered competitively superior to most associated species, although a recent study reported the displacement of long term G. tinctoria stands by Japanese Knotweed (Fallopia japonica) (Gioria et al. 2011). Any competitive potential of Japanese Knotweed for control of G. tinctoria is of no benefit, as it also is a highly invasive alien species.

Flowering reproduction

The massive, erect, cylindrical inflorescence of G. tinctoria is a spike-like panicle up to 100 cm in height that arises directly from the rhizome. It consists of a central rachis or stalk, densely covered with a very large number of slender, catkin-like branches 8-11 cm long, bearing very small, male, female and bisexual, petal-less flowers, each with parts in pairs. The flowers self-pollinate and produce masses of small, red-orange fleshy, drupe fruits. It seeds freely and prolifically in the mild, wet, growing conditions of W Ireland and the Channel Isles, where frosts are infrequent and not very penetrating. Each inflorescence can produce 8,000 seeds and an individual clone could muster 200,000 or more in a season. The edible fruits attract birds that feed on them and disperse the seed and wind and water are also likely to help dispersal to some extent. Webb & Scannell (1983) suggested that in the W of Ireland, seed might also be dispersed by sheep (presumably attached to their woolly coats).

Fermanagh occurrence

It is perhaps fortunate that G. tinctoria has only been recorded three times in Fermanagh, in widely spaced streamside sites in which it does not yet seem to be actively spreading. In addition to the first record above, the details of the other two sites are: Tattinweer Bridge, Tempo River, May 1989, RHN; and on the Crom side of Inisherk Bridge, Upper Lough Erne, February 2010, RHN & HJN.

Irish occurrence

In Ireland, G. tinctoria is more commonly found along roadside banks and ditches in the milder, damper W & SW than elsewhere. It is extensively established and naturalised, forming large dominant, ± single species stands in numerous parts (both coastal and inland) of W Mayo (H27) and W Galway (H16) (Connemara), and particularly frequent around Achill Island and Curraun. It is unquestionably invasive in these two latter VCs at least (Webb & Scannell 1983; Hickey & Osborne 1998; Reynolds 2002). It is distinctly alarming to see this huge, vigorous, nitrogen-fixing, invasive alien, thriving, seeding freely and expanding its territory year by year as an abundant, fully naturalised species in the W of Ireland. It is doing so along the roadside bank overlooking the coast at Leenane in W Galway (H16), a very much visited tourist area of the country, and a site where it was first reported as naturalised and plentiful by Praeger as long ago as 1939 (Praeger 1939). In a few areas of coastal Mayo, dense thickets of G. tintoria up to 3 m high are out of control and cover patches up to 0.5 ha in extent of upland well-drained ground, in which no other species can compete (Jarvis 2011). Sylvia Reynolds (2002), in her Catalogue of Alien Plants in Ireland, lists records from twelve Irish VCs, not including Fermanagh (H33).

Under experimental garden cultivation, G. tinctoria was capable of very similar growth and productivity in the more frost-affected Dublin area when compared with W Ireland, provided only that the water supplied matched the higher rainfall of the west (Campbell & Osborne 1993).

British occurrence

G. tinctoria is an occasional, widely scattered garden escapee and discard in western England, Wales, the Isle of Man and Scotland and, to lesser extent, in parts of C & S England and the Channel Isles. A high proportion of occurrences appear associated with or near the west coast, which fits with the species' restricted temperature limits. Inland sites must be sheltered from severe frost in some other way that allows the plant to survive. Confusion with or misidentification of the Brazilian species, G. manicata, may make the New Atlas hectad map inaccurate to an unknown extent, as for instance the latter species is regarded as the more frequent of the two in Devon (VCs 3, 4) (T.D. Dines, in: Preston et al. 2002).

European and world occurrence

G. tinctoria is native to South America, predominantly in Chile. It is also considered to be native to parts of Argentina, and in the Andean region of Colombia, Venezuela, Peru and Ecuador (Skeffington & Hall 2011; Gioria & Osborne 2013). Having been widely introduced to gardens, it now has a current global distribution spanning both the northern and southern hemispheres. In the northern hemisphere, it is found in England, Scotland and Wales, the Isle of Man, Isles of Scilly, the Channel Islands, France (Osborne et al. 1991), Spain (Sanz Elorza et al. 2001), São Miguel Island in the Azores (Schäfer 2002) and California (Howell 1970). In the southern hemisphere it is found in Tasmania (Duretto 2013), New Zealand (Webb et al. 1988) and the Chatham Islands (de Lange et al. 2011).

Threats

G. tinctoria is too infrequent an escapee to be a problem in Fermanagh, but it is invasive, very conspicuous and now extremely difficult or even impossible with current technology to control in many parts of W and SW Ireland.

Despite their long-known potential to escape and become naturalised and/or invasive, G. tinctoria and G. manicata are still advertised for sale on the internet as giant, tropical species suitable for planting around lakes and ponds in gardens. G. tinctoria received the Award of Garden Merit in 2006 from the Royal Horticultural Society. Responsible horticultural traders should not stock G. tinctoria at all, but its seed remains readily available via the internet and garden centres may inadvertently or in ignorance continue to trade the species. Gunnera tinctoria is listed on Schedule 9 of the Wildlife and Countryside Act in England and Wales, and therefore it is an offence to plant or otherwise to cause it to grow there in the wild.