Gunnera tinctoria (giant rhubarb)

Gunnera tinctoria, a large leaved plant from Chile growing up to 2 m in height, became a very popular ornamental species in gardens and parks in temperate areas from the middle of the nineteenth century. By the 1930s, and more extensively since the 1960s, it became naturalized to form dense monospecific stands on the west coast of Ireland, southwest England, the west coast of Scotland, the Azores, the coast of California and New Zealand where dense local infestations are found on coastal cliffs. Habitats with moderate to heavy rainfall and frost-free conditions for most of the year and relatively small variations in temperature are ideal for plants to naturalize. The species can spread from rhizomes discarded from gardens and from seed disseminated by birds and is a threat to native vegetation. Its sale and further cultivation is now prohibited in Ireland and New Zealand where eradication programmes are underway.

Numerous taxonomic affinities have been suggested for the genus Gunnera. The history of these and current understanding of the phylogeny has been reviewed by Fuller and Hickey (2005). Gunnera was placed in the monogeneric family the Gunneraceae, by the 1830s but other taxonomists later argued that it should be placed in the Haloragaceae because the genus has a distinctive pseudo-polystelic vascular system, which has been claimed to indicate an aquatic ancestry, such as from the Haloragaceae. However, striking differences between Gunnera and haloragaceous taxa have been noted by numerous researchers particularly the floral vasculature of Gunnera considered to be too reduced to provide a useful comparison with Haloragaceae. Nevertheless, Gunneraceae has remained associated with Haloragaceae in much of the taxonomic literature by placement in the same order, despite a long list of character differences between the two families. The International Plant Names Index records this species as Gunnera tinctoria (Molina) Mirb, as a species of the Gunneraceae, based on the treatment of the species by Mirbel (1805). The species name was derived from the basoionym Panke tinctoria Molina, a member of the Saxifragaceae. This nomenclature is also followed by the Integrated Taxonomic Information System. Both Flora Europea and Tropicos (Missouri Botanical Gardens) however place the genus in the Haloragaceae, referring to the species as G. tinctoria Mirb.

This monogeneric family, the Gunneraceae, comprises about 50 species, which show great variation in size. The larger, rhubarb-like, stand forming species (up to 6 m high) are typical of areas ranging from Hawaii to Central America and South America, whereas smaller, more slender species are found in New Zealand, South-East Asia and the southernmost parts of South America. The eleven endemic species of New Zealand are all small, often 10 cm high, and form stolons, whereas the larger species are rhizomatous. Gunnera is traditionally divided into six subgenera that are largely geographically distinct (Fuller and Hickey, 2005). G. tinctoria is included in the sub-genus Panke. This consists of more than 19 species in South America and the Hawaiian and Juan Fernandez Islands These species produce leaves up to 2 meters across, supported by erect petioles that arise near the apex of a pachycaul. Infloresences are large (up to 2 m long) panicles of hermaphroditic, but occasionally, unisexual, flowers. The subgenus Panke lacks stolons.

G. tinctoria or giant rhubarb is not related to rhubarb (Rheum rhabarbarum), but as its English name implies it is similar in appearance.

G. tinctoria is a large herbaceous plant with thorny leaves and stems that forms dense colonies and shades out other plants. Williams et al. (2005) provide the following description: “It is deciduous, with short, stout, horizontal rhizomes which give rise to stout petioles up to 1000 (1500), 1 mm x 45 mm that are studded with short reddish prickles. The leaf lamina measures up to about 0.8 m x 1.0 m with 5–7 lobes. It is very coriaceous, and hairy beneath, especially on the veins. Massive over-wintering buds—up to 250 mm long—accumulate on the rhizomes and they are covered in pinkish, pinnatisect scales that extend to the broad leaf midribs. The flowers are borne on panicles less than or equal to 1 m long; usually three or four per plant. Individual flowers are densely packed, sessile, apetalous, with minute sepals, and only about 1 mm long. Style length is slightly less than the ovary. The drupes are reddish, oblong, and 1.5 mm–2 mm long. Each contains a single ovoid and flanged seed of 1.2 mm x 1–1.5 mm diameter, weighing 4 mg. The hundreds of fruit are regularly arranged and densely packed on the infructescence”.

G. tinctoria is reported as native on both sides of the Andes from Colombia to Chile (Williams et al., 2005) but has an origin in southern Chile between 36 and 42° south. Because of its striking appearance the species was widely introduced as an ornamental, planted by lakes, streams and in bog gardens, initially in North America and Europe but later in New Zealand and Australia. From these introductions it has become naturalised as a garden escape. In UK it is now common in the moist south-west of the country in the Scilly Isles and Cornwall, but is also found in Devon, Dorset, Somerset, Oxfordshire, Berkshire, Surrey and in East Anglia, west to North Wales, Central England, Yorkshire and on the west coast of mainland Scotland and a number of the Western Isles (see Global Biodiversity Information Facility (2008) for records). There is also a record of the plant in the east coast of Scotland. In UK the plant ranges from 49.9 to 58.2° north. G. tinctoria is currently considered invasive on the west coast of Ireland where the most extensive colonies are found west of County Mayo and County Galway (Hickey and Osborne, 2001). These are areas where freezing temperatures are uncommon because of the Gulf Stream current; its eastern expansion appears to be related to intolerance of low temperatures. However it is also found on the east coast of Ireland but it is not considered invasive here (Invasive Species Ireland, 2008).

Elsewhere in Europe the species has proved a successful invader of the Azores archipelago, particularly in San Miguel Island and is also naturalized in France and Spain (Sanz-Elorza et al., 2001; Williams et al., 2005).

In USA it is currently restricted to coastal counties of California (Calflora, 2008). The species is now widespread in New Zealand (Williams et al., 2005) having been introduced as a waterside plant in parks, botanic gardens, and in large public and private gardens throughout the country. It has been found naturalised in Hawke’s Bay, Taranaki, Wanganui, Banks Peninsula, Dunedin and Stewart Island. Williams et al. (2005) show occurrences along the western coasts of both North and South Islands. In these areas it grows from sea level to 380 m above sea level. The areas where it is present cover a range of summer temperatures but lack extremes, and they have abundant and relatively even, year-round precipitation.

The position in Australia is less clear. ISSG (2008) cites “Department of Environment and Conservation” indicating that this is an Alien Invasive Plant in New South Wales. However, a fact sheet warning of the dangers of introducing the species, prepared by the State of Victoria Department of Primary Industries in December 2007 indicated that G. tinctoria, “is not known to have naturalized in Victoria or Australia” (Plant and Robertson, 2008).

Osborn (2006) provides evidence to suggest that G. tinctoria population increases in Britain and Ireland, north west Europe, North America, New Zealand and the Azores have occurred at a similar time, during the early 1960s, irrespective of geographical location. Analysis of environmental correlates associated with the current/past distribution of this species shows that this has been associated with increases in rainfall and temperature and is largely independent of edaphic factors. Future projections, based on current climatic data, suggest that the distribution will increase at all current locations, as well as proposing that new areas may become invaded.

All cases of introduction and spread of G. tinctoria have been through planting as an ornamental followed by the species becoming naturalized as a garden escape. In Britain and Ireland this species was introduced into cultivation around 1850, but was not recorded in the wild until the early 1900s indicating an establishment phase of 60 years. Significant records of this species in the wild were not, however, found until the 1950s indicating a further lag phase of 40 years before any significant spread (Osborne and Gioria, 2005). The earliest known record of a naturalized stand in Ireland is from 1939, at Killary Harbour in County Mayo, although it is likely that it had been naturalized prior to this. Based on pollen identification in conjunction with analysis of soil sequences suggests it could have been present in Ireland for 100 years (Hickey and Osborne, 2001). G. tinctoria was first collected in the wild in New Zealand in 1968. By 1988 it had been found naturalised in Hawke’s Bay, Taranaki, Wanganui, Banks Peninsula, Dunedin and Stewart Island and has continued to spread (Williams et al., 2005).

G. tinctoria is a popular ornamental species, particularly for larger gardens and landscaping by streams, ponds and other water features. It has therefore become widely available through the horticultural trade since being introduced to cultivation during the 1800s. In a suitable habitat and climate it can be spread accidentally beyond gardens in garden waste or contaminated soil. Seeds are also spread in the faeces of birds e.g. blackbird in New Zealand (Williams et al., 2005). The species is readily available through the nursery trade in many countries including Ireland and UK and can be ordered via the internet. It was also available via wholesale nurseries in New Zealand at least until the late 1990s.

The horticultural trade in Ireland has introduced a voluntary ’Code Of Practice‘ which expects member traders to “avoid selling non-native plants that are known to be invasive, and are already posing a threat to native biodiversity or if invasive non-native plants are sold, to ensure they are clearly and correctly named, and labeled to identify the dangers to the wider environment if these plants were to escape from gardens or horticultural premises” (Invasive Species Ireland, 2008). Following concerns at the extent of invasions in County Mayo, the Department of National Parks and Wild Life Service planed to use Section 52(6)(a) of the Wildlife Act 1976 to make regulations in 2008, under which possession or introduction of Gunnera will be prohibited.

The UK Department for Environment, Farming and Rural Affairs published a consultation document on restricting the cultivation or sale of non-native species in November 2007 as part of a review of the Wildlife and Countryside Act, 1981 (www.defra.gov.uk). The proposal is to add G. tinctoria to the list of species whose introduction into the wild is prohibited and to ban further sales of plants. G. tinctoria is classed as an ‘unwanted organism’ under the Biosecurity Act 1997 of New Zealand and is included on the National Pest Plant Accord List making selling, propagating or distributing it illegal throughout the country (Williams et al., 2005). It is now included on regional Pest Plant Management Strategies developed by regional councils. For example in Taranaki, it was made illegal to propagate, distribute or sell the species in the region, and it has been obligatory since July 1, 2003 for land occupiers throughout Taranaki to destroy it (Law, 2003).

Although it is considered as high risk for the State of Victoria and other areas of south eastern Australia the plants and seed of the species are permitted entry into Australia and are not listed as noxious by any state laws (Plant and Robertson, 2008). G. tinctoria is not scheduled as a noxious weed in USA.

G. tinctoria may easily be confused with G. manicata Linden ex André, a closely related species in the sub-genus Panke from Brazil that is also widely cultivated. This is also a large leaved species.

This south Brazilian species can grow even taller than its Chilean relation but it looks very similar and is often mistaken for it. According to Sykes (1969) there are a number of characters that can be used to distinguish between the two. The rhizome of G. manicata is thicker and more massive. A better distinguishing character is the large laciniate or jaggedly cut scales at the base of the leaves and inflorescence. In G. manicata there is a prominent development of membranous ’webbing‘ between the main lobes of the scale whereas in G. tinctoria this is not well developed and so the lobes are often almost free to the main rachis of the scale. The lengths and thickness of the inflorescence branches are also diagnostic. The diameter of central part of main inflorescence axis is 3.0 to 3.3 cm in G. manicata but 4 to 4.5 cm in G. tinctoria. Length of typical inflorescence branches from about half way up the main axis is 9.5-11 cm compared to 5-7 cm, while the diameter of central part of the typical middle inflorescence branches ranges from 3-4 cm and from 5-7 cm in the two species, respectively.

Although G. manicata is not generally considered to be invasive there are indications that it may naturalise in New Zealand. Webb et al. (1988) did not record it as naturalised in New Zealand but Williams et al. (2005) report observations from the early 2000s of seedlings near Wanganui while a few G. manicata seedlings have been found near planted specimens by Pigeon Bay domain, Banks Peninsula. This has led Williams et al. (2005) to suggest that all large-leaved Gunnera species should be banned from propagation and sale in New Zealand.

Williams et al. (2005) is the most comprehensive source of information on habitat. In its native range in Chile, G. tinctoria grows on forest margins adjacent to wetland areas, stream-sides and, particularly, on bluffs and talus slopes. In Ireland it is now found on coastal cliffs, waterways, roadsides, wet meadows and derelict gardens and fields. On the west coast of New Zealand the species is common along stream banks and ’wet cliffs‘ (Williams et al., 2005).

Williams et al. (2005) found no information about predators in their review of the literature. In New Zealand, the plants relative lack of insect pests and diseases made the species attractive to horticulturalists.

G. tinctoria is an environmental weed so its economic impact is largely through the cost of control measures rather than a reduction of agricultural or forest productivity. However there are sites in Ireland where the species has invaded native species-rich native grassland, (Hickey and Osborne, 1998), which reduces the value of the land for grazing. National or regional governments and hence taxpayers in, for example, Ireland, UK, the Azores, Australia and New Zealand have to bear the cost of regulation, raising public awareness and of control measures.

In New Zealand there is concern that G. tinctoria can block drains and streams (Weedbusters, 2003).

Invasion of coastal cliffs in New Zealand has led to a change in the appearance of the landscape (e.g. in areas of high importance to tourism such as Taranaki. The species has been reported here to obstruct access to natural and recreational areas (Weedbusters, 2003).