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.

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.

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 Nationa 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.

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).

Genetics

The chromosome number of G. tinctoria has been determined as 2n = 34 (Dawson, 1983). No information on genetic variation or hybridization in this species has been found. 

Reproductive Biology  

G. tinctoria reproduces sexually via seed as well as vegetatively, from root fragments, rhizomes and even leaf cuttings. Birds and small mammals find the tiny red fruit nutritious and assist greatly in seed dispersal. Plants flower after about 4 to 5 years under New Zealand conditions (Williams et al., 2005) and flowers are either female or hermaphrodite on the same plant and are, therefore, gynomonoecious (Webb et al., 1988). Hymenopterous insects, particularly bees, are probably the main pollinators. A single G. tinctoria may produce 250,000 seeds in a year (Law, 2003). Fresh seed germinates readily and shows no dormancy (Williams et al., 2005).

Physiology and Phenology  

Generally the species lies dormant under winter conditions in Ireland, New Zealand and UK (Williams et al., 2005). It is deciduous in areas with harsh winter frosts. However active leaves may be maintained in some areas of New Zealand.

Nutrition  

The genus Gunnera is unique in the plant kingdom by acquiring nitrogen through symbiosis with nitrogen-fixing cyanobacterium, Nostoc punctiforme (Osborne et al., 1991). The structurally unique stem glands of Gunnera function as the conduit through which cyanobacteria infect the host. As the genus Nostoc is cosmopolitan and common, it is likely to be present in the wet habitats in which Gunnera grows (Bergman and Osborne, 2002).

Environmental Requirements  

These aspects have been covered elsewhere in this data sheet. Further details are given in 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.

Environmental

• Ornamental

Materials

• Dye/tanning

Ornamental

• Propagation material

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.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

Measures are in place in Ireland and New Zealand and are being considered in UK to prohibit the sale and planting of G. tinctoria as an ornamental (see detailed discussion under Risk of Introduction). Public awareness campaigns through production of fact sheets and articles in the media are key tools in preventing further planting of the species e.g. a Victorian Alert Weed Fact Sheet in Australia (Plant and Robertson, 2008), fact sheets in various districts of New Zealand (Timmins and Blood, 2003; Horizons, 2006).

Eradication

As G. tinctoria has ’unwanted organism‘ status under the New Zealand biosecurity law, councils and land-owners have an obligation to undertake eradication where possible. Such programmes are included in regional Pest Plant Management Strategies (RPMS) developed by regional councils. A typical example has been described by Law (2003) for Taranaki and includes mechanical removal of plants and application of herbicides, particularly on coastal cliffs. The Taranaki RPMS lists the species as a ‘containment pest plant’. This recognises that it is already locally abundant but aims to prevent its spread to new areas in the region and, where practicable, to reduce existing infestations. From July 2003, land occupiers have been required to destroy all adult and juvenile plants on their land. Two other regional councils (Environment Waikato and Greater Wellington) specify its control at sites of ecological value, and two more (Horizons Regional Council and Bay of Plenty) have opted for regional surveillance status (Williams et al., 2005).

Control 

Physical/mechanical control

As the seed of G. tinctoria has little dormancy the first action should be to remove flowering spikes from any plants. In New Zealand this has been found to reduce re-infestation from the seed bank after two years (Williams et al., 2005). Follow-up monitoring and further cutting back of plants will be needed. Where mechanical removal is used it is important to uproot the rhizome.

Movement control

Legal restrictions on sale of G. tinctoria are in place in Ireland and New Zealand (see section on Risk of introduction for further details). Plant and Robertson (2008) argue that similar restrictions should be introduced in Australia.

Chemical control

Use of herbicides is recommended in New Zealand, particularly in areas where access for mechanical control operations is difficult and potentially dangerous, such as on unstable wet cliffs. Horizons District Council recommends applying a broadcast application of triclopyr 600 EC with organo-silicone spreader, or metsulfuron-methyl (Horizons, 2006).

Law (2003) reported that Taranaki Regional Council found use of picloram to be a useful treatment. A 1.5-2% solution of glyphosate with a suitable wetter has also been applied by knapsack sprayer. On steep cliffs, abseilers cut the leaves off plants and immediately treat the cut surface with 25% glyphosate. The growing tips are not cut off to avoid them dropping and re-growing. Results to date have been promising; however, some native seedlings have come up in the cleared areas and there was some re-growth on about 4.5% of the treated G. tinctoria plants. After spraying, some rhizomes take up to 18 months to rot.

Control by utilization

Williams et al., (2005) have observed that leaves of G. tinctoria are browsed by cattle and noted a lack of seedlings in pasture - despite the many large plants adjacent and the consequent seed rain. This suggests that grazing will restrict encroachment of the weed in to pasture.

26/06/08 Original text by:

Charlie Riches, Consultant, UK