Giant Rhubarb

Gunnera tinctoria – Gunnaire

Also commonly known as Giant Rhubarb, Gunnera tinctoria is native to South America. It was introduced as a garden plant and popular across Ireland in Victorian times.  This introduced species that has become invasive in the west of Ireland. Although the exact date of its introduction to Ireland is unknown, it was first recorded in the wild in Ireland in 1939 on Achill Island. Pollen analysis suggests that it could have been present on Achill Island for 70-100 years.

Giant Rhubarb is found growing in damp, low-lying grasslands, the banks of rivers and lakes and in quarries and cliffs and have a preference for anthropogenic habitats. They are predominantly found in the west coast and is believed to be because of local climates. Gunnera tinctoria is not very tolerant of frost as it impacts seedling germination. It is believed that recent warmer winters has led to an increase in invasiveness of G. tinctoria in Ireland.

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Giant Rhubarb

Distribution

(National Invasive Species Database, National Biodiversity Data Centre, Ireland, Giant-rhubarb (Gunnera tinctoria), image, accessed 22 November 2022)

Species Recorded: 

Giant Rhubarb (Gunnera tinctoria)

Invasiveness rating:

Classified as a high impact invasive species by the Irish National Biodiversity Centre. Third Schedule listed species under Regulations 49 & 50 in the European Communities (Birds and Natural Habitats) Regulations 2011.

Status:

Established

Identification: 

  • Large herbaceous perennial which can grow up to 2 meters tall.
  • Leathery leaves of up to 2 meters in diameter.
  • Leaves borne on stems coming from large rhizomes.
  • It is deciduous with the leaves dying off in autumn (October) leaving the large brown rhizomes exposed.
  • Inflorescence development occurs early in the spring  with the fruits maturing in late summer/early autumn.

Reproduction: 

  • Plant flowers after 4-5 years.
  • It can reproduce by both sexual (seed) and asexual (vegetative) means.
  • Large numbers (up to 250 000 seeds per mature plant) of red or orange seeds are produced.
  • Small fragments of the rhizome have the potential to establish new plants.

Impacts:

To date there has not been a detailed assessment of the impact of Gunnera on native ecosystems. Previous studies indicate there is a reduced number of native species growing underneath large colonies.

In grassland sites it was found that former species-rich sites were replaced with a sparse cover of dicotyledonous species not found in un-colonised sites. Of particular concern are impacts associated with peat bog and waterside vegetation, as large dense colonies can rapidly dominate and displace important native species. On coastal cliffs, the main impact is caused by increasing the threat of erosion and loss of maritime species.

With the recent warmer winters and climate change, it is predicted that G. tinctoria will increase in range across the country.

Treatment:

Mechanical: The removal of seed heads helps to stop the spread of Gunnera tinctoria. Each large seed head produces up to 80,000 seeds, and by removing the seed head before the seeds are viable, can help prevent further spread. Removal (or ‘dead-heading’) has proven successful in controlling the spread of Gunnera on previous works contracts operated by JKI. JKI’s proposed method of treatment is removing the seed heads in June as a way of preventing further spread. – while treating the source plant chemically from mid-August onwards.

Chemical: Chemical control can be effective in treating large areas and is generally considered efficient and cost effective. JKI found that effective reductions in the growth of Gunnera tinctoria can be achieved with the use of glyphosate-based herbicides (RoundUp Biactive). The plant is best treated at the end of the season which is from mid-August onwards. Plants treated at the end of the growing season (Aug-Sept) showed no re-growth after one year, but after two years re-growth was observed. Despite no leaf growth the rhizomes do remain in the ground. The presence of a viable rhizome indicates a potential for regrowth and reapplications of herbicide will be required. Treatments and monitoring should continue over a four-to-five-year programme.   The major drawback of using chemicals is the impacts they can have on the environment, affecting not only the target species but other species in the neighbouring area. The use of chemical control on Gunnera has been investigated both in Ireland and New Zealand.

More photos: 

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