Gunnera Control in the Western Isles
Gunnera tinctoria, or “Giant Rhubarb” is a non-native invasive plant that was introduced to Hebridean gardens in the 1980’s. Since then it has spread rapidly in the wild and is now threatening our crofts, gardens and native habitats.
The North Harris Trust is co-ordinating a campaign to control Gunnera and encourages all residents to take action now.
Mature Gunnera plants are easily recognisable. In early spring the plant sprouts rapidly from thick sturdy surface roots called rhizomes. Supported by fleshy stems as thick as your wrist, the umbrella shaped leaves extend to more than a meter across. The stems and supporting veins on the undersides of the leaves are covered in small spikes. Within 3 years Gunnera matures and the dense clumps formed can reach over 2 meters in height.
Growth & Reproduction
Gunnera plants spread via their root system or rhizomes and entire new plants can regenerate from small fragments of broken root. They also disperse by seed. The tiny flowers are borne on erect cone-like seed heads up to 1m long growing from the base of the plant.
By August the seeds are ripe as can be seen by the orange/red colour. Up to 250,000 seeds can be produced by a single plant. Birds and running water are the natural carriers of Gunnera seed. However, in the Western Isles one of the main distributors of Gunnera has been man; either gardeners not understanding the potential risk or contractors transporting seed with soil and aggregates used for roadworks and landscaping.
With an established store of food in their rhizomes, Gunnera plants emerge early in the season and grow rapidly, overshadowing and out competing our native flora. It is particularly fond of roadsides and riverbanks and can quickly block drainage ditches. At the end of the growing season Gunnera plants die back. The resultant brown rotting vegetation is unattractive and the bare soil exposed to heavy winter rainfall is susceptible to erosion.
The maritime climate and poorly drained soils of the Hebrides provide idea growing conditions for Gunnera, combine this with the absence of natural enemies and a very efficient dispersal system and it is not difficult to see why the plant is spreading and how it has the potential to become a problem for land management and local biodiversity. In other parts of the world with similar growing conditions, including New Zealand and the west coast of Ireland. Gunnera is already a serious pest and the subject of control campaigns.
In August 2017, Gunnera tinctoria was included on the list of species of Union concern and is subject to EU Invasive Alien Species (IAS) Regulation (1143/2014). Strict restrictions apply to these species (subject to exemptions) so they cannot be imported, kept, bred, transported, sold, used or exchanged, allowed to reproduce, grown or cultivated, or released into the environment. Gunnera, is not prohibited if it is already in your garden. However, you must act responsibly and not allow or encourage it to grow or spread outside your garden.
The most effective way to apply herbicide is spraying. Before spraying check the forecast and make sure it will be calm and dry. Any rainfall within 24 hours of spraying may wash the herbicide off the plant rendering it ineffective. Calm conditions are required to prevent the spray drifting, missing the target plant and damaging the wider environment.
When spraying, the entire plant needs to be covered until the liquid begins to run off. Try to spray the underside of the leaves as well.
If the Gunnera is very close to waterways, beside other valued plants, or impossible to spray safely due to large size, herbicide can be applied as a stump treatment. To do this, cut all the leaves just above ground level and then paint the herbicide onto the exposed wounds. The herbicide solution used for stump treatment is much stronger than that used for spraying, but overall much less chemical is required. When you are finished cover the treated stumps with cut the cut leaves to protect your work from rainfall and see below for advice on how to handle the cut seed heads.
Herbicide is most effective when applied to Gunnera late in the growing season when the plants are fully grown and right up until they begin to die back for winter. This period normally starts late July and extends until early October. It can take a month or more before the herbicide application results in any visible effects. If you spray right at the end of the season the Gunnera will die back just like an untreated plant, but by next spring the herbicide should have done its job and the treated plant will not re-grow.
So far herbicide trails carried out by the North Harris Trust have concluded that glyphosate based products such as “Round Up” are successful and readily available.
Whichever herbicide product you use, remember herbicides are potentially dangerous to you, those around you, livestock and the environment. Always follow the manufacturer’s instructions and wear appropriate safety clothing.
This is not the most efficient method, especially when dealing with many plants. None-the-less digging up is an option if conditions prohibit the safe use of herbicides. Using a sharp spade to dig up seedlings and young plants is surprisingly easy. The Gunnera rhizome is often right on the surface. Be aware that the whole rhizome needs to be removed and do not leave any pieces on the ground or the plant will regenerate. It is best to bag-up the rhizome fragments and dispose of at a recycling centre or put them into your organic wheelie bin.
Removal of Seed Heads
NHT has always encouraged people to bag seed heads and dispose of them at the re-cycling depot, or in your organic wheelie bin. However, if you are dealing with large quantities, or transporting the seed heads to the re-cycling depot is a problem, we have discovered that once cut from the plant and left on the ground, they will rot very quickly- killing the seeds in the process. Once cut from the plant, the seed heads should be neatly piled up on the ground and completely covered. Leaves cut from the parent Gunnera plant are ideal for this. Neatly piling the fruiting bodies and covering them, reduces airflow and speeds up the rotting process. It also hides the seeds from birds that will potentially transport them elsewhere. Make sure these piles are not placed beside watercourses or on bare gravelly ground. An area of level grassland is best.
Just removing the seed heads will not kill the parent plant, but it will stop dispersal of seed, preventing the establishment of new populations. So even if you don’t want to tackle the whole plant this stop-gap measure is worthwhile.
Whatever control method you use, always check the location the following season for re-growth and seedlings. Re-treat as necessary and your dedication will pay off.
For further information on Gunnera, please follow the links below:
North Harris Trust Gunnera Treatment Advice Leaflet