Gunnera, commonly known as ‘Giant Rhubarb’ is native to South America and was introduced to Harris as an ornamental garden plant around 20 years ago. However, it has become an invasive species, spreading out of gardens, onto neighbouring crofts and hill ground in some areas. Growing to a height of 2m and with its large leaves, Gunnera can shade out and out-compete native plants and can also cause problems by blocking drainage ditches and access tacks. The plant thrives in our mild wet climate and can spread rapidly. Some research into the species has revealed that Gunnera has been causing problems in Western Ireland, where the climate and habitat is similar to ours.
Having tested numerous treatment methods on the plant across Harris we have produced the following leaflet as a guide to give some more information on the plant itself and how best to eradicate it. Gunnera Control in the Western Isles
We have recently teamed up with Scottish Natural Heritage to carry out a project to quantify the extent of the problem of Gunnera across the Western Isles and how best to tackle it. The first stage of this is to map the spread of the plant and although we have done a good idea of the plant distribution in Harris we are looking for residents across the Western Isles to help up map the spread of Gunnnera in other areas. Please submit all reports of the plants along with details as to its location to the Scottish Environment Website via the following link Invasive Non-native Species Reporting
The following tried and tested methods do work and NHT encourages anyone with Gunnera growing on their property to deal with it quickly to prevent further spread.
Use of Herbicides
The most effective and widely available chemical for spraying Gunnera is ‘Round-Up’, although other glyphosate based herbicides can be used. Always ensure good spray coverage of the entire plant (until ‘run-off’ occurs) and try get spray onto the undersides of the leaves and the buds found at the base of the plant. We’ve found the greater the leaf area of the plant the more effective spraying is. So don’t be tempted to cut off some of the leaves before spraying.
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. i.e. cut all of 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 fruiting bodies.
Gunnera following stump treatment with herbicide (blue dye)
So far our experience suggests the best time for treating Gunnera with chemicals is late in the growing season (from late July until early October), when the plants are fully grown, but before the leaves die back.
Always be aware that glypohsate and other herbicides are potentially dangerous to you and the environment. Fine dry, calm weather is required to prevent herbicide being washed off by rainfall into watercourses and to prevent spray drift. Whenever using herbicides always follow the manufacturer’s instructions for mixing and conditions of use and always wear the recommended safety equipment.
Although physically demanding, simply digging the plants up is effective on seedlings and young plants and it is often surprisingly easy to dig up mature plants using a sharpened spade. Just be aware that the whole of the rhizome needs to be removed or the plant will regenerate. Do not leave fragments of the rhizome on the ground or they will take root. Put them in your organic bin or take them to the Urgha re-cycling depot. If fruiting heads are mature these should be carefully treated as described below.
Follow up treatments
Whatever method is used, follow up treatments are often required. Gunnera is very resilient and if the plants have reached maturity their seeds will be in the surrounding area and are likely to germinate. However, every time the plant is treated it will get weaker, it will produce little if any seed and will be more susceptible to the next treatment. If the plant is allowed a full season to fully recover, your initial effort will have been wasted. So don’t be disheartened, if the plant re-grows, treat it again!
Removal of Fruiting Heads
Fruiting Head in flower. Seeds turn red/orange when ripe.
NHT has always encouraged people to bag fruiting heads and dispose of them at the re-cycling depot. This is still the best practice. However, if transporting the fruiting 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 actually rot very quickly- killing the seeds in the process. Once cut from the plant, the fruiting heads should be neatly piled up on the ground and completely covered up. Leaves cut from the 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 fruiting 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.
For further information on Gunnera, please follow the links below: