American Skunk Cabbage Project
”At times our volunteers were knee deep in squelching mud with huge grins on their faces!
Background to project
American skunk cabbage is a tall, aquatic herbaceous plant native to Western North America. It gets its name from its reputation of smelling quite foul! Its flowers have a distinctive odour likened to a combination of smells from skunk, carrion and garlic! However, its not its unique smell which is of immediate concern here in the UK. American skunk cabbage is an invasive plant which can over time dominate aquatic and marginal habitats, pushing out native species and reducing biodiversity, so much so it is now listed as an Invasive Alien Species (IAS) of Union concern (Regulation (EU) 1143/2014).
In 2018 Yorkshire Wildlife Trust (YWT) secured funding from the EU-funded Rapidlife project to undertake catchment-scale control of invasive species American skunk cabbage. The project focused on the Aire and Calder catchment in West Yorkshire. At the beginning of the project there were less than 10 records of this species within the catchment but it was suspected that this species was vastly under recorded, its true impact is still unknown. It also provided an opportunity to tackle this species before it became more established and widespread. YWT actively chose to undertake this project using a volunteer-led approach, by utilising the existing volunteer programme and developing new roles for volunteers to support the control of the species.
Volunteers have always been an incredibly important and valued part of YWT’s Invasive Non- Native Species Project. Over the last 5 years volunteers have mainly been involved with undertaking INNS surveys along Yorkshire’s watercourses, during this time surveying over a staggering 150km of Yorkshire’s rivers. With this project we were keen to explore new opportunities for volunteers to support all aspects of invasive species management, from surveying, to control, to restoration of effected habitats.
The project started with initial training workshops in spring of 2018. Volunteers were invited to learn how to identify some of our most common INNS, including American skunk cabbage. They were trained to record these species methodically through walkover surveys and how to upload any records to INNS Mapper to share these records with wider stakeholders. After an initial classroom workshop volunteers were invited for a field surveying day in Leeds to try out their identification and surveying techniques. During this day volunteers recorded a number of American Skunk Cabbage plants within the area – a great start!
Next volunteers were strategically deployed to survey watercourses where existing records of American skunk cabbage had been identified. Volunteers uploaded their records to the online web-tool INNS Mapper, providing up to date data for species distribution and size of populations, allowing an assessment to be made about the overall spread of this species in the catchment and which populations could be prioritised for control
Based on the results of volunteer surveys it became clear that Meanwood Beck, North of Leeds, was a hotspot within the Aire and Calder catchment for American skunk cabbage. The project decided to focus much of its activities within the Meanwood Valley.
Two distinct sites were identified as priorities for action. The project staff engaged with the landowners at each site who were receptive to the project’s aims and understood the need to reduce the impacts of this species in the Meanwood Valley.
The population towards the top of the catchment is a horticultural collection. Despite the EU regulation applying to this species, exiting populations can still be cultivated as long as the population is strictly controlled and it does not expand or spread further.
The second population towards the middle of the catchment began as a horticultural collection many years ago but has since expanded quite extensively throughout this area. We worked closely with the landowner and after a number of site visits were able to begin control of this population.
There are a number of ways to control American Skunk Cabbage from manual excavation to use of appropriate herbicides. You can find out more about best practice guidance here: Rapid American Skunk Cabbage Good Practice Guidance
However there isn’t a huge amount of evidence to suggest which method is most effective and how specific methods may be best applied to different scenarios. The project was keen to add to the available evidence for the control of this species. We decided to test the effectiveness of manually digging these plants out versus the use of herbicide application.
With support of our volunteers we set up a number of quadrats at locations within the Meanwood Valley and in other locations in the Aire & Calder catchment which we would be working. 2x2m quadrats were placed randomly within and around the edges of the area infected by American Skunk Cabbage. The number of quadrats used was dependent on local factors such as the size of infection and site accessibility. Vegetation coverage surveys were undertaken using the DOMIN scale to assess the abundance of different plant species.
The quadrat surveys were undertaken in May and June 2019 and will be repeated at the same time in 2020 to assess changes in vegetation types and the effectiveness of control measures.
In May and June 2019 we spent a number of days working with a large group of volunteers, including YWT volunteers, the Friends of Meanwood Park (link), Leeds City Council Rangers and YWTs Tomorrow’s Natural Leaders to dig out the population in the middle of the catchment.
Manual excavation is challenging work, particularly in damp and boggy conditions which the skunk favours. At times our volunteers were knee deep in squelching mud with huge grins on their faces. We found that the extensive and fragile roots of the plant were at times difficult to extract fully, particularly for larger individuals within the population. Skunk Cabbage can propagate from fragment of root, so it was important that we removed as much as possible to prevent regrowth later that year.
Plant material which was removed were collected together and composted at a number of suitable locations on site, away from watercourses or damp ground and away from well walked areas of the site.
A small amount of herbicide application was undertaken to populations of skunk cabbage by staff with the appropriate training and qualifications. One round of glyphosate application was applied at 6 l/ha during July and August.
At time of writing (May 2020) repeat vegetation coverage surveys have not been completed, so we do not have this data available as yet.
However, from observations in early autumn 2019, it appeared there was some initial regrowth from areas under manual excavation compared to areas control using herbicide. Perhaps those difficult to remove roots had successfully propagated towards the end of the summer. Regrowth observed was small and a repeated manual excavation may now be easier.
We are planning on continuing to monitor the areas treated, and repeat vegetation coverage surveys at the original quadrats. We will also be continuing to work with our volunteers to control this species at these sites.
We are also keen to expand our knowledge of this species elsewhere in Yorkshire and with the support of volunteers, we will be conducting further surveys throughout the region to understand it’s extent and prioritise future control efforts.
How can you help?
We would like you to submit any records of American Skunk Cabbage in Yorkshire to our webtool INNS Mapper. You can also get in touch with the team directly with records by emailing a photo, description and location to Invasives@ywt.org.uk
If you would like to support the American Skunk Cabbage or our wider Invasive Non-Native Species project please get in touch with email@example.com