Getting to grips with an alien

It looks like a plant from another planet. The European Union has placed American skunk cabbage on a list of invasive alien species of concern.  Stan Whitaker, our non-native species adviser, looks at the implications for nature conservationists and gardeners. 

American skunk cabbage in flower. © Dick Shaw

American skunk cabbage in flower. © Dick Shaw

The striking yellow, lantern-shaped flowers of American skunk cabbage appear before most of our native plants have started to grow. Their skunk-like odour smells like rotting flesh, which attracts flies to pollinate them.  Skunk cabbages are often planted beside garden ponds and burns, and have a habit of escaping into the wild.  The berries are transported downstream by water but, in its native range in North America, they are also dispersed by birds, squirrels and bears.

More than half of the British flora is alien, that is, plants that were introduced by people. The vast majority coexist harmlessly with our native plants.  Only a small minority become invasive and seriously affect our native wildlife and environment.

The extensive colony at West Loch Tarbet. © David Knott

The extensive colony at West Loch Tarbet. © David Knott

A visit to West Loch Tarbet in Argyll will convince you that the American skunk cabbage can become invasive. Almost a hectare of marsh at the head of the loch has been taken over by one of the most extensive colonies of the plant in Europe.  Mature skunk cabbage colonies form large dense patches, which suppress the native ground flora through shade.  A student at Aberdeen University is researching whether skunk cabbage may also produce chemicals which discourage other plants from growing nearby.

In contrast to most other alien plants, skunk cabbage is restricted to habitats that are of high nature conservation value, such as wet woodlands and fens. Initial colonisation is slow, which gives the impression that the species is less of a threat than it really is.  But as numbers build up the population grows exponentially, ending up with almost complete ground cover.

American skunk cabbage is a non-native species management priority in Scotland. Although it is fairly widespread, most populations in the wild are small and still feasible to control.  It’s already being controlled on around a dozen protected nature sites and, in 2017, the Tweed Forum launched an initiative to control it across the entire river catchment.  To protect vulnerable wetlands, we need to know where it grows and find any new populations quickly.

You can get involved by reporting sightings of skunk cabbage growing in the wild.

Look out for the yellow lanterns in the early spring and the giant cabbage leaves later in the year. See our webpage for information on how to report sightings. Not every sighting will be a priority for control but your records will help us to build our knowledge and to prioritise future action.

Recording invasive plants can be fun! © SNH

Recording invasive plants can be fun! © SNH

Gardeners can get involved too by moving their plants away from running water and always dead-heading them after flowering. Although not specifically aimed at skunk cabbage, the Scottish Government’s Be Plant Wise campaign provides relevant advice for gardeners on how to stop the spread and compost with care.

From August 2017, garden centres were no longer permitted to sell American skunk cabbage. Gardeners who had the plants before the ban are allowed to keep them, but must act responsibly and not allow them to grow or spread outside their garden.  Dawyck Botanic Gardens on upper Tweed has removed the plant entirely from their collection, as a precaution.  A non-invasive alternative, the Asian skunk cabbage with white lantern-shaped flowers, is still available to buy from nurseries.

Download this leaflet for further information on how to identify skunk cabbage, where to report sightings, and how to manage any plants on your land, or in your garden.

Invasive Species Week 2018 runs from 26 to 29 March. For more information on what’s happening, search of the hash tag #InvasivesWeek.

If you liked this story, find out about the eradication of the highly invasive floating pennywort on Scotland’s Environment blog.

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