Whether you call it - American skunk cabbage, swamp cabbage or yellow arum - this plant is causing controversy, as it has recently been classified as an invasive alien species!

American skunk cabbage (Lysichiton americanus), named for its sap and flowers’ unpleasant aroma, is used as an ornamental plant in streamside or damp gardens. Skunk cabbage first shows itself in April, its magnificent goblet yellow inflorescences and club-like flower spikes erupting from the earth, followed shortly by the emergence of large green lance-shaped leaves.

This plant originates from the western side of North America along the Pacific coast, ranging from Alaska to California. Naturally these plants grow in the transitional zone between semi-aquatic habitats such as swamps, wet woods, along streams and riverbanks, lakesides and ponds and in boggy and other wet areas.

Skunk cabbage is the mainstay of many a damp garden. It requires damp humus-rich soil in full sun or semi-shade and thrives best in nutrient-rich clay soils. These plants are tough: they can grow completely submerged in water, are hardy to -15°C and tolerate both oxygen-poor and nutrient-poor conditions. Skunk cabbages are also masters of reproduction. Beginning in July, the magnificent yellow inflorescences lower to the ground and develop into impressive seed heads which resemble unripe cobs of corn. Each seed head in turn contains between 100 and 650 berries. These seed heads quickly disintegrate; the berries either fall directly to the ground, germinating beneath the mother plant, or are quickly dispersed downstream.

It could be argued that these plants are a victim of their own success, so successful that they have become invasive. Outside of ornamental landscapes, these plants are able to out-compete native vegetation and over time, threaten surrounding biodiversity. EU regulations on invasive alien species became law in 2015. In March this year the EU commission published a list of species of EU-wide concern and skunk cabbage was among them. This once beautiful and reliable ornamental went from a highly prized botanical specimen to an invasive weed overnight.

Skunk cabbage was included on the March 2016 list, and as such requires very strict levels of control, including a ban on keeping, growing, cultivating, transporting, exchange or release into the wild. Thankfully gardeners who already have skunk cabbages in their gardens are not at risk from prosecution for possession as the regulation is not retrospective, but it must be controlled and not be allowed to spread.

So what can be done? Skunk cabbage has a tendency to grow in inaccessible streamside locations; combined with its toughness and deep roots, it is hard to control and eradication is extremely difficult. At RHS Garden Harlow Carr, we utilise a number of methods of control to stop the plant

spreading: we remove old flower heads to stop the development of seeds, and regularly inspect and remove seedlings. Chemical control is not an option because it would potentially cause damage to surrounding aquatic plants. Over time, the plan is to completely remove the skunk cabbage Lysichiton americanus from the streamside and throughout Harlow Carr. But don’t despair: a sterile hybrid is available known as Lysichiton x hortensis. Lysichiton x hortensis is a result of hybridisation between the yellow-flowered L. americanus and its white-flowered Asian cousin L. camtschatcensis. Since there is no seed, this plant can be planted without taking over, either within the garden or the wider environment.

Jobs for the Week

* Plant spring flowering bulbs.

* Divide herbaceous perennials.

* Collect and sow seed from perennials and hardy annuals.

* Clean out cold frames and greenhouses so that they are ready for use in the autumn.

With thanks to Matthew Brewer, Horticulturist at RHS Garden Harlow Carr

DIARY DATES

Saturday 1 – Sunday 2 October: The Autumn Food & Wood Festival Enjoy the very best the season has to offer with a food market specialising in local delights, cookery demonstrations using seasonal produce and live music. See handcrafted wooden items being created using traditional woodworking techniques. Younger members of the family can make their own willow wand while mum and dad watch demonstrations of a range of new cordless gardening gadgets from STIHL. Normal garden admission. Buy garden tickets online & save 10%.

Saturday 15 – Sunday 16 October: Alpine Weekend The Alpine Weekend will showcase the very best of these hardy plants in association with the Alpine Garden Society. Pick up interesting varieties and get advice from a host of UK alpine plant growers. On Saturday, enjoy an Alpine Garden Society show. On Sunday, the West Yorkshire group will be competing for the best locally grown alpines. Normal garden admission.