Invasive Species Week: Garden Escapees
Our theme for Invasive Species Week 2023 is Garden Escapees. These are ornamental plants originally bought to enhance our gardens, but which have since spread into the wild where they are invasive and are causing biodiversity issues by out-competing native plants.
We’ll start with a common ornamental plant which has increasingly been causing problems for homeowners in Scotland – Bamboo.
Unlike many of the other invasive plants which were also originally introduced to parks during the 19th century, bamboo is still very popular as an ornamental plant and can be found in garden centres around the UK – including the running varieties which cause the most issues for homeowners (and their neighbours). However, as it rarely flowers, spread into the wild mainly occurs by fly-tipping of contaminated soil/garden waste.
The plant and rhizomes can be mechanically excavated, including by hand-digging in tighter garden spaces, but care must be taken to remove all the rhizomes as fragments have the potential to propagate into new plants. Additional chemical treatment may be required in hard-to-reach areas where excavation is not possible.
Native to coastal regions of East Asia, this woody perennial resembles the native dog rose and was introduced to parks and gardens in the UK during the mid-19th century. It has since spread to UK coastal habitats, such as sand dunes, both directly from gardens and via birds eating the seeds. Once established, it flourishes and forms dense thickets, which can smother whole dune systems. It out-competes native plants like marram grass, significantly reducing biodiversity and bare sand habitats for dune-dwelling birds and animals, threatening some rare species.
Due to the unstable and ecologically sensitive coastal habitats where Japanese Rose is often found, it is not always suitable to use heavy machinery for mechanical removal. In these areas herbicide treatment is required to limit the plant’s spread.
Montbretia is a plant with similar leaves to irises, but with smaller, clustered, bright orange flowers. Although originally from South Africa, it was first imported into the UK from France during the 19th century. However, it quickly spread from parks and gardens into the wild and today is widespread across much of the UK.
Once established, Montbretia forms dense clusters of growth which can cover wide areas. They are winter hardy and can thrive in poorer soils, outcompeting native plants, which can have a significant impact on some ecosystems. The plants produce dense clumps of fleshy corms, similar to bulbs, which can propagate into new plants. The plant can be spread when garden waste containing corms is fly tipped or carelessly disposed of by gardeners.
Montbretia plants can be excavated, but it is essential all the corms and roots are removed so the plants can’t regenerate. This often requires mechanical excavation to at least a metre. It can also be treated with herbicides but this needs to be done whilst the plants are actively growing during spring and summer to ensure it reaches all parts of the plant.
Curly waterweed is an aquatic plant which originates from South Africa and – despite being listed as an invasive plant – it is still widely sold in aquatics shops across the UK as an oxygenating pond plant.
It has strongly curved dark green leaves in a spiral arrangement and can grow up to 3m in water. It reproduces by fragmentation – detached stems sink and root, establishing new growth. It grows as a dense “canopy” in slow moving water bodies like canals, impeding water flow and blocking out light, outcompeting native plants. In areas of dense growth it can even reduce dissolved oxygen levels and harm aquatic life.
Remediation can be difficult, due to the way the plant spreads by fragmentation and the negative effect herbicide control might have on native plants. However, it can be controlled with an appropriate site-specific treatment and management plan. Any machinery, equipment and clothing must be checked for plant fragments before leaving the site.
American Skunk Cabbage
The final garden escapee featuring in our Invasive Species Week series is American Skunk Cabbage. Native to North America as its name suggests, American Skunk Cabbage was introduced as an ornamental pond plant in the 20th century but was still on sale in garden centres as recently as 2009.
It’s easily identified by its rosette of large leathery green leaves and yellow hood-like flowers, which have a strong skunk odour, giving the plant its name. It forms dense stands in wet, muddy areas surrounding ponds, streams and marshy woodlands, and can outcompete many of the sensitive native species growing in these habitats. It spreads by rhizomes and by seeds, which are carried by water, birds and animals.
As American Skunk Cabbage often grows in sensitive habitats, such as alongside water courses, herbicide use can be limited due to its impact on native plants. It can be removed mechanically, but the marshy habitat it prefers can make it difficult for heavy equipment to access. The plant is also capable of regenerating from its rhizome network, so care needs to be taken when digging to ensure no rhizome fragments are missed.
For more information on these and other invasive weeds, or to book a survey for your site our land, please visit our Invasive Weeds page.