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28 Aug 2019

Tackling invasives

Written by Hannah Patterson
A close-up of Rhododendron ponticum, showing a cluster of purple flowers and glossy green leaves.
‘Rhododendron ponticum’: one of Scotland’s invasive species
At number 81 on our 100 Ways list, we’re tackling invasive species, with the help of Hannah Patterson, Ranger for our Perthshire properties.

Invasive species essentially refer to plants or animals that are often not originally from here but thrive and spread rapidly causing negative impacts on our native wildlife. This can happen through increased competition for food, territory and other essential resources. 

Once invasive species become established in an area, their removal can be expensive and time-consuming. However, it’s very necessary to protect our indigenous species. The North Perthshire Ranger Service has been working in partnership with the Scottish Invasive Species Initiative to tackle four species found along the banks of the River Tay in Dunkeld: Himalayan balsam, Japanese knotweed, Rhododendron ponticum and American mink. Here are a few tips on how to identify and appropriately manage these species.

A large coverage of Himalayan balsam, with green leafy stems and delicate purple flowers.
Himalayan balsam

Himalayan balsam 

This plant is identifiable by the reddish tinge to the stalk and leaves. Found between June–October, this plant has large pink flowers. Although bees love Himalayan balsam, this isn’t really a good thing as it means they will favour it over other plants, including our native species. As a result, less indigenous plants are being pollinated, which reduces their populations and overall biodiversity. Himalayan balsam can be removed by simply pulling it from the ground. Once removed from the ground, the stems should be snapped and the plants can be left on site but off the ground, ie on a tree stump, to avoid the roots restabilising. Plants can emerge from May and often germinate in spaces that have been cleared, so it’s best to monitor the area throughout this time. Don’t pull Himalayan balsam between August–September as this could result in accidental dispersal of seeds. It may take several years of pulling for the plant to be eradicated from the area but it will work, eventually.

A stand of Japanese knotweed growing through bracken in woodland. The stems look a little like bamboo with green spade-shaped leaves.
Japanese knotweed

Japanese knotweed 

This plant has hollow stems that look similar to bamboo and bright green leaves in the shape of a spade. It spreads through a large rhizome (a modified subterranean plant stem that sends out roots and shoots from its nodes). Left untreated it can grow in dense clusters, leaving no space or light for other ground flora. It should not be dug out as this could encourage the species to spread. Instead, the stems should be injected with neat pesticide. This is then absorbed into the root system and eventually kills off the plant. This can take multiple sessions from July–September over several years, depending on how well-established the plant is in the area. Please remember that the use of pesticides should only be done by a trained individual with the appropriate equipment and protective clothing.

A close-up of a rhodendron branch, showing the leaves and a cluster of purple flowers.
‘Rhododendron ponticum’

‘Rhododendron ponticum’

Rhododendrons are much-loved garden plants but the ponticum variety is highly invasive. Like Himalayan balsam, they produce large purple flowers that attract bees, consequently reducing the pollination of native species. Their large leaves block sunlight from reaching the ground, resulting in a vast reduction of flora species. Rhododendrons have thin waxy leaves that are not palatable to our native species and are not a suitable habitat for nesting or other animals. Consequently, where rhododendron is present it’s difficult for other species to survive. To manage this species effectively, the roots must be removed by either being dug out by hand or with machinery like a stump grinder. Alternatively, pesticides can be used. As rhododendrons are a type of tree with a solid timber trunk, not hollow stems like the knotweed, holes must be drilled for the pesticide to be delivered to ensure it reaches the root system.

A lady places an American mink trap on a riverbank.
American mink traps

American mink

Since their release from fur farms in the 1930s, American mink populations have grown across Scotland. Highly adaptable and opportunistic predators that thrive both on land and in rivers, mink are direct competition for otters and have had a catastrophic impact on water vole populations. The Scottish Invasive Species Initiative has provided us with mink rafts across our properties that help us monitor the presence of mink in the area. They work by floating in the water which attracts the curious mink. When mink enter the raft, they step on a clay pad inside, leaving footprints behind. The rafts are regularly checked, and upon confirmation of mink footprints the clay pad is replaced with a trap, which is then checked every 24 hours. Once a mink has been trapped, it’s illegal to release it due to its non-native invasive status, so it must be humanely dispatched by a trained individual.