This week is national Invasive Species Week. Today’s blog, a collaboration between SNH and the Scottish Invasive Species Initiative (SISI), takes a look at invasive non-native species living on our riverbanks and lochsides, and explains the control methods used to tackle these problematic plants.
SNH is the lead partner in the SISI project, working with other partners including Fishery Boards and Trusts, and the University of Aberdeen. SISI operates across northern Perthshire, Angus, Aberdeenshire, Moray and Highland. The project is developing invasive species management skills among a network of enthusiastic volunteers and partners, who are supported, trained and equipped to take on invasive species control at a local level.
In Scotland, there are three invasive plants that often cause the most damage to riverbanks and lochsides: Japanese knotweed, giant hogweed and Himalayan balsam (images L-R).
Japanese knotweed was introduced into the UK in the early 19th century as an ornamental plant. Since then, it has spread quickly and is now common across much of the country. It can be very imposing to look at, with long bamboo-like stems that can grow to over 7 feet tall. But it can develop into very dense stands that don’t allow native plants to grow. This species is difficult, but not impossible, to eradicate. The best method for treatment is to use stem injection. ‘Stem injectors’ hold a cartridge of herbicide which delivers a measured dose through a fine needle (imagine a high-tech water pistol). Although this method is time consuming, it is very effective at delivering the herbicide directly into the stem, where it is taken down into the rhizome. Application of herbicide is strictly controlled and can only be done by qualified personnel, so SISI has been investing in local people and enabling them to achieve their herbicide qualification.
Japanese knotweed being treated
Giant hogweed was first recorded in the wild in the late 19th century and was also originally introduced as an ornamental plant. It can grow into a huge plant – with flowering stems typically 2-3m tall, flower heads up to 80cm across, and lower leaves can be more than 1m wide. Each plant can produce 20,000-30,000 seeds and some seeds can survive more than 3 years in the soil. This species also grows in such dense stands that native plants are prevented from growing. The most effective control method is through herbicide treatment. This is applied by knapsack sprayer, using a spot spraying technique where a controlled amount is applied directly to the leaves of the plant. Great care must be taken as there is a risk of skin burns from the plant’s toxic sap.
Volunteers are key to controlling these plants
Volunteer days are hosted by SISI project officers and local fishery trusts where teams of volunteers get suited up and stuck in to treating infestations of giant hogweed. While this is a serious business, the emphasis is on having an enjoyable day with plenty of tea breaks and the essential chocolate biscuits. It is these volunteers who are key to the long-term control of this plant. Inspired, enthused and trained, the hope is they will continue tackling their local stretch of river for years to come until the hogweed is finally gone.
Different plants require different control methods
Himalayan balsam was introduced to the UK as a garden plant in the early 19th century and first recorded in the wild in 1855. It has very pretty flowers, which is no doubt one of the main reasons that it was introduced to the UK! This plant grows up to 2m tall and has explosive seed capsules, which can eject seed up to 7m away from the parent plant. It can grow in such high densities that when it dies down in winter, it can expose riverbanks that are bare of plants because native species haven’t had the opportunity to grow. Himalayan balsam has a very shallow room system and if water levels rise during the winter and spring months, bare riverbanks can be at risk of erosion.
Unlike Japanese knotweed and giant hogweed, the shallow root system of Himalayan balsam means it is easy to pull plants out by hand. SISI is working with communities throughout northern Scotland to tackle infestations of this plant. The model for this is to work in partnership with local groups (e.g. angling associations, Britain in Bloom groups, community payback and wildlife groups) to organise conservation volunteering days and encourage local people to come along and help pull out the plant. These have been successful with good numbers of people coming out, lots of plants being removed, lots of fun had, and awareness of Himalayan balsam and its impacts being raised locally.
Communities are encouraged to continue this vital work
Initially, events are organised and coordinated by the local SISI project officer or other partner staff. However there is a gradual shift with the emphasis for organising moving over to the local community, who are keen to take on this work long-term, creating a sustainable model for successful long-term control of this species.
These invasive non-native plants can quickly take over an area. Image courtesy of the Tweed Forum.
In Scotland, several organisations coordinate the approach to tackling invasive non-native species. SNH is the lead for terrestrial species (which includes those on riverbanks and lochsides). As part of our work in this area, SNH’s Freshwater and Wetland Group recently supported a student of Stirling University (Dr Alex Seeney) in investigating responses of young salmon and trout, and aquatic and terrestrial invertebrates, to Japanese knotweed and Himalayan balsam. The project was carried out on small streams across central Scotland. This research showed a consistently negative impact of these species on the diversity of both terrestrial and aquatic invertebrates but suggested that the young fish may have been more resilient, due to their opportunistic feeding methods. These findings support management of these invasive non-native species, as heavily invaded sites were of a lower ecological quality, and intervention work would prevent these species spreading.
SNH’s Freshwater and Wetland Group supported research by Dr Alex Seeney
For more Information on invasive non-native species associated with riverbanks and lochsides, you can also visit:
Scottish Natural Heritage – invasive non native plants
GB Non-Native Species Secretariat
Papers from Dr Alex Seeney’s research project
Scottish Invasive Species Initiative
SEPA practical guidance on the Controlled Activities Regulations
Scotland's Nature 
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