Along the canals, invasive plant species pose a threat as they can cause navigation and water control problems, a reduction in habitat availability, the quality of the water and, in some cases, are a threat to human health.
Himalayan Balsam (botanical name Impatiens glandulifera) is an invasive plant introduced to Britain in the mid 19th Century by Victorian gardeners. It is the tallest annual plant in the UK, growing to a height of over three metres.
Himalayan Balsam grows in dense stands, crowding out native plants. It can take over whole areas of river and canal bank over spring and summer before dying back in the winter. When Himalayan Balsam dies back it leaves banks bare, that it previously dominated, having crowded out native species. With no roots left to strengthen the bank, the bank becomes more susceptible to erosion.
This is a problem for local wildlife as the natural biodiversity of the area is reduced because native plants cannot compete with Himalayan Balsam. Biodiversity is further impacted by the way in which bees are drawn to Himalayan Balsam over other plants, reducing the pollination of native species.
In addition, the loss of cover on the banks over winter is the loss of a habitat for animals. Animal habitats can also be negatively impacted by the increased bank erosion, which in turn leads to increased sedimentation that can suffocate fish spawning beds.
Himalayan Balsam can impact boaters in a number of ways. Most obviously its spread along whole sections of river or canal bank can make accessing the bank from the towpath or water difficult during spring and summer. The increased bank erosion in winter can lead to navigation problems and an increased need for dredging.
Himalayan Balsam can be found across much of England and Wales. It spreads quickly as it has up to 800 seeds per plant, which are released explosively from seedpods and can travel for up to seven metres from the plant. If the seeds land in a stream, river or canal they will be taken downstream where they will start a new colony, one of the reasons this plant is so difficult to control.
Giant Hogweed (Heracleum mantegazzianum) is an invasive species that was introduced to Britain in the 19th Century as an ornamental plant. Heracleum mantegazzianum is the name usually used to refer to giant hogweeds but there are as many as four other species in Britain as well as the native hogweed, which is much smaller.
Giant Hogweed poses a threat to human health as it has phytotoxic sap. If this sap comes into contact with skin it can cause severe burns and blistering as it makes the skin extremely sensitive to sunlight. This usually happens within 48 hours of skin coming into contact with the sap.
In the event of skin contact with the sap, the affected area should initially be covered to reduce exposure to sunlight, only uncovering the skin to wash it with soap and water, which should be done as soon as possible. Medical advice should be sought if a rash develops, blistering occurs or you begin to feel unwell.
Native hogweed can cause rashes but theses are generally less severe than the damage caused by Giant Hogweed.
Giant Hogweed will die back after seeding, where it has taken over whole areas of bank this can contribute to bank erosion as the bank is weakened without plant roots to contribute to its integrity.
Giant Hogweed is widespread across the lowlands of the UK and continues to spread rapidly. Each plant produces between 10,000 and 50,000 seeds, this abundance leads to its quick spread, although plants only seed once. Seeds like moist, fertile soils in partial shade, such as river and canal banks and are highly germinable.
Japanese Knotweed (Fallopia japonica) is a non-native invasive plant originating from Japan, Taiwan and Northern China. Bought over here in the 19th century, the plant was grown and sold as an ornamental plant. Japanese Knotweed is a tall herbaceous perennial plant, which forms dense bamboo like thickets. This species is widespread across the UK, with noticeable infestations.
Able to survive naturally within the UK, Japanese Knotweed has unfortunately become a common sight across much of the UK, even colonizing alongside our waterways. Japanese Knotweed can grow in dense thickets which can outcompete and shade out native species, reducing biodiversity.
Japanese Knotweed contributes to bankside erosion as well as increase the likelihood of flooding. The plant is able to find weak spots in man-made materials such as cracks and defects and can often be seen growing through asphalt, patios, landscaped areas and even concrete. This can pose a threat to buildings, structures and our waterways, potentially degrading locks, walls and banks, affecting integrity of a waterway.
Despite only being able to spread through vegetative means in the UK, all it takes are a few fragments from the stem or rhizome to sprout in a new area, making it extremely difficult to effectively control the spread of this plant. Japanese Knotweed grows particularly well on disturbed areas of land which can include urban areas, waste sites, road sites, railways and riverbanks.
Floating Pennywort (Hydrocotyle ranunculoides) is a species native to North America. It was first discovered naturalised in Essex in 1990 and had been introduced to the UK as an ornamental plant for ponds. Since April 2014 it has not been available to buy in the UK.
It is now common in the south-east of England and occasionally found in the north-west of England and Wales.
Floating Pennywort is found emergent or floating on still or slow moving waterways and prefers areas of full sunlight.
This plant can outcompete native species by blocking sunlight, reducing water temperature and preventing air-breathing insects from reaching the water's surface. It can deoxygenate the water by reducing available light to waterweeds and algae, then causing nutrient overload when it dies back.
Floating pennywort can grow up to 20cm a day forming dense mats of vegetation that quickly dominate waterways making navigation difficult and impeding water flow, also increasing flood risk.
Spreads easily via plant segments that break off and move through the waterways via currents or attached to boats. Roots form at the plant nodes, allowing it to quickly regenerate. During the summer it can double it's biomass between 4-7 days.
Floating pennywort is listed under Schedule 9 of the Wildlife and Countryside Act 1981 with respect to England, Wales and Scotland. As such, it is an offence to plant or otherwise cause this species to grow in the wild.
Australian Swamp Stonecrop (Crassula helmsii) is also known as New Zealand Pigmyweed. It was widely sold by garden centres and aquatic centres for its excellent oxygenating properties. However once it had escaped into the environment it has proven to be a problematic non-native invasive species and since April 2014 has been banned from sale.
Crassula helmsii is suited to a wide range of slow moving freshwater systems and forms dense mats of vegetation year-round. It out-competes our native plants by depleting the oxygen levels in water, killing submerged plants such as algae and waterweeds and suppressing the germination of other native species. The resulting deoxygenated water has an adverse effect on invertibrates such as great crested newts, and fish.
Crassula helmsii is not harmful to humans but dense mats can be mistaken as dry land and therefore presents a potential hazard to people, dogs and livestock when present on public access sites.
The dense mats of vegetation can reduce the flow of water along waterways, increasing the risk of flooding and can impact the tourism and recreation of a waterway by preventing the passage of boats and making it impossible for anglers to fish.
Blue-green algae are a number of different species of a type of bacteria (known as cyanobacteria) with the ability to use the sun’s energy to make food in the same way that many plants do. Under certain conditions blooms of algal growth can appear forming thick scums on the surface of the water.
These are of concern to waterways users as one quarter of algal blooms are known to produce toxins. These toxins can result in illness in humans but have been known to kill cows, sheep and dogs that have been drinking from the water’s edge.
Humans face a health risk from exposure to blue-green algae toxins. Illnesses including skin rashes, eye irritation, vomiting, diarrhoea, fever and muscle and joint pain have occurred in people who’ve swallowed or swam through algal scum. These haven’t led to long-term effects or death but, in some cases, the illnesses can be severe.
When the blooms decay they use up the oxygen in the water and can lead to the death of aquatic organisms and a decline in biodiversity.