Look out: How to identify invasive plants on your local waterway

You may think that having a wide variety of plant species on your local waterway is a good thing. But although variation is undeniably good for biodiversity, the types of plant that grow in an ecosystem are extremely important. Indeed, every species is not suited to every habitat.

If a plant that isn’t native to an area takes root, it is called an invasive species. Far from bringing biodiversity benefits, invasive plants can actually cause harm. In the UK, we have a number of invasive plants that can be dangerous or disruptive and many of these are found on and around our waterways. As you go out to assess water quality, use this guide to help you identify these troublemaking plants

Himalayan Balsam

Native to the Himalayas mountain range spanning Pakistan, India, and western Nepal, the Himalayan Balsam was brought to the UK in 1839. It was destined for greenhouses and warm, managed gardens, but the Himalayan Balsam had other ideas. Within a few years, it had spread into the wild.

Key characteristics

The Himalayan Balsam is most commonly found around riverbanks, streams, ponds and lakes, as well as in ditches and damp meadows.
The flowers, often described as being shaped like a policeman’s helmet, are pink and purple.
There are small red ‘teeth’ at the end of each leaf, which tend to grow in threes.
Stem of each plant is noticeably red in colour.
The seed pods ‘pop’ when they’re touched.

Threats

The Himalayan Balsam spreads quickly, forming dense thickets that alter the ecological balance of wetland habitats. The plants utilise the waterways where they grow to disperse seeds. Its seeds drop into the water, flow downstream and take route on new land and riverbanks. With the ability to spread seeds up to four metres away, it is little wonder that the Himalayan Balsam is so successful. However, this means that it takes nutrients and sunlight from other plant species and often dominates riverbank ecosystems. As other species can no longer survive, this reduces the overall biodiversity of an area. This is especially problematic as the Himalayan Balsam dies back in the winter to leave riverbanks bare. Without plant cover, these banks are vulnerable to erosion.

Japanese Knotweed

The Japanese Knotweed is native Japan, China and Korea but was originally introduced in the UK as an ornamental garden plant. It is versatile, growing near waterways, in ditches and even near urban environments.

Key characteristics

During Spring, the plant develops red and purple tinged shoots. By Summer, these grow into pink buds.
In Summer, Japanese Knotweed grows at a rapid pace. It grows into tall canes, similar to bamboo, that can exceed 2 metres in height. Flecked with purple, these canes are incredibly distinctive.
By late summer, the plant produces white flower tassels that reach up to 15cm in length.
The leaves are also catching. Growing to 14cm long, they are shaped like hearts and protrude from the stem in a zig-zag pattern.
In the Winter, the stems die and return to ground level, but dry canes often live for several more months.

Threats

Japanese Knotweed is powerful and strong. Even sturdy, man-made infrastructure can be infiltrated – reports have found Japanese Knotweed undermining the foundations of buildings. The strength of the plant lies in its root system which is capable of growing through paving, tarmac and flood defences.

American Skunk Cabbage

As the name suggests, this plant is native to America. Growing from Northern California to Alaska, the American Skunk Cabbage can survive a range of weather conditions. Although it was first brought to the UK in 1901, it didn’t escape into the wild until 1947.

Key characteristics

The defining feature of American Skunk Cabbage are its huge, leathery leaves. They range from 40cm to 1.5 metres in length.
The plant’s bright, yellow flowers first appear in early Spring. They can grow up to 45cm.
At the centre of the plant is a large spike, covered by tiny flowers. These flowers release seeds, enabling the American Skunk Cabbage to spread.

Threats

The American Skunk Cabbage thrives in wet habitats. Often found on pond margins, the edges of streams, in bogs or wet woodlands, it is likely that you will encounter this species during your water quality assessments.

Studies conducted in Hampshire have reported declines in other species in the areas where American Skunk Cabbage has established itself. Although unconfirmed, there have been reports of similarly negative impacts in Germany.

Floating Pennywort

Native to America, Floating Pennywort was first introduced to the UK in the 1980s as an ornamental, aquatic plant. However, it didn’t stay confined to our ponds. The plant quickly spread into our waterways and is still considered by many as the worst aquatic weed in the UK.

Key characteristics

Floating Pennywort is most commonly found near water – usually lakes and ponds.
The plant grows horizontal stems that can float on water.
Leaves can grow up to 35cm long, and are usually kidney-shaped.
The flowers are small and can be a greenish-white to light yellow. Usually, these flowers occur in clusters of 5 to 13.

Threats

Floating Pennywort can cause a range of issues to our ecosystem. Firstly, the plant can spread over large areas as it produces a seed-carrying fruit that can float on water. Secondly, it can quickly overwhelm a habitat, reducing the overall availability of oxygen for other plants and fish species. Finally, this species can also harm human infrastructure. For example, blocking drainage systems and pipe networks.

Giant Hogweed

Giant Hogweed was introduced to Europe in the 1800s after it was collected in the Caucasus Mountains. The earliest record of Giant Hogweed in the UK was in 1817 when a sample was brought to Kew Gardens. Since then, it has colonised river banks across the UK.

Key characteristics

Giant Hogweed has thick, bristled stems that are often purple and blotchy.
It has white, flat-topped flowers that grow in clusters facing upwards towards the sun.
These flower heads can each grow to be 60 cm across and the plant itself can reach heights of 3.5m.
Giant Hogweed only lives for two years, but changes significantly in the course of its lifetime. In year one, it has a rosette of jagged leaves but by the second year it forms a large flower spike.

Threats:

When Giant Hogweed dies, the flowers release seeds that flow in river water until they take root in a new location. Grow close together, Giant Hogweed can prevent other plants from accessing sunlight and, as such, it quickly dominates an area.

If you identify Giant Hogweed on a river bank, do not try to remove it. The plant produces a sap that can cause burns if it comes into contact with human skin. This is particularly painful if you are simultaneously exposed to sunlight – it can cause you to develop recurring blisters that can last for months.

Curly Waterweed

Native to Southern Africa, Curly Waterweed was first recorded in the UK in 1944. Unsurprisingly, this plant thrives in watery environments and was intended as an ornamental garden pond plant. However, like other invasive species, it soon spread into the wild.

Key characteristics

Curly Waterweed prefers warmer waters. While it can survive the Winter in Southern England, it is far less likely to do so in the North.
It prefers still or slowly flowing water, such as canals, ponds or lakes.
This is a long plant, with stems sometimes reaching 3 metres in length. This allows it to take root in the mud at the bottom of a waterway but still reach the surface with its shoots.
The curved leaves of this plant can grow in spirals, especially the ones lower down the stem.

Threats

Curly Waterweed grows fast so tends to outcompete most native species. When many plants grow together, they can cause fluctuations in the availability of oxygen in the water which is harmful to fish populations. In densely packed groups, Curly Waterweed can even prevent water flow into drainage systems and cause flooding.

Water Primrose

Like many invasive species, the water primrose was originally introduced to the UK as an ornamental water garden plant. It originates from Australia, New Zealand, North America, and South America but has rapidly (and problematically) established itself in the UK.

Key characteristics:

With bright yellow flowers and willowy leaves, the water primrose is a striking plant that is hard to miss.
The Water Primrose has long, powerful roots that grow horizontally for 4-5 metres around waterways.
These plants also grow vertically to around 1 metre above the water’s surface. This can create the illusion of dry land.
Water Primroses can easily grow from very small fragments, meaning that severe infestations can easily occur.

Threats

The plant can have a hugely negative effect on native habitats. Its excessive growth can outcompete native species, clog waterways and increase risk of flooding. Banned from sale in 2014, the removal of Water Primroses is taken seriously in the UK. It is even on a rapid response list, meaning that when it is spotted, it must be reported for immediate eradication.

Himalayan Knotweed

The Himalayan Knotweed is very similar to the Japanese Knotweed, yet it originated from western Asia. It was introduced to the UK as a garden plant in the Victorian era before rapidly spreading to wild habitats. Although you can still purchase it today, it is increasingly less common.

Key characteristics

Himalayan Knotweed grows beside streams, on woodland edges, roadsides, railway banks and waste ground.
It can be distinguished from Japanese Knotweed by its leaves. They are very narrow and usually their width is about half of their length.
The Himalayan Knotweed develops red shoots in Spring that quickly develop into a thick foliage. By Summer, this often reaches around 2 metres high.
In autumn, the foliage wilts and the leaves turn yellow. By Winter, only the stalks remain.

Threats

Himalayan Knotweed can grow in dense stands. Similarly to other invasive plants, these dense gatherings block sunlight from other, lower-lying plants and prevent them from growing. It can also damage drains and sewers.

Water Fern (Azolla filculoides)

Water Fern, also known as Azolla filculoides, is a small aquatic fern that is native to North America. It was first introduced to the UK in the 19th century as an ornamental pond plant, but quickly escaped into the wild.

Key Characteristics

Water fern is a free-floating fern that forms dense mats on the surface of still or slow-moving water bodies.
The fern is small and grows up to 3 cm long, with tiny overlapping leaves that form a dense mat on the surface of the water.
The leaves are bright green and resemble tiny feathers.
The plant has small roots that hang down into the water, but they are not very effective at anchoring the plant to the bottom of the water body.

Threats

Water fern is a fast-growing plant that can quickly cover the surface of a water body. The dense mats of fern can block out sunlight, reducing oxygen levels in the water and affecting the survival of other aquatic plants and animals. The mats can also clog up waterways, making them difficult to navigate and affecting water quality. In addition, the fern can spread rapidly, as small fragments can break off the mats and start new colonies in other areas of the water body.

Duckweed (Lemnoideae family)

Duckweed is a tiny, free-floating plant that is native to the UK and found in slow-moving or still waters like ponds, lakes, and canals. However, it can become invasive if the conditions are right. It is important to identify duckweed early on to prevent it from taking over waterways.

Key Characteristics

Duckweed is a small plant with a single, round or oval-shaped leaf that can be as small as 1 mm in diameter.
It has no stem or true roots, but instead, it floats on the surface of the water with hair-like roots dangling below.
It reproduces quickly and can double its population in just a few days, forming a dense mat of plants that can cover the entire surface of a body of water.
Duckweed is often mistaken for algae or even young mosquito larvae due to its small size and floating habit.

Threats

Although duckweed can provide some benefits to aquatic ecosystems, such as providing food and cover for fish and insects, it can also become invasive and harm the environment. Duckweed can block out sunlight from underwater plants, disrupting the balance of an ecosystem. A dense mat of duckweed can also lead to reduced oxygen levels in the water, creating hypoxic conditions that can be deadly to fish and other aquatic organisms. Additionally, if duckweed is not managed, it can clog up waterways and impede recreational activities like boating and fishing.

Parrot’s Feather

Parrot’s Feather is native to South America, and it was introduced to the UK in the 1960s as an aquarium plant. It is highly invasive and can spread quickly if not controlled.

Key characteristics

Parrot’s Feather is a submerged aquatic plant with feather-like leaves that can grow up to 5 cm in length.
The leaves are bright green and arranged in a whorl pattern, with up to six leaves per whorl.
It has a distinctive stem, which can reach up to 2 metres in length and is often red or green.
The plant can reproduce both through small white flowers that grow on the surface of the water and through fragmentation.

Threats

Parrot’s Feather is a threat to native aquatic plant and animal species. It can form dense mats on the surface of the water, which can reduce light penetration and oxygen levels, leading to a decrease in biodiversity. The mats can also impede water flow, causing flooding and erosion. The plant can easily spread from one water body to another via water currents, and even small fragments can establish new populations.

New Zealand Pigmyweed aka Australian swamp-stonecrop

New Zealand Pigmyweed, also known as Australian swamp-stonecrop, is an invasive aquatic plant that is native to the southern hemisphere. It was first introduced to the UK in the 1950s as an ornamental plant in garden ponds, but it soon escaped into the wild.

Key characteristics:

New Zealand Pigmyweed is an aquatic plant that is usually found in slow-moving or stagnant water bodies, such as ponds, lakes, canals, and ditches.
It has small, oval-shaped leaves that are typically less than 1cm in length.
The leaves are bright green and form dense mats on the surface of the water.
The plant also produces small, white flowers in the summer.

Threats:

New Zealand Pigmyweed can grow rapidly and form dense mats on the surface of the water, blocking out sunlight and reducing oxygen levels. This can cause harm to other aquatic plants, fish, and other wildlife that depend on the water body.

The plant is able to spread easily through fragments of stems and leaves, and it can quickly colonise new water bodies, making it difficult to control and eradicate.

New Zealand Pigmyweed is also able to tolerate a wide range of environmental conditions, including low temperatures and high nutrient levels, which makes it highly adaptable and resilient.

What should I do if you find an invasive plant?

The Government website contains practical advice for dealing with invasive plants:

How to prevent the spread of harmful invasive plants

You can cut plants back to prevent seed dispersal or you can dig them out to stop regrowth. Whatever your method, you should burn the debris to prevent the plant from taking root elsewhere.

Important safety information:

Do not attempt to remove Giant Hogweed due to the potential burns and blisters it can cause to skin.
Do not wade into fast-flowing water to remove plants.
Avoid slippery plants, especially if working alone.