Invasive Water Plants Affecting Scotland
Whilst common invasive non-native plant species like Japanese Knotweed, Giant Hogweed and Himalayan Balsam are known to grow on the edges of lochs and watercourses in Scotland, often less attention is paid to invasive non-native species that grow in the water. However, many such invasive non-native water plants are already widespread across Scotland whilst some are better established further south in the UK and are just starting to spread to Scottish rivers and lochs.
Some of these invasive non-native plants have escaped (or been fly tipped) from garden ponds and aquariums. Another common method of spread is via fragments which can hitchhike on leisure boats/canoes, fishing equipment, animals and aquaculture/maintenance equipment.
Species Affecting Scotland
As most invasive non-native water plants initially escaped into the wild much further south, it has taken longer for them to spread to and across Scotland, despite the amount of leisure visitors coming to our lochs and rivers. Many plants have much lower distribution profiles across Scotland compared to parts of England and Wales. However, these four plants are already well established across Scotland:
Canadian Pondweed / Waterweed (Elodea canadensis) is native to temperate parts of Canada and northern America. It was first recorded in Berwickshire in 1842 and today it is endemic across the whole of the UK and Ireland with only some remote parts of the Highlands remaining unaffected.
It has thread-like stems growing up to 3m in length, with branching stems around 30cm long, bearing whorls of three curved round ended leaves 4.5-17mm long. Unlike other invasive non-native water plants, it is tolerant of faster flowing water, although cannot form dense beds there. It reproduces vegetatively as all plants are female in the UK. Fragmented stems sink and take root, establishing new growth. A range of native fish and birds eat it, but these don’t have a significant control effect and may even help it spread.
The most effective method of remediation is the physical removal of the plants, such as hand pulling and raking. However, as the plant reproduces and spreads by fragmentation, care needs to be taken to ensure all vegetation is removed. This can make complete eradication difficult, following the initial treatment up with an ongoing management plan is the best way to prevent regrowth and re-establishment in affected sites.
Nuttall’s Pondweed / Waterweed (Elodea nuttallii) is another plant native to the temperate North American region. It was first recorded in a ditch in Oxfordshire in 1966. Since then it has spread widely across England and Wales, and is also present across Scotland, though mostly concentrated in the central belt and south.
Nuttall’s Pondweed looks very similar to Canadian Pondweed, except the leaves have pointed ends, and it also reproduces and spreads vegetatively through fragmentation as all plants are female in the UK. Again, the best method of control is by physically removing the plants, being sure to not leave any plant fragments, followed up with an ongoing management plan.
New Zealand Pygmyweed
New Zealand Pygmyweed / Australian Swamp Stonecrop (Crassula helmsii) is native to Australasia and was introduced to the UK in 1911 as an oxygenating plant for ponds. It is now widely spread across England and Wales and is present throughout Scotland, except for northern and remote regions of the Highlands, although much more sporadically than down South.
New Zealand Pygmyweed is an emergent or marginal evergreen perennial. With 2cm long succulent leaves in pairs opposite each other, bearing a single white or pale pink flower with four petals. It can produce seeds but these are not viable therefore, the main method of spread is vegetatively via fragmentation. A single node <5mm of stem can regenerate into a new plant. The plant can tolerate less water than full aquatics and will grow in ditches and marshy margins of ponds and lochs, as well as submerged in the water. It can form dense mats up to 3m deep, shading out native plants and leading to deoxygenation of the water which is harmful to aquatic life.
In areas where the plant is out of the water, New Zealand Pygmyweed can be treated with herbicides, but for submerged growth physical removal must be used, taking care to remove all plant fragments. It can be very difficult to eradicate as tiny fragments can regenerate, so initial removal must be followed up with an ongoing management plan.
Curly Waterweed (Lagarosiphon major) is native to southern Africa. First noted in the wild in Bedfordshire in 1944, today it is widespread across England and Wales. In Scotland, it is still largely consigned to the central belt and the south, although it has been reported in parts of the Highlands and Orkney too.
The plant has strongly curved dark green leaves in a spiral arrangement and can grow up to 3m in water. It also reproduces vegetatively by fragmentation. It grows as a dense “canopy” in slow moving water bodies like canals and ponds, it impedes water flow and blocks out light outcompeting native plants. In areas of dense growth it can deoxygenate the water and harm aquatic life.
As Curly Waterweed is another submerged plant that spreads by fragmentation, it is best treated initially by removing the plants, followed up with an ongoing management plan.
Ones to Watch Out For
The following two plants are not yet widespread in Scotland but do have isolated established populations in some areas and are already very widespread throughout England, so will definitely pose a threat to Scotland’s lochs and rivers in the future if care isn’t taken to prevent their spread:
Parrot’s Feather (Myriophyllum aquaticum) is native to the lowlands of South America and has been cultivated in the UK since 1878. It was first recorded in the wild in 1960 and is now widespread in the midlands and south of England and parts of Wales. It has been recorded in Dumfries and Galloway and the northern Highlands but has not spread to other parts of Scotland for now.
Parrot’s Feather has many branches with whorls of leaves with many fine divides, giving them a feathery appearance. The branches can grow up to 13cm above the water. It forms dense growth in still water such as canals, ponds and ditches but can also grow in flowing water. Significant infestations can block out the light and cause localised oxygen depletion.
Cutting, pulling and dredging can be effective methods for controlling smaller areas but care must be taken to avoid fragmentation as this is how the plant vegetatively reproduces. Parrot Feather stands growing above the surface can also be treated using herbicides. As with the other invasive non-native water plants, an ongoing management plan is required to prevent it becoming re-established.
Floating Pennywort (Hydrocotyle ranunculoides) is native to the Americas. It was first recorded in the UK in Essex in 1990 and has since spread across southeast England and the Midlands, with patches in Wales. Currently it is only found in a couple of locations in the northern Highlands of Scotland but is highly invasive and has the potential to spread further.
Floating Pennywort has blunt-toothed, kidney-shaped, flat leaves growing from a fleshy stem which hold them above the water. It does flower but the main method of reproduction is vegetatively by fragmentation and it can regrow from very small fragments. It prefers still or slow-moving water and can rapidly take over ponds and canals in late summer – even to the point where it affects boat users. These dense mats block out the light, out-compete native plants and can cause localise deoxygenation.
Physical removal using rakes or weed removal boats is effective at reducing the volume of Floating Pennywort but as it can regrow from very small fragments, an ongoing management plan is required to prevent it recolonising previously treated areas. Herbicides also work well, but in areas of dense growth it cannot reach all the leaves, so a combination of treatments is often used in non-sensitive locations.
Remediating invasive Water Plants
Because many of these plants grow in and under the water, it is difficult to treat them with herbicides. Outside of man-made watercourses and smaller ponds which have been completely taken over, they share the same environment as many sensitive native plants which would also be affected by chemical control.
The best method of control in most cases is to physically remove the plants, which can be done by hand pulling and cutting in small or sensitive sites, or using machinery such as long-reach excavators, dredgers and weed removal boats on larger areas. However, as many of these aquatic invasive weeds reproduce and spread by fragmentation, extra care must be taken to ensure all pieces of the plant are removed and do not spread downstream where they can colonise new areas. Once removed from the water, many plants will dry out and die, and start to decompose. However, dredged material will contain mud and water and may stay damp for several weeks, allowing rhizomes or stem fragments to survive. Care must be taken that this material is disposed of properly (at a suitably licenced facility for Schedule 9 plants ) and not accidentally spread to another water body.
As these plants are hardy and will regrow from any remaining plant fragments, complete eradication can be very difficult. Often combinations of control methods with regular repeat treatments over several years will be required, and the methods will vary from location to location depending on the plant, the type of waterbody and the surrounding flora and fauna. For this reason, a site-specific treatment and management plan is essential to prevent regrowth and re-establishment of these invasive weeds in affected area.
If you have a site that is affected by one of the invasive non-native water plants above and you would like to know more about ERS’ remediation services or arrange a site survey, please contact our Invasive Weeds team specialist Lianne Cameron on email@example.com.
How can I Help Prevent the Spread of Invasive Water Plants in our Rivers and Lochs?
Due to the difficulties remediating invasive water plants, prevention really is better than cure. Everyone can help keep an eye on the spread of invasive plants in and out of the water by reporting sightings to the GB Non-native Species Secretariat (NNSS). You can record sightings online using the iRecord website or by emailing firstname.lastname@example.org.
For people who enjoy water sports and fishing, follow the NNSS’ “Check, Clean, Dry” initiative to prevent the spread of invasive plants to new rivers and lochs:
Check your equipment, boat, clothing and pets for mud, plants and aquatic animals. Remove anything you find and leave it at the site.
Clean equipment thoroughly as soon as you can using hot water if possible. Pay attention to damp areas and don’t forget places that are hard to reach/access.
Dry everything properly and leave for as long as you can before using it elsewhere. Some invasive plants and animals can survive for over 2 weeks in damp conditions.
There are also local volunteering initiatives across Scotland, so if you’re lucky enough to live near a watercourse or loch that you’re concerned may be affected by invasive plants in and out of the water, ask your local community group, fishing club, River Trust or landowner if there are any opportunities nearby.