As we march towards summer most aquatic or water weeds will be starting to spring to life, and most will have some form of environmental impact. Catching them at this early stage is vital if you want to control them and avoid an explosion of growth that has the ability to wipe out other water borne species and damage our natural waterways. In this article, we will look at the UK’s three most common species; Water Fern (Azolla Filiculoides), Floating pennywort, and Parrot’s feather, and some other common offenders.
methods of control
Further on in this article we’ll discuss the various methods of control that we use, and why adopting an integrated approach is best.
But first, what are they and why are they a problem? Aquatic weeds are plant species that grow in or near water bodies such as rivers, lakes, and ponds, they can be either floating, partially or fully submerged. They can cause several problems, including the disruption of water flow, reduction of oxygen levels, and the formation of dense mats that creates other problems for aquatic life and impedes recreational activities, making it difficult for anglers and boaters to navigate the water.
Water fern (Azolla Filiculoides)
Azolla is a genus of seven species of aquatic ferns in the family Salviniaceae most frequently found in ponds, lakes, canals, ditches and slow flowing rivers. A floating aquatic plant that forms dense mats on the water surface it can form a layer up to 30 cm deep, providing shelter for insects and fish that thrive in low-light conditions. A green small leaved plant consisting of rosettes up to a few cm in diameter, which turn a deep shade of red in autumn and winter. Azolla alters the pH of water which poses a risk to other native species, and due to the dense nature of its growth pattern can also be a hazard to livestock mistaking it for solid land as it creates a carpet that appears solid. This density blocks out light, preventing photosynthesis in aquatic plants, compromises oxygen levels and prevents amphibians and invertebrates from reaching the surface. One of the most effective methods of control is the introduction of Stenopelmus rufinasus, an Azolla-feeding weevil.
Floating pennywort (Hydrocotyle Ranunculoides)
Floating pennywort, also known as Pennywort, is a perennial aquatic plant that is native to North America but has become invasive in the UK. It is commonly found in slow-moving or still water bodies, such as ponds and canals. Pennywort has circular leaves that can reach up to 10cm in diameter, with scalloped edges and a bright green colour. The leaves are arranged in a rosette formation and are supported by long stems that can grow up to two meters in length. This causes problems for fish and other aquatic species as their environment becomes limited and movement up and down rivers and waterways becomes increasingly difficult.
Parrot’s feather (Myriophyllum Aquaticum)
Parrot’s feather is a submerged aquatic plant that is native to South America but has become invasive in the UK. It is commonly found in ponds and slow-moving water bodies and can grow up to two meters in length. Parrot’s feather can be identified by its bright green, feathery leaves that are arranged in whorls around the stem. The leaves are usually around 2cm long and are finely divided, giving them a delicate appearance. Parrot’s feather will easily re-establish from its roots that remain in the bed even when the above water growth has died back. It is highly invasive, choking water bodies and out competing native vegetation by blocking out their natural light source.
Other Common Aquatic Weeds in the UK
FLOATING species:
Duckweed (Lemna Minor)
Duckweed (from the family Araceae) is one of the smallest flowering plants in the world but with an extreme growth pattern. One plant produces two more plants and those grow to produce two more and so on which creates the characteristic dense colony (as shown in the main picture at the top of this article). A floating aquatic plant with small, round leaves that cover the water surface, reducing light penetration and oxygen levels, it is highly invasive and its ability to completely smother the water’s surface leads to death of fish and beneficial algae.
Water hyacinth (Eichhornia Crassipes)
A floating aquatic plant with showy purple or blue flowers, it may appear attractive but it invades lakes, rivers and marshlands and can completely block waterways which causes issues with flood control and depletes other submersed plants and aquatic invertebrates. When allowed to spread its mass impacts fishing and transportation, irrigation, and in some cases hydropower infrastructures. Despite being a threat to biodiversity worldwide, its biomass has been found useful for the absorption of toxic waste from polluted water. Biological methods of control are often the most effective, such as introducing arthropods to the affected area, which feed the leaves of water hyacinth. The most common and effective arthropods are weevils, but pathogens, bacteria, fungus, and viruses can also be effective. We recommend using a range of integrated control methods which are more ecologically sustainable than just using herbicide treatments.
SUBMERGED species:
Nuttall’s water weed (Elodea Nuttallii)
Nuttall’s water weed, often confused with Canadian waterweed, is a submerged aquatic plant that is native to North America but has become invasive in the UK. It is commonly found in rivers, lakes, and canals and can grow up to three meters in length. Nuttall’s water weed has green, whorled leaves that are around 1cm long and are arranged in groups of three or four around the stem. It reproduces through fragmentation, whereby fragments of the plant break off and form new plants. Nuttall’s water weed can be identified by its long, slender stems and bright green leaves that are pointed at the tip. It forms dense mats that float on the water surface, which can reduce light penetration, and oxygen levels in the water.
Water soldier (Stratiotes Aloides)
A submerged aquatic plant with stiff, spiky leaves that grow in rosettes on the water surface, often likened to a pineapple. The leaves float on the water surface, and become slimy in winter, secreting calcium carbonate. Water soldier is a good source of food for several species such as dragon flies, however if left to grow will quickly out-compete other native species. However, as mechanical removal is the optimum method here, care should be taken to weigh up the benefits of the removal against the potential negative impacts of disturbing other aquatic life it shares.
Waterweed (Elodea Canadensis)
Often called Canadian pondweed, this submerged aquatic plant has long, green stems and whorled leaves that are arranged in groups of three. These leaves overlap, creating a dense carpet that if left uncontrolled, over a few seasons can cover hundreds of acres, severely impeding natural water flow.
Hornwort (Ceratophyllum Demersum)
A submerged rootless aquatic plant with stiff and brittle yet feathery looking leaves that can grow up to 2 meters in length. A natural floating oxygenator, however it excretes substances that inhibit the growth of phytoplankton and blue-green algae. Because it grows in dense mats it will out-compete other native species if left uncontrolled.
OTHER species:
Crassula Helmsii
Often referred to as New Zealand Pigmyweed or Australian swamp stonecrop is recognisable when growing at the water’s edge by its narrow, fleshy leaves and small succulent like white flowers grows rapidly to form an extensive lush-green carpet that floats on freshwater or may be submerged. Crassula can tolerate extreme environmental conditions and, as such, management can be challenging however, the result of a study undertaken by CABI in 2010 showed the mite, Aculus crassulae gave some success as it affects the plant’s reproductive system, which in turn reduces plant growth.
Common reed (Phragmites Australis)
A tall, reed-like plant that grows along the banks of rivers and lakes, providing shelter for birds and fish. Their roots need to be submerged in water to thrive. If left alone, reed beds of Common reed will naturally be encroached by scrub and succeed to woodland. However, as they tend to be cut and used for thatching buildings, this stops overgrowth and maintains reed beds as a natural habitat for many species.
Controlling Aquatic (or water) Weeds
Aquatic weeds can be challenging to control, and prevention is often the best approach. Simple measures such as removing plant fragments from boats, fishing gear, and other equipment can help to prevent the spread of invasive aquatic plants. Regular monitoring of water bodies can also help to identify invasive species early, allowing for prompt action to be taken.
However, several methods are successful in the control of problematic aquatic weeds, including mechanical, chemical, and biological. Mechanical control involves physically removing the plants from the water, either manually or using machines such as dredgers or aquatic weed harvesters, or if suitable for the environment excavation can be used. Chemical control involves the use of herbicides to kill the plants, but this method must be used with caution and only by a company holding an Aqua Licence, as it can also harm aquatic life. Biological control involves the introduction of natural enemies, such as insects or fish, to control the growth of invasive species, this being the most natural option as the invasive plants literally get eaten.
methods
Management of aquatic weeds is complex, and the best method varies on the species concerned, its location, and how much is present. Even for species that are widespread in the there is no definitive control method. Treatments will vary from species to species and there may be a cross over or multi-faceted approach. Rather than use a single treatment approach, many management plans now use “integrated pest management”, an approach that includes the following:
- Correctly identifying the invasive or nuisance plant(s)
- Identifying desired vegetation to achieve fish and wildlife habitat goals
- Establishing tolerable levels of any single plant species, including target nuisance plant(s)
- Making decisions based on site-specific information
- Using ecosystem, watershed, and cost-benefit perspectives to determine long-term management strategies
- Developing an on-going system of integrated control methods that include mechanical, cultural, biological, and chemical treatments as needed
- Educating local managers and the public about the importance of protecting water resources from invasive weeds to maintain healthy water quality and fish and wildlife habitat
- Assessing results of invasive weed control programs (including quantitative documentation of results of all control strategies), and re-evaluating management options
The best management option is selected based on these considerations and can take a variety of forms, including cutting, hand pulling, biological, mechanical, physical and herbicide. For instance, the cutting of the truly aquatic species can actually exacerbate a weed problem, as many of the species are able to regenerate new plants from stem fragments. A variety of control measures exists including reducing light levels, altering the nutrient regime, use of chemicals (including ones which change water colour in addition to herbicides) and biological control agents.
The choice of appropriate control method has to be carefully considered, on a case-by-case basis, as control methods may be as potentially damaging to other wildlife in the water body as the invasive plant.
Wrapping it up
Aquatic weeds can cause significant problems to UK water bodies, affecting aquatic life and impeding recreational activities such as boating and fishing. Several species are prevalent during the spring and summer months, including Nuttall’s water weed, Pennywort, and Parrot’s feather. By taking action to control invasive aquatic plants, we can help to protect our aquatic environments and their ecosystems.
If you think you may have a problem with invasive aquatic weeds, get our advice; use our free online identification service or arrange to speak with our Aquatic Weed expert by using the contact form below: