Michael Austin

Are we overlooking small invasive plants because they are not big enough to be seen, out of sight and therefore out of mind?

Large invasive species can cause problems for the economy, the environment or are a health hazard. But what about the smaller invasive plants that are also causing environmental or economic damage and can be just as difficult to control or eradicate and sometimes more so?

The Big Three

Concern over invasive plants often centres on the ‘Big Three’:

Japanese knotweed, Himalayan balsam and giant hogweed, which are large, widespread, and difficult to control and eradicate, along with being destructive to property, health hazards and environmentally damaging.

Giant hogweed produces sap that contains a phototoxin that can leave people scarred and suffering extreme reactions to sunlight.

Japanese knotweed can grow within riparian and terrestrial areas, causing lasting damage to property by growing through pipes, cracks and foundations. Himalayan balsam can outcompete native species and can lead to increased bank erosion when it dies back.

All three species were brought to the UK during the Victorian times as wondrously large, ornate garden plants. These days they are all invasive and most riparian and aquatic plant control focuses on these species, predominantly using herbicides and physical removal.

What about smaller plants?


However, what about the lesser-known non-native species that are causing just as much damage environmentally?

The following aquatic plants are much smaller than the Big Three.

Despite this, due to their means of growth, they can spread rapidly and easily while out-competing native flora, leading to decreases in native species and potentially the organisms that depend on them.

New Zealand pygmyweed – Crassula helmsii, first recorded in the wild in 1956, is a small plant with delicate little white flowers from New Zealand and Australia.

Pygmyweed grows aquatically as both an emergent and submerged plant but also in damp ground and even terrestrially over riverbanks.

It can grow throughout the year and can quickly smother native species, effectively creating a plant carpet that can be dangerous to people and wildlife by hiding the presence of water beneath it.  The plant itself is fragile and this lends itself to its dispersal as it mainly reproduces vegetatively through the establishment of new plants from broken fragments.

Controlling this plant is extremely difficult as success is highly site-specific, meaning a successful control strategy and method on one site does not equal success on another.

This, coupled with the fact it can grow amongst native species, means it is difficult to eradicate without affecting native species. Integrated management strategies are often the most successful combining several control methodologies into a site-specific plan.

Canadian and Nuttall’s waterweed – Elodea canadensis and Elodea nuttallii, were introduced to the UK in 1842 and 1966, respectively, from North America. They have been sold as oxygenators for aquariums and ponds and this is likely the cause for escape.

The plants are spread through vegetative means and are easily broken up by boats, angling, and other water activities. As a result, they can quickly colonise lakes and displace native plants, leading to lower biodiversity.

Dependent on its extent and density, eradication is unlikely in heavily infested areas due to its ability to quickly re-establish, therefore the emphasis on this species should be on control and reduction.

As a result of their perennial nature and submerged growth, common methods of control such as herbicide application have limited use and success. Control is therefore commonly through physical means, which in turn can inadvertently cause spread through the production of fragments.

Floating pennywort – Hydrocotyle ranunculoides, is originally from the Americas and first found in the UK in 1990, likely as an escapee from aquaculture. In peak summer pennywort can grow up to 30cm a day and spreads via fragments. It is a very delicate plant and can form large floating rafts leading to canal blockages which in turn has social impacts but also can be a flood risk.

Its dense floating mats can restrict light penetration and lead to a decrease in native submerged plants and their associated fauna. Due to its preference for slow-flowing water bodies, canals are ideal habitat and boat traffic has been a major source of spread due to the breaking up of plants by propellers.

Its long growing season, fast growth rates and vegetative reproduction mean it spreads quickly, often outpacing control actions, particularly in heavily infested areas.

Control has been increasing in recent years and while the use of herbicide can be successful combining it with physical removal has yielded the best results and lowers the amount of chemicals used.

Are these smaller plants more damaging to the environment than the ‘Big Three’? This remains to be seen. It is the difficulty in control and eradication that is a major concern, leaving populations to increase and colonise new areas.

Many operators provide guarantees for Japanese knotweed for several years; however, no such guarantees exist for the smaller plants and few operators have experience in controlling aquatic and riparian species outside of the ‘Big Three’.

On a positive note, we are now seeing more and more effort in controlling the smaller plants for the benefit of biodiversity and to reduce the risk of spread.

How we can help

APEM has extensive experience working on Invasive and Non-Native Species (INNS), with a dedicated team able to call upon a broad range of specialist knowledge and expertise. We work with a broad range of clients across marine, terrestrial, and freshwater sectors aiding in meeting requirements on INNS management using this approach.

Our specialist INNS team can provide but not limited to the following services:

  • INNS identification and monitoring including aerial surveys
  • Advice on policy and regulatory review and implementation
  • Asset and pathway risk assessment
  • Eradication and control advice, planning and implementation
  • Biosecurity plan advice and development

If you have an INNS related question or issue, please visit our INNS webpage for more information.



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