We all want our plants to thrive and grow in and around our ponds, but there are some kinds which do so rather too well, and left to their own devices, have a tendency to take over. Many species can grow just too quickly or too large to be comfortably accommodated in a typical domestic garden if conditions suit them, but as a general rule, getting around the problem only involves some radical pruning or removal, with no real harm done otherwise. There are however, some non-native and highly invasive types of aquatic plants which pose a much more serious threat, not just to our own water features, but to the ponds, lakes, rivers and waterways of Britain.
The British countryside has a long history of gaining alien species, either deliberately introduced or escapees-gone-wild, and, as everyone knows happened in the case of both grey squirrels and Japanese knotweed, things can very rapidly get out of hand. While our Victorian forebears were free to experiment with the plants and animals being brought back from different parts of the world, these days we take protecting our vulnerable wildlife and habitats much more seriously – and legislate accordingly.
The principal piece of law is the Wildlife and Countryside Act 1981, which makes it illegal to plant or “cause to grow” a whole range of species included on Schedule 9 of the Act – and a number of water plants were newly added to the list in April 2010. According to the ‘Be Plant Wise’ initiative from the Non-Native Species Secretariat (NNSS) – the organisation responsible for co-ordinating the UK’s approach to dealing with such aggressively colonising, non-indigenous species – the top five are:
- Azolla filiculoides (Water Fern) – originally from tropical America, this tiny plant with its 2.5mm leaves can double its biomass every two days, covering ponds and lakes in no time, reducing oxygen and light levels. The surface cover is so dense that children and animals may be tempted to try to walk on it.
- Crassula helmsii (New Zealand Pigmy-weed, also called Australian Swamp Stonecrop) – forms dense mats of vegetation and aggressively out-competes native plants, resulting in significant changes to the oxygen and light levels in the water, as well as posing an increased risk of flooding if it blocks ditches, drains or water courses.
- Hydrocotyle ranunculoides (Floating Pennywort) – forms thick, light-blocking mats which can grow as much as 20cm per day. The solid-looking surface cover may also lure unsuspecting children and livestock into danger.
- Ludwigia grandiflora (Water-Primrose) – introduced from South America for their showy, bright yellow flowers, they can rapidly grow into thick carpets of vegetation which quickly clog watercourses. There are few currently recorded problems with them in the UK, but elsewhere in Europe, they have already become a major nuisance.
- Myriophyllum aquaticum (Parrot’s Feather) – A distinctive plant with bluish green feathery leaves, the over-wintering ability of this fast growing species belies its Central South American origins and goes some way to explaining why it can rapidly dominate watercourses. It is able to re-grow from fragments under 1cm in length.
Avoiding the Problem
Obviously the simplest way to avoid the problem is not to grow any of these plants in the first place, but achieving that is not always as easy as it seems. Even if you don’t actively go out and buy one of these five – or any of the other plants that can cause trouble in our wild ecosystems – they can still arrive as uninvited hitch-hikers on ‘innocent’ purchases, or gifts from fellow pond keepers. Part of the problem with many of these species is that they can rapidly re-grow from even quite small fragments, so you don’t need much to find its way into your pond to end up unwittingly playing host to a serious potential nuisance.
Fortunately, however, if you employ a bit of simple, back garden ‘bio-security’, you shouldn’t have too many problems. Wash any newly obtained plants or cuttings thoroughly in a bucket of water before you add them to your pond to rinse off any unwanted hangers-on – making sure that once you’ve finished, you empty the contents onto the ground a good way from the pond itself, or from any drains or natural watercourses.
Don’t Help The Spread
It’s just as important to be careful how you dispose of excess plant material from your pond, particularly if you’re not 100 per cent sure of the exact species of everything you’re growing. Although these days, plants bought new from reputable dealers are almost invariably correctly labelled, for long-term residents in an inherited pond, or those obtained from friends, the identification may be rather less sure. Unless you are absolutely certain what you’re dealing with, it pays to play it safe, particularly since some of the most invasive species can easily be confused with much more innocuous varieties.
Any unwanted material should be composted or put out as green waste, if there is a suitable collection scheme in your area. Bear in mind that surprisingly small bits of plant can still represent a danger to native wildlife and habitats, so take particular care to clean any equipment you’ve used while managing your pond – and don’t forget your wellies – to avoid the chance of cross contamination.
As the NNSS say, in the end it’s all about being plant wise – knowing what you grow and how best to look after it. The good news is, with so many suitable varieties to choose from, avoiding the would-be invaders certainly won’t leave you stuck for choice.
Invasive aquatic species can be a major threat to the health and biodiversity of ponds and waterways.
When non-native species are introduced into these environments, they can outcompete native species, disrupt food webs, and alter the balance of the ecosystem. In the UK, several invasive species have been identified as problematic when planted in ponds. Some of these species can rapidly form dense mats, reducing oxygen levels and blocking sunlight from reaching other aquatic plants and animals.
Others can outcompete native species, reduce biodiversity, and alter the physical structure of ponds and waterways. In this list, we will explore some of the most problematic invasive species that are commonly planted in UK ponds and explain why they pose a threat to these fragile ecosystems.
Australian swamp stonecrop/New Zealand pigmyweed (Crassula helmsii): This invasive plant species can quickly form dense mats that shade out and displace native plant species in ponds and waterways. It also has the ability to grow and spread on both land and in water, making it particularly challenging to control.
Parrot’s feather (Myriophyllum aquaticum): This invasive aquatic plant species is known for its rapid growth rate and ability to quickly form dense mats in ponds and waterways, blocking light and nutrients from reaching other plant species and reducing oxygen levels in the water. Parrot’s feather can also clog waterways and interfere with recreation activities like fishing and boating.
Floating pennywort (Hydrocotyle ranunculoides): This invasive aquatic plant species can form dense mats on the surface of ponds and slow-moving waterways, preventing sunlight and oxygen from reaching other aquatic plants and animals. It can also cause flooding and erosion by clogging waterways and interfering with water flow.
Japanese knotweed (Fallopia japonica): This invasive plant species is notorious for its fast growth rate and ability to outcompete native plant species, crowding out other vegetation and reducing biodiversity. It can also cause structural damage to buildings and infrastructure, as well as harm natural ecosystems by altering the soil and nutrient balance.
Waterferns (Azolla filiculoides) & (A. caroliniana): These invasive aquatic ferns can quickly form dense mats on the surface of ponds and slow-moving waterways, blocking sunlight and oxygen from reaching other aquatic plants and animals. They can also release toxins that harm native aquatic species and disrupt food webs.
Indian/Himalayan balsam (Impatiens glandulifera): This invasive plant species can grow up to two meters tall and outcompete native vegetation, reducing biodiversity and altering ecosystems. Its explosive seed pods also contribute to its spread, allowing it to colonize new areas quickly.
Water lettuce (Pistia stratiotes): This invasive aquatic plant species can form dense mats on the surface of ponds and slow-moving waterways, blocking sunlight and oxygen from reaching other aquatic plants and animals. It can also reduce water flow and interfere with navigation and recreation activities.
Giant salvinia (Salvinia molesta): This invasive aquatic plant species can form dense mats on the surface of ponds and waterways, blocking sunlight and oxygen from reaching other aquatic plants and animals. It can also clog waterways and interfere with navigation and recreation activities.
Water hyacinth (Eichornia crassipes): This invasive aquatic plant species can form dense mats on the surface of ponds and slow-moving waterways, blocking sunlight and oxygen from reaching other aquatic plants and animals. It can also reduce water flow and interfere with navigation and recreation activities.
Water chestnut (Trapa natans): This invasive aquatic plant species can form dense mats on the surface of ponds and slow-moving waterways, blocking sunlight and oxygen from reaching other aquatic plants and animals. It can also reduce water flow and interfere with navigation and recreation activities.
Canadian waterweed (Elodea canadensis): This invasive aquatic plant species can outcompete native aquatic plants, reducing biodiversity and altering ecosystems. It can also clog waterways and interfere with navigation and recreation activities.
Nuttall’s waterweed (E. nuttallii): This invasive aquatic plant species can form dense mats on the surface of ponds and slow-moving waterways, blocking sunlight and oxygen from reaching other aquatic plants and animals. It can also outcompete native species and alter ecosystemsLast Modified: March 31, 2023