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Invasive Plants to Avoid

By: Dr Gareth Evans - Updated: 18 Nov 2014 | comments*Discuss
 
Plants Invasive Non-native Species Water

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.

Alien Invasion!

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.

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