Your grandchildren have started to come along and so you have decided that perhaps the time has come to fill in your garden pond – transform it into a sandpit perhaps? You give away the fish and gather up all that healthy-looking pondweed to throw into the pond in the nearby common... STOP! Don’t do it! That vegetation that is so innocuous-looking in your garden water feature could be – or harbour – a voracious plant alien, just waiting for the chance to invade our countryside.

Over-dramatic? Not a bit of it. According to Plantlife, the charity that campaigns on behalf of our wild plants and plant conservation, non-native invasive plant species will over-run the British countryside unless urgent measures are taken to halt their introduction and spread here.

According to their 2000 report, At War with Aliens, species associated with aquatic habitats – sold for use in garden ponds and aquaria – are the chief culprits. Australian Swamp Stonecrop (also called New Zealand Pigmyweed) Crassula helmsii, Parrot’s Feather Myriophyllum aquaticum and Floating Pennywort Hydrocotyle ranunculoides are the three main aliens threatening our native pond species.

Freed of predators in their native homeland, aliens can thrive unimpeded and research is being carried out worldwide to find some way of controlling them. We’ve all heard of the plague of rabbits in Australia, but equally serious is the water hyacinth, the purple-flowered plant that is choking lakes in Africa and thought to have been transported there, courtesy of air travellers, far from its Amazonian home.

Anne Larigauderie of the Diversitas Secretariant based in Paris studied the water hyacinth for her doctorate and explains how the plant, whose leaves float on the water, blocks sunlight and kills plants and fish below. African nations have tried everything to combat their spread, including mashing them up with machines called “swamp devils” and introducing beetles to prey on them. So far British winters have been too cold for the water hyacinth to gain a foothold in the wild, but with global warming it could pose a major threat in the future, as it has done in parts of the US.

Countries such as New Zealand, Australia, South Africa and the US have been concerned with plant aliens for years, introducing strong legislation to prohibit their sale, and even informal trade of certain plants between gardeners. However, in England and Wales only Japanese knotweed and the hazardous giant hogweed, which is found along waterways and whose sap can cause severe burns to humans, come under Schedule 9 of the Wildlife and Countryside Act of 1981. It remains the UK’s strongest piece of anti-alien legislation. In Scotland the schedule was updated in June 2005 and now includes nine invasive aquatic plants and four more terrestrial plants.

The Royal Horticultural Society has banned seven species from its shows and gardens and Plantlife has listed 20 plants on its website that it would like to see legally banned (see list below). You can help Plantlife campaign for tighter controls by looking out for these plant invaders and letting them know where you see them (in gardens, garden centres, local parks, along waterways) by filling in the online survey form. They urge everyone to spread the word and tell fellow gardeners about these problem plants.

Until a ban is set in legislation, says Dick Shaw, an invasive weed specialist with CABI Bioscience, “the greatest impact on the control of these aliens will be garden centres and individual gardeners acting responsibly in the interests of the environment.” The Environment Agency has somewhat belatedly developed a new control programme to help developers indentify knotweed, parrot’s feather, Himalayan balsam and some animal invaders, with cost-effective ways of destroying them.

If you realise you’ve got them, how do you dispose of them? “Householders should not try to dispose of them by composting or taking them to council recycling depots,” says Surrey Wildlife Trust river specialist Chris Matcham, writing in the spring 2006 issue of Surrey Nature. “The only certain way to destroy them is to burn them... Japanese knotweed is classified as ‘hazardous waste’ – these plants are not to be trifled with.”

Banned by the RHS

In 2003 the RHS banned the following seven species from its shows and gardens:

1. Japanese knotweed:A single clone of Fallopia japonica, which arrived in the UK in 1850, has spread far and wide and would cost an estimated £1.5 billion to eradicate. It can generate from a piece of rhizome as small as a pea and is strong enough to break apart drains, concrete paving and tarmac. It can grow up to 3.7m (12ft) in a few weeks.

2. Himalayan balsam (impatiens glandulifera) has pretty flowers and explosive seedpods and has turned riverbanks across Europe into ‘balsam highways’. It dominates fragile ecosystems by shading out competitors and may draw native pollinators away from their proper jobs.

3. Giant hogweed (Heracleum mantegazzianum) arrived from the Caucasus mountains in 1828 but has only quite recently been recognised as a risk to human health. Its sap causes severe blistering on contact, which worsens with exposure to sunlight. Again found particularly along watercourses.

4. The four aquatic species are very fast-growing and can quickly dominate ponds and slow-moving streams and rivers, increasing flooding risks, sometimes interfering with navigation and reducing native biodiversity. Each has escaped from garden ponds. They are: Swamp stonecrop, fairy fern, parrot’s feather and floating pennywort.

In addition to the ‘RHS Seven’, Plantlife believes the following should also be banned from sale:

• Fanwort Cabomba caroliniana
• Cotoneaster microphyllus
• Cotoneaster simonsii
• Water hyacinth Eichhornia crassipes
• Elodea candensis
• Elodea nuttallii
• Giant rhubarb Gunnera tinctoria
• Spanish bluebell Hyacinthoides hispanica
• Garden or hybrid bluebell Hyacinthoides hispanica x H. non-scripta
• Curly waterweed Lagarosiphon major
• Water lettuce Pistia stratiotes
• False acacia Robinia pseudoacacia
• Rhododendron Rhododendron ponticum
• Rhododendron ponticum x catawbiense
• Rhododendron ponticum x maximum
• Giant salvinia Salvinia molesta
  

Useful websites to help you identify plant aliens

RHS – www.rhs.org.uk
Plantlife – www.plantlife.org.uk
CABI Bioscience – www.cabi.org This is a not-for-profit organisation owned by member countries including the UK. There are many pages on invasive species and their control.
Helpful PDFs on the control and recognition of invasive aquatic and waterside weeds can be found on the website of the Centre for Aquatic Plant Management – www.ceh.ac.uk/sections/wq/CAPMInformationsheets.htm