In 1847 the Japanese knotweed was a gold-medal-winning plant with its unusually vigorous growth and pretty white flowers. Things are considerably different today, with British house buyers being denied mortgages if this invasive alien is found on a property.

It has also taken hold here. Knotweed can penetrate through the slightest crack in a wall or tarmacadam and grow up to 2cm a day, extending 7m horizontally and 3m deep.

Japanese knotweed is one of an alarmingly high number of unwanted and sometimes dangerous invasive species that have a foothold here.

Invasive species are ones that are largely attributable to human intervention. Some have been deliberately released, while others, such as the American mink and giant rhubarb, have escaped from gardens and farms. Others, such as the New Zealand flatworm, arrive as stowaways in the soil of imported plants.

Dr Noeleen Smyth, conservation botanist at the National Botanic Gardens, explains that non-natives can appear to present no problem at first. It is only after a couple of generations that some become so well-adapted to local conditions that their numbers take off.

In Ireland we have about 2,500 nonnative species of plants, mammals, fish and insects; four-fifths of these are plants. Natives of Asia and Africa are now beginning to thrive here.

The majority don’t present a problem, and the benefits are great when nature works in harmony, says Colette O’Flynn, research officer with the National Biodiversity Data Centre in Waterford. A non- native becomes an issue only “when it has a significant negative impact on the health of our wildlife”.

These invasives commonly out- compete native plants or animals and lack natural controls such as diseases and predators. Currently 51 non-natives are seen as invasive, with a further 78 considered to be worth monitoring.

This year sees the launch of an EU-wide attempt to deal with the problem of invasive alien species (IAS), which is causing millions of euro worth of damage throughout Europe. The EU IAS Regulation involves a phased introduction of various strategies, including an early-warning and rapid-response system with border controls in member states.

Ireland, for example, will likely have to try to prevent the escape of our grey squirrel (once an alien itself) and carpet sea squirt, a revolting-looking sea animal also known as “sea snot”. But first, member states have to compile an inventory of notifiable species that cannot be traded or imported into Europe.

Since 2011 it has been an offence to release into the wild certain non-native species such as Hottentot fig or Siberian chipmunk, listed in regulation 49 of the European Communities (Birds and Natural Habitats) Regulations. And by released into the wild, O’Flynn also means gardens, as a “garden is effectively the wild”.

The regulations cover not just plants but animals that might have become too much for owners, who might think it is a kindness to release them rather than rehome them.

However, regulation 50 of the same EC legislation – which prohibits the import, advertising or sale of certain species – is not yet in effect, although Ireland is ahead of some other European states in its legislative efforts, according to O’Flynn. “We expect it to be enacted in the summer of 2015,” she says.

According to the Department of Heritage, the delay in bringing section 50 into effect has been caused by the need to consult with groups that may be affected by the new law. It is also necessary to carry out risk assessments on potentially invasive species that are traded.

“The assessments were completed on contract last year and the department will shortly send the file to the Minister for decision,” the department says.

Potential invaders

The list of potential invaders continues to grow. The next could be the quagga mussel, which was found in Britain last year. Its impact would likely be similar to that caused by the infamous zebra mussel, which can dominate an area of water and reduce the amount of food available for other species. The quagga could be worse, because it tolerates a much wider selection of habitats.

Then there is the well-named “killer shrimp”, with its ferocious appetite. Also likely to reach us is the large Asian hornet. It is described as a highly aggressive predator of native insects that poses a significant threat to honey bees and other pollinators.

Sometimes unwanted invaders can be made to provide a benefit as a resource, something achieved by a group of Swedish students. Lake Victoria, in Kenya, is suffering from an invasive water hyacinth. The students combined the absorptive powers of the plant with the need to provide menstrual pads for Kenyan girls who were missing school, in the process inventing the Jani biodegradable pad.

THE PRICE OF INVASION: OVER €200M A YEAR

The estimated annual cost of invasive species to the Irish economy is almost €203 million, and to the island as a whole about €261 million.

It was feared that one species, Curly pondweed (lagarosiphon major) had the potential to close Lough Corrib back in 2005. By 2013 more than €2.2 million had been spent in controlling the plant.

“We realised that the threat was very significant to the lake and the whole tourist industry around the lake,” says Joe Caffrey, an invasives expert who recently retired from Inland Fisheries Ireland. Inland Fisheries worked on the clean-up project with support from the EU, the National Parks and Wildlife Service and the Office of Public Works. Some €250,000 annually is still being spent “just to keep the problem at bay”, he says.

In the next few weeks the National Roads Authority will launch a pilot study into eradicating non-native species such as Japanese knotweed, a plant so tough it can even damage roads. An NRA report points out that giant hogweed, Himalayan balsam and Japanese knotweed are often found together outcompeting native riverbank plants. These invasives die back during winter, exposing the soil and leading to erosion and collapse, which in turn contributes to flooding.

Public authorities are already putting resources into trying to eradicate various aliens, often with significant input from volunteers. The National Botanic Gardens has been running various projects on invasive species control and habitat restoration.

More about invasive species and how you might help at invasives.biodiversityireland.ie and invasivespeciesireland.com